signal: Make the si_code check in rt_[tg]sigqueueinfo stricter

PATCH In review Amanieu d'Antras 2 participants 5 messages Oct 12, 2015 – Oct 13, 2015 x86

Share & tools

Amanieu d'Antras

author

2015-10-12 15:34 UTC #0 lore raw

PATCH 1/1 signal: Make the si_code check in rt_[tg]sigqueueinfo stricter

rt_sigqueueinfo and rt_tgsigqueueinfo check the value of si_code to prevent a process from spoofing a kernel-generated signal or one generated by kill/tgkill.

Unfortunately this check failed to take into account the fact that the si_code value seen by a user process is only the low 16 bits of the value in the kernel. It was still possible to spoof any si_code by ORing 0xffff into the top 16 bits.

The check is tightened by checking the value of si_code that will be seen by a user program instead of the one in the kernel.

Signed-off-by Amanieu d'Antras <[email protected]>

---
 kernel/signal.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

kernel/signal.c +2 -2

index 0f6bbbe..f3d4f39 100644
@@ -2989,7 +2989,7 @@ static int do_rt_sigqueueinfo(pid_t pid, int sig, siginfo_t *info)
 	/* Not even root can pretend to send signals from the kernel.
 	 * Nor can they impersonate a kill()/tgkill(), which adds source info.
 	 */
-	if ((info->si_code >= 0 || info->si_code == SI_TKILL) &&
+	if (((short)info->si_code >= 0 || (short)info->si_code == SI_TKILL) &&
 	    (task_pid_vnr(current) != pid))
 		return -EPERM;

@@ -3037,7 +3037,7 @@ static int do_rt_tgsigqueueinfo(pid_t tgid, pid_t pid, int sig, siginfo_t *info)
 	/* Not even root can pretend to send signals from the kernel.
 	 * Nor can they impersonate a kill()/tgkill(), which adds source info.
 	 */
-	if ((info->si_code >= 0 || info->si_code == SI_TKILL) &&
+	if (((short)info->si_code >= 0 || (short)info->si_code == SI_TKILL) &&
 	    (task_pid_vnr(current) != pid))
 		return -EPERM;

Signature · 1 line

2.6.1

Oleg Nesterov

2015-10-12 15:58 UTC #1 lore raw

Quoted 9 lines · Amanieu d'Antras

rt_sigqueueinfo and rt_tgsigqueueinfo check the value of si_code to prevent a process from spoofing a kernel-generated signal or one generated by kill/tgkill.
Unfortunately this check failed to take into account the fact that the si_code value seen by a user process is only the low 16 bits of the value in the kernel. It was still possible to spoof any si_code by ORing 0xffff into the top 16 bits.

Confused...

Quoted 12 lines

kernel/signal.c +1 -1

@@ -2989,7 +2989,7 @@ static int do_rt_sigqueueinfo(pid_t pid, int sig, siginfo_t *info)
 	/* Not even root can pretend to send signals from the kernel.
 	 * Nor can they impersonate a kill()/tgkill(), which adds source info.
 	 */
-	if ((info->si_code >= 0 || info->si_code == SI_TKILL) &&
+	if (((short)info->si_code >= 0 || (short)info->si_code == SI_TKILL) &&
 	    (task_pid_vnr(current) != pid))
 		return -EPERM;

kernel/signal.c static int do_rt_tgsigqueueinfo(pid_t tgid, pid_t pid, int sig, siginfo_t *info) · line 3042

Quoted 7 lines

kernel/signal.c +1 -1

@@ -3037,7 +3037,7 @@ static int do_rt_tgsigqueueinfo(pid_t tgid, pid_t pid, int sig, siginfo_t *info)
 	/* Not even root can pretend to send signals from the kernel.
 	 * Nor can they impersonate a kill()/tgkill(), which adds source info.
 	 */
-	if ((info->si_code >= 0 || info->si_code == SI_TKILL) &&
+	if (((short)info->si_code >= 0 || (short)info->si_code == SI_TKILL) &&
 	    (task_pid_vnr(current) != pid))
 		return -EPERM;

Yes, copy_siginfo_to_user() does __put_user((short)from->si_code). But SI_FROMUSER/SI_FROMKERNEL are internal kernel checks, we mostly use them in copy_siginfo_to_user().

