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From: ebiederm@xmission.com (Eric W. Biederman)
To: Oleg Nesterov <oleg@redhat.com>
Cc: Jann Horn <jann@thejh.net>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Roland McGrath <roland@hack.frob.com>,
	John Johansen <john.johansen@canonical.com>,
	James Morris <james.l.morris@oracle.com>,
	"Serge E. Hallyn" <serge@hallyn.com>,
	Paul Moore <aul@paul-moore.com>,
	Stephen Smalley <sds@tycho.nsa.gov>,
	Eric Paris <eparis@parisplace.org>,
	Casey Schaufler <casey@schaufler-ca.com>,
	Kees Cook <keescook@chromium.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Janis Danisevskis <jdanis@google.com>,
	Seth Forshee <seth.forshee@canonical.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Benjamin LaHaise <bcrl@kvack.org>,
	Ben Hutchings <ben@decadent.org.uk>,
	Andy Lutomirski <luto@amacapital.net>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Krister Johansen <kjlx@templeofstupid.com>,
	linux-fsdevel@vger.kernel.org,
	linux-security-module@vger.kernel.org, security@kernel.org
Subject: Re: [PATCH v3 1/8] exec: introduce cred_guard_light
Date: Fri, 04 Nov 2016 10:00:38 -0500	[thread overview]
Message-ID: <87k2cjuw6h.fsf@xmission.com> (raw)
In-Reply-To: <87k2cj2x6j.fsf@xmission.com> (Eric W. Biederman's message of "Fri, 04 Nov 2016 08:26:28 -0500")

ebiederm@xmission.com (Eric W. Biederman) writes:

