diff --git a/pkgs/servers/x11/xorg/fix_segfault.patch b/pkgs/servers/x11/xorg/fix_segfault.patch
new file mode 100644
index 000000000000..b3a7d2ed5e00
--- /dev/null
+++ b/pkgs/servers/x11/xorg/fix_segfault.patch
@@ -0,0 +1,63 @@
+From 7cc7ffd25d5e50b54cb942d07d4cb160f20ff9c5 Mon Sep 17 00:00:00 2001
+From: Martin Peres <martin.peres@linux.intel.com>
+Date: Fri, 17 Jul 2015 17:21:26 +0300
+Subject: [PATCH] os: make sure the clientsWritable fd_set is initialized
+ before use
+
+In WaitForSomething(), the fd_set clientsWritable may be used unitialized when
+the boolean AnyClientsWriteBlocked is set in the WakeupHandler(). This leads to
+a crash in FlushAllOutput() after x11proto's commit
+2c94cdb453bc641246cc8b9a876da9799bee1ce7.
+
+The problem did not manifest before because both the XFD_SIZE and the maximum
+number of clients were set to 256. As the connectionTranslation table was
+initalized for the 256 clients to 0, the test on the index not being 0 was
+aborting before dereferencing the client #0.
+
+As of commit 2c94cdb453bc641246cc8b9a876da9799bee1ce7 in x11proto, the XFD_SIZE
+got bumped to 512. This lead the OutputPending fd_set to have any fd above 256
+to be uninitialized which in turns lead to reading an index after the end of
+the ConnectionTranslation table. This index would then be used to find the
+client corresponding to the fd marked as pending writes and would also result
+to an out-of-bound access which would usually be the fatal one.
+
+Fix this by zeroing the clientsWritable fd_set at the beginning of
+WaitForSomething(). In this case, the bottom part of the loop, which would
+indirectly call FlushAllOutput, will not do any work but the next call to
+select will result in the execution of the right codepath. This is exactly what
+we want because we need to know the writable clients before handling them. In
+the end, it also makes sure that the fds above MaxClient are initialized,
+preventing the crash in FlushAllOutput().
+
+Thanks to everyone involved in tracking this one down!
+
+Reported-by: Karol Herbst <freedesktop@karolherbst.de>
+Reported-by: Tobias Klausmann <tobias.klausmann@mni.thm.de>
+Signed-off-by: Martin Peres <martin.peres@linux.intel.com>
+Tested-by: Martin Peres <martin.peres@linux.intel.com>
+Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=91316
+Cc: Ilia Mirkin  <imirkin@alum.mit.edu>
+Cc: Martin Peres <martin.peres@linux.intel.com>
+Cc: Olivier Fourdan <ofourdan@redhat.com
+Cc: Adam Jackson <ajax@redhat.com>
+Cc: Alan Coopersmith <alan.coopersmith@oracle.com
+Cc: Chris Wilson <chris@chris-wilson.co.uk>
+---
+ os/WaitFor.c | 1 +
+ 1 file changed, 1 insertion(+)
+
+diff --git a/os/WaitFor.c b/os/WaitFor.c
+index 431f1a6..993c14e 100644
+--- a/os/WaitFor.c
++++ b/os/WaitFor.c
+@@ -158,6 +158,7 @@ WaitForSomething(int *pClientsReady)
+     Bool someReady = FALSE;
+ 
+     FD_ZERO(&clientsReadable);
++    FD_ZERO(&clientsWritable);
+ 
+     if (nready)
+         SmartScheduleStopTimer();
+-- 
+2.4.5
+
diff --git a/pkgs/servers/x11/xorg/overrides.nix b/pkgs/servers/x11/xorg/overrides.nix
index 2036e62ee945..c1a8fec596f3 100644
--- a/pkgs/servers/x11/xorg/overrides.nix
+++ b/pkgs/servers/x11/xorg/overrides.nix
@@ -282,7 +282,8 @@ in
         inputproto xextproto randrproto renderproto presentproto
         dri2proto dri3proto kbproto xineramaproto resourceproto scrnsaverproto videoproto
       ];
-      commonPatches = [ ./xorgserver-xkbcomp-path.patch ];
+      # fix_segfault: https://bugs.freedesktop.org/show_bug.cgi?id=91316
+      commonPatches = [ ./xorgserver-xkbcomp-path.patch ./fix_segfault.patch ];
       # XQuartz requires two compilations: the first to get X / XQuartz,
       # and the second to get Xvfb, Xnest, etc.
       darwinOtherX = overrideDerivation xorgserver (oldAttrs: {