Update.

	* sysdeps/unix/sysv/linux/check_fds.c: New file.
	* sysdeps/generic/check_fds.c: Check that file opened is really
	/dev/null.

1 files changed, 25 insertions, 5 deletions

diff --git a/sysdeps/generic/check_fds.c b/sysdeps/generic/check_fds.c
index 4eea168794..9891b9c865 100644
--- a/sysdeps/generic/check_fds.c
+++ b/sysdeps/generic/check_fds.c
@@ -20,6 +20,8 @@
 #include <fcntl.h>
 #include <paths.h>
 #include <unistd.h>
+#include <sys/stat.h>
+#include <sys/sysmacros.h>
 
 /* Try to get a machine dependent instruction which will make the
    program crash.  This is used in case everything else fails.  */
@@ -38,11 +40,22 @@ check_one_fd (int fd, int mode)
   if (__builtin_expect (__libc_fcntl (fd, F_GETFD), 0) == -1
       && errno == EBADF)
     {
+      struct stat64 st;
+
       /* Something is wrong with this descriptor, it's probably not
 	 opened.  Open /dev/null so that the SUID program we are
 	 about to start does not accidently use this descriptor.  */
       int nullfd = __libc_open (_PATH_DEVNULL, mode);
-      if (__builtin_expect (nullfd, 0) == -1)
+      /* We are very paranoid here.  With all means we try to ensure
+	 that we are actually opening the /dev/null device and nothing
+	 else.  */
+      if (__builtin_expect (nullfd, 0) == -1
+	  || __builtin_expect (__fxstat64 (_STAT_VER, nullfd, &st), 0) != 0
+	  || __builtin_expect (S_ISCHR (st.st_mode), 1) == 0
+#if defined DEV_NULL_MAJOR && defined DEV_NULL_MINOR
+	  || st.st_rdev != makedev (DEV_NULL_MAJOR, DEV_NULL_MINOR)
+#endif
+	  )
 	/* We cannot even give an error message here since it would
 	   run into the same problems.  */
 	while (1)
@@ -55,8 +68,15 @@ check_one_fd (int fd, int mode)
 void
 __libc_check_standard_fds (void)
 {
-/* Check all three standard file descriptors.  */
-  check_one_fd (STDIN_FILENO, O_RDONLY);
-  check_one_fd (STDOUT_FILENO, O_RDWR);
-  check_one_fd (STDERR_FILENO, O_RDWR);
+  /* This is really paranoid but some people actually are.  If /dev/null
+     should happen to be a symlink to somewhere else and not the device
+     commonly known as "/dev/null" be bail out.  We can detect this with
+     the O_NOFOLLOW flag for open() but only on some system.  */
+#ifndef O_NOFOLLOW
+# define O_NOFOLLOW	0
+#endif
+  /* Check all three standard file descriptors.  */
+  check_one_fd (STDIN_FILENO, O_RDONLY | O_NOFOLLOW);
+  check_one_fd (STDOUT_FILENO, O_RDWR | O_NOFOLLOW);
+  check_one_fd (STDERR_FILENO, O_RDWR | O_NOFOLLOW);
 }