[PATCH 0/3] fix SIGNAL_UNKILLABLE && SIGKILL interaction

[PATCH 0/3] fix SIGNAL_UNKILLABLE && SIGKILL interaction

[Oleg Nesterov]

On 11/02, Oleg Nesterov wrote:
>
> I need to write the changelog, and perhaps even split this small patch for
> better documentation.

OK, it is not clear if I answered Eric's concerns or not, let me send the
patches for review anyway. I tried to document every change in signal.c.

Oleg.

 kernel/signal.c | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

[PATCH 1/3] protect the traced SIGNAL_UNKILLABLE tasks from SIGKILL

[Oleg Nesterov]

The comment in sig_ignored() says "Tracers may want to know about
even ignored signals" but SIGKILL can not be reported to debugger
and it is just wrong to return 0 in this case: SIGKILL should only
kill the SIGNAL_UNKILLABLE task if it comes from the parent ns.

Change sig_ignored() to ignore ->ptrace if sig == SIGKILL and rely
on sig_task_ignored().

SISGTOP coming from within the namespace is not really right too
but at least debugger can intercept it, and we can't drop it here
because this will break "gdb -p 1": ptrace_attach() won't work.
Perhaps we will add another ->ptrace check later, we will see.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/signal.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index 800a18f..b9aebe1 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -94,13 +94,15 @@ static int sig_ignored(struct task_struct *t, int sig, bool force)
 	if (sigismember(&t->blocked, sig) || sigismember(&t->real_blocked, sig))
 		return 0;
 
-	if (!sig_task_ignored(t, sig, force))
-		return 0;
-
 	/*
-	 * Tracers may want to know about even ignored signals.
+	 * Tracers may want to know about even ignored signal unless it
+	 * is SIGKILL which can't be reported anyway but can be ignored
+	 * by SIGNAL_UNKILLABLE task.
 	 */
-	return !t->ptrace;
+	if (t->ptrace && sig != SIGKILL)
+		return 0;
+
+	return sig_task_ignored(t, sig, force);
 }
 
 /*
-- 
2.5.0

[PATCH 2/3] protect the SIGNAL_UNKILLABLE tasks from !sig_kernel_only() signals

[Oleg Nesterov]

Change sig_task_ignored() to drop the SIG_DFL && !sig_kernel_only()
signals even if force == T. This simplifies the next change and this
matches the same check in get_signal() which will drop these signals
anyway.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/signal.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index b9aebe1..8fc0182 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -78,7 +78,7 @@ static int sig_task_ignored(struct task_struct *t, int sig, bool force)
 	handler = sig_handler(t, sig);
 
 	if (unlikely(t->signal->flags & SIGNAL_UNKILLABLE) &&
-			handler == SIG_DFL && !force)
+	    handler == SIG_DFL && !(force && sig_kernel_only(sig)))
 		return 1;
 
 	return sig_handler_ignored(handler, sig);
-- 
2.5.0

[PATCH 3/3] remove the no longer needed SIGNAL_UNKILLABLE check in complete_signal()

[Oleg Nesterov]

complete_signal() checks SIGNAL_UNKILLABLE before it starts to destroy the
thread group, today this is wrong in many ways.

If nothing else, fatal_signal_pending() should always imply that the whole
thread group (except ->group_exit_task if it is not NULL) is killed, this
check breaks the rule.

After the previous changes we can rely on sig_task_ignored(); sig_fatal(sig)
&& SIGNAL_UNKILLABLE can only be true if we actually want to kill this task
and sig == SIGKILL OR it is traced and debugger can intercept the signal.

This should hopefully fix the problem reported by Dmitry. This test-case

	static int init(void *arg)
	{
		for (;;)
			pause();
	}

	int main(void)
	{
		char stack[16 * 1024];

		for (;;) {
			int pid = clone(init, stack + sizeof(stack)/2,
					CLONE_NEWPID | SIGCHLD, NULL);
			assert(pid > 0);

			assert(ptrace(PTRACE_ATTACH, pid, 0, 0) == 0);
			assert(waitpid(-1, NULL, WSTOPPED) == pid);

			assert(ptrace(PTRACE_DETACH, pid, 0, SIGSTOP) == 0);
			assert(syscall(__NR_tkill, pid, SIGKILL) == 0);
			assert(pid == wait(NULL));
		}
	}

triggers the WARN_ON_ONCE(!(task->jobctl & JOBCTL_STOP_PENDING)) in
task_participate_group_stop(). do_signal_stop()->signal_group_exit()
checks SIGNAL_GROUP_EXIT and return false, but task_set_jobctl_pending()
checks fatal_signal_pending() and does not set JOBCTL_STOP_PENDING.

