Fix detection of getrandom()

ChangeLog

@@ -28,6 +28,8 @@ Features
      errors on use of deprecated functions.
 
 Bugfix
+   * Fix detection of support for getrandom() on Linux (reported by syzzer) by
+     doing it at runtime (using uname) rather that compile time.
    * Fix handling of symlinks by "make install" (found by Gaël PORTAY).
    * Fix potential NULL pointer dereference (not trigerrable remotely) when
      ssl_write() is called before the handshake is finished (introduced in

library/entropy_poll.c

@@ -86,27 +86,46 @@ static int getrandom_wrapper( void *buf, size_t buflen, unsigned int flags )
 {
     return( syscall( SYS_getrandom, buf, buflen, flags ) );
 }
-#endif /* SYS_getrandom */
-#endif /* __linux__ */
 
-#if defined(HAVE_GETRANDOM)
+#include <sys/utsname.h>
-
+/* Check if version is at least 3.17.0 */
-#include <errno.h>
+static int check_version_3_17_plus( void )
-
-int platform_entropy_poll( void *data,
-                           unsigned char *output, size_t len, size_t *olen )
 {
-    int ret;
+    int minor;
-    ((void) data);
+    struct utsname un;
+    const char *ver;
 
-    if( ( ret = getrandom_wrapper( output, len, 0 ) ) < 0 )
+    /* Get version information */
-        return( POLARSSL_ERR_ENTROPY_SOURCE_FAILED );
+    uname(&un);
+    ver = un.release;
+
+    /* Check major version; assume a single digit */
+    if( ver[0] < '3' || ver[0] > '9' || ver [1] != '.' )
+        return( -1 );
+
+    if( ver[0] - '0' > 3 )
+        return( 0 );
+
+    /* Ok, so now we know major == 3, check minor.
+     * Assume 1 or 2 digits. */
+    if( ver[2] < '0' || ver[2] > '9' )
+        return( -1 );
+
+    minor = ver[2] - '0';
+
+    if( ver[3] >= '0' && ver[3] <= '9' )
+        minor = 10 * minor + ver[3] - '0';
+    else if( ver [3] != '.' )
+        return( -1 );
+
+    if( minor < 17 )
+        return( -1 );
 
-    *olen = ret;
     return( 0 );
 }
-
+static int has_getrandom = -1;
-#else /* HAVE_GETRANDOM */
+#endif /* SYS_getrandom */
+#endif /* __linux__ */
 
 #include <stdio.h>
 
@@ -117,6 +136,22 @@ int platform_entropy_poll( void *data,
     size_t ret;
     ((void) data);
 
+#if defined(HAVE_GETRANDOM)
+    if( has_getrandom == -1 )
+        has_getrandom = ( check_version_3_17_plus() == 0 );
+
+    if( has_getrandom )
+    {
+        int ret;
+
+        if( ( ret = getrandom_wrapper( output, len, 0 ) ) < 0 )
+            return( POLARSSL_ERR_ENTROPY_SOURCE_FAILED );
+
+        *olen = ret;
+        return( 0 );
+    }
+#endif /* HAVE_GETRANDOM */
+
     *olen = 0;
 
     file = fopen( "/dev/urandom", "rb" );
@@ -135,7 +170,6 @@ int platform_entropy_poll( void *data,
 
     return( 0 );
 }
-#endif /* HAVE_GETRANDOM */
 #endif /* _WIN32 && !EFIX64 && !EFI32 */
 #endif /* !POLARSSL_NO_PLATFORM_ENTROPY */