* [PATCH] signal: Make the si_code check in rt_[tg]sigqueueinfo stricter
@ 2015-10-12 15:33 Amanieu d'Antras
2015-10-12 15:54 ` Oleg Nesterov
0 siblings, 1 reply; 5+ messages in thread
From: Amanieu d'Antras @ 2015-10-12 15:33 UTC (permalink / raw
To: linux-kernel
Cc: Oleg Nesterov, Andrew Morton, Paul E. McKenney,
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.
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 <amanieu@gmail.com>
---
kernel/signal.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/kernel/signal.c b/kernel/signal.c
index 0f6bbbe..f3d4f39 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -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;
--
2.6.1
^ permalink raw reply related [flat|nested] 5+ messages in thread
* Re: [PATCH] signal: Make the si_code check in rt_[tg]sigqueueinfo stricter
2015-10-12 15:33 [PATCH] signal: Make the si_code check in rt_[tg]sigqueueinfo stricter Amanieu d'Antras
@ 2015-10-12 15:54 ` Oleg Nesterov
2015-10-12 16:37 ` Amanieu d'Antras
0 siblings, 1 reply; 5+ messages in thread
From: Oleg Nesterov @ 2015-10-12 15:54 UTC (permalink / raw
To: Amanieu d'Antras; +Cc: linux-kernel, Andrew Morton, Paul E. McKenney
On 10/12, Amanieu d'Antras wrote:
>
> 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...
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -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;
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.
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] signal: Make the si_code check in rt_[tg]sigqueueinfo stricter
2015-10-12 15:54 ` Oleg Nesterov
@ 2015-10-12 16:37 ` Amanieu d'Antras
2015-10-13 14:54 ` Oleg Nesterov
0 siblings, 1 reply; 5+ messages in thread
From: Amanieu d'Antras @ 2015-10-12 16:37 UTC (permalink / raw
To: Oleg Nesterov; +Cc: linux-kernel, Andrew Morton, Paul E. McKenney
On Mon, Oct 12, 2015 at 4:54 PM, Oleg Nesterov <oleg@redhat.com> wrote:
> 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?
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] signal: Make the si_code check in rt_[tg]sigqueueinfo stricter
2015-10-12 16:37 ` Amanieu d'Antras
@ 2015-10-13 14:54 ` Oleg Nesterov
2015-10-13 15:40 ` Amanieu d'Antras
0 siblings, 1 reply; 5+ messages in thread
From: Oleg Nesterov @ 2015-10-13 14:54 UTC (permalink / raw
To: Amanieu d'Antras; +Cc: linux-kernel, Andrew Morton, Paul E. McKenney
On 10/12, Amanieu d'Antras wrote:
>
> On Mon, Oct 12, 2015 at 4:54 PM, Oleg Nesterov <oleg@redhat.com> wrote:
> > 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?
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.
--- x/arch/x86/kernel/signal_compat.c
+++ x/arch/x86/kernel/signal_compat.c
@@ -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)
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] signal: Make the si_code check in rt_[tg]sigqueueinfo stricter
2015-10-13 14:54 ` Oleg Nesterov
@ 2015-10-13 15:40 ` Amanieu d'Antras
0 siblings, 0 replies; 5+ messages in thread
From: Amanieu d'Antras @ 2015-10-13 15:40 UTC (permalink / raw
To: Oleg Nesterov; +Cc: linux-kernel, Andrew Morton, Paul E. McKenney
On Tue, Oct 13, 2015 at 3:54 PM, Oleg Nesterov <oleg@redhat.com> wrote:
> 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
^ permalink raw reply [flat|nested] 5+ messages in thread
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2015-10-12 15:33 [PATCH] signal: Make the si_code check in rt_[tg]sigqueueinfo stricter Amanieu d'Antras
2015-10-12 15:54 ` Oleg Nesterov
2015-10-12 16:37 ` Amanieu d'Antras
2015-10-13 14:54 ` Oleg Nesterov
2015-10-13 15:40 ` Amanieu d'Antras
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