> Oleg Nesterov <oleg@redhat.com> writes:
>
>> On 11/02, Jann Horn wrote:
>>>
>>> On Wed, Nov 02, 2016 at 07:18:06PM +0100, Oleg Nesterov wrote:
>>> > On 10/30, Jann Horn wrote:
>>> > >
>>> > > This is a new per-threadgroup lock that can often be taken instead of
>>> > > cred_guard_mutex and has less deadlock potential. I'm doing this because
>>> > > Oleg Nesterov mentioned the potential for deadlocks, in particular if a
>>> > > debugged task is stuck in execve, trying to get rid of a ptrace-stopped
>>> > > thread, and the debugger attempts to inspect procfs files of the debugged
>>> > > task.
>>> >
>>> > Yes, but let me repeat that we need to fix this anyway. So I don't really
>>> > understand why should we add yet another mutex.
>>>
>>> execve() only takes the new mutex immediately after de_thread(), so this
>>> problem shouldn't occur there.
>>
>> Yes, I see.
>>
>>> Basically, I think that I'm not making the
>>> problem worse with my patches this way.
>>
>> In a sense that it doesn't add the new deadlocks, I agree. But it adds
>> yet another per-process mutex while we already have the similar one,
>>
>>> I believe that it should be possible to convert most existing users of the
>>> cred_guard_mutex to the new cred_guard_light - exceptions to that that I
>>> see are:
>>>
>>>  - PTRACE_ATTACH
>>
>> This is the main problem afaics. So "strace -f" can hang if it races
>> with mt-exec. And we need to fix this. I constantly forget about this
>> problem, but I tried many times to find a reasonable solution, still
>> can't.
>>
>> IMO, it would be nice to rework the lsm hooks, so that we could take
>> cred_guard_mutex after de_thread() (like your cred_guard_light) or
>> at least drop it earlier, but unlikely this is possible...
>>
>> So the only plan I currently have is change de_thread() to wait until
>> other threads pass exit_notify() or even exit_signals(), but I don't
>> like this.
>>
>>>  - SECCOMP_FILTER_FLAG_TSYNC (sets NO_NEW_PRIVS on remote task)
>>
>> I forgot about this one... Need to re-check but at first glance this
>> is not a real problem.
>>
>>> Beyond that, conceptually, the new cred_guard_light could also be turned
>>> into a read-write mutex
>>
>> Not sure I understand how this can help... doesn't matter.
>>
>> My point is, imo you should not add the new mutex. Just use the old
>> one in (say) 4/8 (which I do not personally like as you know ;), this
>> won't add the new problem.
>>
>>
>>> It seems to me like SECCOMP_FILTER_FLAG_TSYNC doesn't really have
>>> deadlocking issues.
>>
>> Yes, agreed.
>>
>>> PTRACE_ATTACH isn't that clear to me; if a debugger
>>> tries to attach to a newly spawned thread while another ptraced thread is
>>> dying because of de_thread() in a third thread, that might still cause
>>> the debugger to deadlock, right?
>>
>> This is the trivial test-case I wrote when the problem was initially
>> reported. And damn, I always knew that cred_guard_mutex needs fixes,
>> but somehow I completely forgot that it is used by PTRACE_ATTACH when
>> I was going to try to remove from fs/proc a long ago.
>>
>> 	void *thread(void *arg)
>> 	{
>> 		ptrace(PTRACE_TRACEME, 0,0,0);
>> 		return NULL;
>> 	}
>>
>> 	int main(void)
>> 	{
>> 		int pid = fork();
>>
>> 		if (!pid) {
>> 			pthread_t pt;
>> 			pthread_create(&pt, NULL, thread, NULL);
>> 			pthread_join(pt, NULL);
>> 			execlp("echo", "echo", "passed", NULL);
>> 		}
>>
>> 		sleep(1);
>> 		// or anything else which needs ->cred_guard_mutex,
>> 		// say open(/proc/$pid/mem)
>> 		ptrace(PTRACE_ATTACH, pid, 0,0);
>> 		kill(pid, SIGCONT);
>>
>> 		return 0;
>> 	}
>>
>> The problem is trivial. The execing thread waits until its sub-thread
>> goes away, it should be reaped by the tracer, the tracer waits for
>> cred_guard_mutex.
>
> There is a bug here but I don't believe it has anything to do with
> the cred_guard_mutex.
>
> If we reach zap_other_threads fundamentally the tracer should not
> be able to block the traced thread from exiting.  Those are the
> semantics described in the comments in the code.
>
> I have poked things a little and have a half fix for that but
> the fix appears to be the wrong, but enlightening.
>
> AKA the following prevents the hang of your test case.
> diff --git a/kernel/signal.c b/kernel/signal.c
> index 75761acc77cf..a6f83450500e 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -1200,7 +1200,7 @@ int zap_other_threads(struct task_struct *p)
>  		if (t->exit_state)
>  			continue;
>  		sigaddset(&t->pending.signal, SIGKILL);
> -		signal_wake_up(t, 1);
> +		signal_wake_up_state(t, TASK_WAKEKILL | __TASK_TRACED);
>  	}
>  
>  	return count;
>
> It looks like somewhere on the exit path the traced thread is blocking
> without setting TASK_WAKEKILL.

Apologies there was a testing mistake and that patch does not actually
help anything.

The following mostly correct patch modifies zap_other_threads in
the case of a de_thread to not wait for zombies to be reaped.  The only
case that cares is ptrace (as threads are self reaping).  So I don't
think this will cause any problems except removing the strace -f race.

Not waiting for zombies to be reaped in de_thread keeps the kernel from
holding the cred_guard_mutex while waiting for userspace.  Which should
mean we don't have to move it.

