[artful/linux bionic/linux 1/1] compat: fix 4-byte infoleak via uninitialized struct field

[Andy Whitcroft]

From: Jann Horn <jannh at google.com>

Commit 3a4d44b61625 ("ntp: Move adjtimex related compat syscalls to
native counterparts") removed the memset() in compat_get_timex().  Since
then, the compat adjtimex syscall can invoke do_adjtimex() with an
uninitialized ->tai.

If do_adjtimex() doesn't write to ->tai (e.g.  because the arguments are
invalid), compat_put_timex() then copies the uninitialized ->tai field
to userspace.

Fix it by adding the memset() back.

Fixes: 3a4d44b61625 ("ntp: Move adjtimex related compat syscalls to native counterparts")
Signed-off-by: Jann Horn <jannh at google.com>
Acked-by: Kees Cook <keescook at chromium.org>
Acked-by: Al Viro <viro at zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>

(cherry picked from commit 0a0b98734479aa5b3c671d5190e86273372cab95)
CVE-2018-11508
Signed-off-by: Andy Whitcroft <apw at canonical.com>
---
 kernel/compat.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/compat.c b/kernel/compat.c
index d1cee656a7ed..3ead2efff78e 100644
--- a/kernel/compat.c
+++ b/kernel/compat.c
@@ -34,6 +34,7 @@ int compat_get_timex(struct timex *txc, const struct compat_timex __user *utp)
 {
 	struct compat_timex tx32;
 
+	memset(txc, 0, sizeof(struct timex));
 	if (copy_from_user(&tx32, utp, sizeof(struct compat_timex)))
 		return -EFAULT;
 
-- 
2.17.0