And his should fix the minor security problem reported by Kyle,
SECCOMP_RET_TRACE can miss fatal_signal_pending() the same way if
the task is the root of a pid namespace.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Reported-by: Kyle Huey <me@kylehuey.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/signal.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index 8fc0182..7e15b56 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -931,9 +931,9 @@ static void complete_signal(int sig, struct task_struct *p, int group)
 	 * then start taking the whole group down immediately.
 	 */
 	if (sig_fatal(p, sig) &&
-	    !(signal->flags & (SIGNAL_UNKILLABLE | SIGNAL_GROUP_EXIT)) &&
+	    !(signal->flags & SIGNAL_GROUP_EXIT) &&
 	    !sigismember(&t->real_blocked, sig) &&
-	    (sig == SIGKILL || !t->ptrace)) {
+	    (sig == SIGKILL || !p->ptrace)) {
 		/*
 		 * This signal will be fatal to the whole group.
 		 */
-- 
2.5.0

Re: [PATCH 3/3] remove the no longer needed SIGNAL_UNKILLABLE check in complete_signal()

[Kees Cook]

On Fri, Nov 3, 2017 at 11:42 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> complete_signal() checks SIGNAL_UNKILLABLE before it starts to destroy the
> thread group, today this is wrong in many ways.
>
> If nothing else, fatal_signal_pending() should always imply that the whole
> thread group (except ->group_exit_task if it is not NULL) is killed, this
> check breaks the rule.
>
> After the previous changes we can rely on sig_task_ignored(); sig_fatal(sig)
> && SIGNAL_UNKILLABLE can only be true if we actually want to kill this task
> and sig == SIGKILL OR it is traced and debugger can intercept the signal.
>
> This should hopefully fix the problem reported by Dmitry. This test-case
>
>         static int init(void *arg)
>         {
>                 for (;;)
>                         pause();
>         }
>
>         int main(void)
>         {
>                 char stack[16 * 1024];
>
>                 for (;;) {
>                         int pid = clone(init, stack + sizeof(stack)/2,
>                                         CLONE_NEWPID | SIGCHLD, NULL);
>                         assert(pid > 0);
>
>                         assert(ptrace(PTRACE_ATTACH, pid, 0, 0) == 0);
>                         assert(waitpid(-1, NULL, WSTOPPED) == pid);
>
>                         assert(ptrace(PTRACE_DETACH, pid, 0, SIGSTOP) == 0);
>                         assert(syscall(__NR_tkill, pid, SIGKILL) == 0);
>                         assert(pid == wait(NULL));
>                 }
>         }
>
> triggers the WARN_ON_ONCE(!(task->jobctl & JOBCTL_STOP_PENDING)) in
> task_participate_group_stop(). do_signal_stop()->signal_group_exit()
> checks SIGNAL_GROUP_EXIT and return false, but task_set_jobctl_pending()
> checks fatal_signal_pending() and does not set JOBCTL_STOP_PENDING.
>
> And his should fix the minor security problem reported by Kyle,
> SECCOMP_RET_TRACE can miss fatal_signal_pending() the same way if
> the task is the root of a pid namespace.
>
> Reported-by: Dmitry Vyukov <dvyukov@google.com>
> Reported-by: Kyle Huey <me@kylehuey.com>
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>

Reviewed-by: Kees Cook <keescook@chromium.org>

Thanks for digging through this! Two birds, one stone, etc. :)

-Kees

> ---
>  kernel/signal.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/signal.c b/kernel/signal.c
> index 8fc0182..7e15b56 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -931,9 +931,9 @@ static void complete_signal(int sig, struct task_struct *p, int group)
>          * then start taking the whole group down immediately.
>          */
>         if (sig_fatal(p, sig) &&
> -           !(signal->flags & (SIGNAL_UNKILLABLE | SIGNAL_GROUP_EXIT)) &&
> +           !(signal->flags & SIGNAL_GROUP_EXIT) &&
>             !sigismember(&t->real_blocked, sig) &&
> -           (sig == SIGKILL || !t->ptrace)) {
> +           (sig == SIGKILL || !p->ptrace)) {
>                 /*
>                  * This signal will be fatal to the whole group.
>                  */
> --
> 2.5.0
>
>



-- 
Kees Cook
Pixel Security

Re: [PATCH 0/3] fix SIGNAL_UNKILLABLE && SIGKILL interaction

[Oleg Nesterov]

ping...

Dmitry confirms this actually fixes the problem reported by syzkaller
we discussed in another thread.


On 11/03, Oleg Nesterov wrote:
>
> On 11/02, Oleg Nesterov wrote:
> >
> > I need to write the changelog, and perhaps even split this small patch for
> > better documentation.
> 
> OK, it is not clear if I answered Eric's concerns or not, let me send the
> patches for review anyway. I tried to document every change in signal.c.
> 
> Oleg.
> 
>  kernel/signal.c | 18 ++++++++++--------
>  1 file changed, 10 insertions(+), 8 deletions(-)

Re: [PATCH 0/3] fix SIGNAL_UNKILLABLE && SIGKILL interaction

[Kyle Huey]

On Mon, Nov 13, 2017 at 6:53 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> ping...
>
> Dmitry confirms this actually fixes the problem reported by syzkaller
> we discussed in another thread.
>
>
> On 11/03, Oleg Nesterov wrote:
>>
>> On 11/02, Oleg Nesterov wrote:
>> >
>> > I need to write the changelog, and perhaps even split this small patch for
>> > better documentation.
>>
>> OK, it is not clear if I answered Eric's concerns or not, let me send the
>> patches for review anyway. I tried to document every change in signal.c.
>>
>> Oleg.
>>
>>  kernel/signal.c | 18 ++++++++++--------
>>  1 file changed, 10 insertions(+), 8 deletions(-)
>

I can confirm that your patches fix the issue I reported as well.

- Kyle