Not waiting for zombies to be reaped should also speed of mt-exec.  So I
think this is a benefit all around.


diff --git a/kernel/exit.c b/kernel/exit.c
index 9d68c45ebbe3..8c8556cab655 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -109,7 +109,8 @@ static void __exit_signal(struct task_struct *tsk)
 		 * If there is any task waiting for the group exit
 		 * then notify it:
 		 */
-		if (sig->notify_count > 0 && !--sig->notify_count)
+		if ((sig->flags & SIGNAL_GROUP_EXIT) &&
+		    sig->notify_count > 0 && !--sig->notify_count)
 			wake_up_process(sig->group_exit_task);
 
 		if (tsk == sig->curr_target)
@@ -690,6 +691,10 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
 	if (tsk->exit_state == EXIT_DEAD)
 		list_add(&tsk->ptrace_entry, &dead);
 
+	if (!(tsk->signal->flags & SIGNAL_GROUP_EXIT) &&
+	    tsk->signal->notify_count > 0 && !--tsk->signal->notify_count)
+		wake_up_process(tsk->signal->group_exit_task);
+
 	/* mt-exec, de_thread() is waiting for group leader */
 	if (unlikely(tsk->signal->notify_count < 0))
 		wake_up_process(tsk->signal->group_exit_task);
diff --git a/kernel/signal.c b/kernel/signal.c
index 75761acc77cf..a3a5cd8dad0f 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1194,7 +1194,9 @@ int zap_other_threads(struct task_struct *p)
 
 	while_each_thread(p, t) {
 		task_clear_jobctl_pending(t, JOBCTL_PENDING_MASK);
-		count++;
+		if ((t->signal->flags & SIGNAL_GROUP_EXIT) ||
+		    !t->exit_state)
+			count++;
 
 		/* Don't bother with already dead threads */
 		if (t->exit_state)


Eric

  reply	other threads:[~2016-11-04 15:08 UTC|newest]

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-10-30 21:46 [PATCH v3 0/8] Various fixes related to ptrace_may_access() Jann Horn
2016-10-30 21:46 ` [PATCH v3 1/8] exec: introduce cred_guard_light Jann Horn
2016-11-02 18:18   ` Oleg Nesterov
2016-11-02 20:50     ` Jann Horn
2016-11-02 21:38       ` Ben Hutchings
2016-11-02 21:54         ` Jann Horn
2016-11-03 18:12       ` Oleg Nesterov
2016-11-03 21:17         ` Jann Horn
2016-11-04 13:26         ` Eric W. Biederman
2016-11-04 15:00           ` Eric W. Biederman [this message]
2016-11-04 18:04             ` Oleg Nesterov
2016-11-04 18:45               ` Oleg Nesterov
2016-11-05 14:56                 ` Oleg Nesterov
2016-11-09  0:34                   ` Eric W. Biederman
2016-11-16 20:03                   ` Eric W. Biederman
2016-11-08 22:02                 ` Kees Cook
2016-11-08 22:46                   ` Eric W. Biederman
2016-11-08 22:56                     ` Benjamin LaHaise
2016-11-08 23:33                       ` Eric W. Biederman
2016-10-30 21:46 ` [PATCH v3 2/8] exec: add privunit to task_struct Jann Horn
2016-10-30 21:46 ` [PATCH v3 3/8] proc: use open()-time creds for ptrace checks Jann Horn
2016-10-30 21:46 ` [PATCH v3 4/8] futex: don't leak robust_list pointer Jann Horn
2016-10-30 21:46 ` [PATCH v3 5/8] proc: lock properly in ptrace_may_access callers Jann Horn
2016-10-30 21:46 ` [PATCH v3 6/8] fs/proc: fix attr access check Jann Horn
2016-10-30 21:46 ` [PATCH v3 7/8] proc: fix timerslack_ns handling Jann Horn
2016-10-30 21:46 ` [PATCH v3 8/8] Documentation: add security/ptrace_checks.txt Jann Horn
2016-11-01 23:57 ` [PATCH v3 0/8] Various fixes related to ptrace_may_access() Linus Torvalds
2016-11-02 18:38   ` Oleg Nesterov
2016-11-02 21:40     ` Jann Horn
2016-11-03 19:09   ` Andrew Morton
2016-11-03 20:01     ` Jann Horn
2016-11-04  0:57   ` James Morris

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