[PATCH] x86: gettimeofday() vDSO: fix segfault when tv == NULL

[John Wright <john.wright@hp.com>]

According to the gettimeofday(2) manual:

       If either tv or tz is NULL, the corresponding structure is not
       set or returned.

Since it is legal to give NULL as the tv argument, the code should make
sure tv is not NULL before trying to dereference it.

This issue manifests itself on x86_64 when vdso=0 is not on the kernel
command-line and libc uses the vDSO for gettimeofday() (e.g. glibc >=
2.7).  A simple reproducer:

  #include <stdio.h>
  #include <sys/time.h>

  int main(void)
  {
      struct timezone tz;

      gettimeofday(NULL, &tz);

      return 0;
  }

See http://bugs.debian.org/466491 for more details.

Signed-off-by: John Wright <john.wright@hp.com>
---
 arch/x86/vdso/vclock_gettime.c |   12 +++++++-----
 1 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/arch/x86/vdso/vclock_gettime.c b/arch/x86/vdso/vclock_gettime.c
index d9d3582..6a40b78 100644
--- a/arch/x86/vdso/vclock_gettime.c
+++ b/arch/x86/vdso/vclock_gettime.c
@@ -104,11 +104,13 @@ notrace int __vdso_gettimeofday(struct timeval *tv, struct timezone *tz)
 {
 	long ret;
 	if (likely(gtod->sysctl_enabled && gtod->clock.vread)) {
-		BUILD_BUG_ON(offsetof(struct timeval, tv_usec) !=
-			     offsetof(struct timespec, tv_nsec) ||
-			     sizeof(*tv) != sizeof(struct timespec));
-		do_realtime((struct timespec *)tv);
-		tv->tv_usec /= 1000;
+		if (likely(tv != NULL)) {
+			BUILD_BUG_ON(offsetof(struct timeval, tv_usec) !=
+				     offsetof(struct timespec, tv_nsec) ||
+				     sizeof(*tv) != sizeof(struct timespec));
+			do_realtime((struct timespec *)tv);
+			tv->tv_usec /= 1000;
+		}
 		if (unlikely(tz != NULL)) {
 			/* Avoid memcpy. Some old compilers fail to inline it */
 			tz->tz_minuteswest = gtod->sys_tz.tz_minuteswest;
-- 
1.6.2.3