And note that if ->si_code < 0 we simply do __copy_to_user(), so userspace can't see something which looks like "from kernel", in this case we do not truncate ->si_code.

So I do not think this patch is right.

Oleg.

Amanieu d'Antras

author

2015-10-12 16:37 UTC #2 lore raw

Quoted 7 lines · Oleg Nesterov

Yes, copy_siginfo_to_user() does __put_user((short)from->si_code). But SI_FROMUSER/SI_FROMKERNEL are internal kernel checks, we mostly use them in copy_siginfo_to_user().
And note that if ->si_code < 0 we simply do __copy_to_user(), so userspace can't see something which looks like "from kernel", in this case we do not truncate ->si_code.

Ah my bad, I seem to have missed that case. So copy_siginfo_to_user seems to be safe, but it is still an issue for copy_siginfo_to_user32 which doesn't have this check.

Maybe this should be fixed in the compat code instead?

Oleg Nesterov

2015-10-13 14:58 UTC #3 lore raw

Quoted 10 lines · Amanieu d'Antras

Quoted 7 lines · Oleg Nesterov
Yes, copy_siginfo_to_user() does __put_user((short)from->si_code). But SI_FROMUSER/SI_FROMKERNEL are internal kernel checks, we mostly use them in copy_siginfo_to_user().
And note that if ->si_code < 0 we simply do __copy_to_user(), so userspace can't see something which looks like "from kernel", in this case we do not truncate ->si_code.

Quoted 5 lines

Ah my bad, I seem to have missed that case. So copy_siginfo_to_user seems to be safe, but it is still an issue for copy_siginfo_to_user32 which doesn't have this check.
Maybe this should be fixed in the compat code instead?

I agree, this doesn't look right... But I'm afraid it is too late to change this, this logic predates the git history.

And unless I missed something copy_siginfo_to_user32() assumes that "from" was filled by copy_siginfo_from_user32(), but this is not true if the sender is 64bit task. And this all doesn't match the behaviour with 32bit kernel.

Personally, I think we should leave this ancient code alone. But I agree that something like below looks right... I dunno.

And I have no idea why copy_siginfo_to_user32() abuses '>> 16', it seems that "si_code & __SI_MASK" could work just fine.

Oleg.

arch/x86/kernel/signal_compat.c +2 -1

@@ -17,13 +17,14 @@
 		   3 ints plus the relevant union member.  */
 		put_user_ex(from->si_signo, &to->si_signo);
 		put_user_ex(from->si_errno, &to->si_errno);
-		put_user_ex((short)from->si_code, &to->si_code);
 
 		if (from->si_code < 0) {
+			put_user_ex(from->si_code, &to->si_code);
 			put_user_ex(from->si_pid, &to->si_pid);
 			put_user_ex(from->si_uid, &to->si_uid);
 			put_user_ex(ptr_to_compat(from->si_ptr), &to->si_ptr);
 		} else {
+			put_user_ex((short)from->si_code, &to->si_code);
 			/*
 			 * First 32bits of unions are always present:
 			 * si_pid === si_band === si_tid === si_addr(LS half)

Amanieu d'Antras

author

2015-10-13 15:41 UTC #4 lore raw

Quoted 10 lines · Oleg Nesterov

I agree, this doesn't look right... But I'm afraid it is too late to change this, this logic predates the git history.
And unless I missed something copy_siginfo_to_user32() assumes that "from" was filled by copy_siginfo_from_user32(), but this is not true if the sender is 64bit task. And this all doesn't match the behaviour with 32bit kernel.
Personally, I think we should leave this ancient code alone. But I agree that something like below looks right... I dunno.

I am currently working on another patch set that updates copy_siginfo_{to,from}_user32 to match the behavior of 32-bit kernels. I will fix the si_code encoding there with a change similar to yours.

Amanieu d'Antras