From: ebiederm@xmission.com (Eric W. Biederman)
To: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Aleksa Sarai <asarai@suse.com>,
Andy Lutomirski <luto@amacapital.net>,
Attila Fazekas <afazekas@redhat.com>, Jann Horn <jann@thejh.net>,
Kees Cook <keescook@chromium.org>,
Michal Hocko <mhocko@kernel.org>,
Ulrich Obergfell <uobergfe@redhat.com>,
linux-kernel@vger.kernel.org, linux-api@vger.kernel.org
Subject: Re: [RFC][PATCH 2/2] exec: If possible don't wait for ptraced threads to be reaped
Date: Sun, 02 Apr 2017 13:53:11 -0500 [thread overview]
Message-ID: <877f32k5ew.fsf@xmission.com> (raw)
In-Reply-To: <20170402153517.GA12637@redhat.com> (Oleg Nesterov's message of "Sun, 2 Apr 2017 17:35:17 +0200")
Oleg Nesterov <oleg@redhat.com> writes:
> On 04/01, Eric W. Biederman wrote:
>>
>> --- a/fs/exec.c
>> +++ b/fs/exec.c
>> @@ -1052,6 +1052,7 @@ static int de_thread(struct task_struct *tsk)
>> struct signal_struct *sig = tsk->signal;
>> struct sighand_struct *oldsighand = tsk->sighand;
>> spinlock_t *lock = &oldsighand->siglock;
>> + bool may_hang;
>>
>> if (thread_group_empty(tsk))
>> goto no_thread_group;
>> @@ -1069,9 +1070,10 @@ static int de_thread(struct task_struct *tsk)
>> return -EAGAIN;
>> }
>>
>> + may_hang = atomic_read(&oldsighand->count) != 1;
>> sig->group_exit_task = tsk;
>> - sig->notify_count = zap_other_threads(tsk);
>> - if (!thread_group_leader(tsk))
>> + sig->notify_count = zap_other_threads(tsk, may_hang ? 1 : -1);
>
> Eric, this is amazing. So with this patch exec does different things depening
> on whether sighand is shared with another CLONE_SIGHAND task or not. To me
> this doesn't look sane in any case.
It is a 99% solution that makes it possible to talk about and review
letting the exec continue after the subthreads are killed but not
reaped.
Sigh I should have made may_hang say:
may_hang = (atomic_read(&oldsignand->count) != 1) && (sig->nr_threads > 1)
Which covers all know ways userspace actually uses these clone flags.
> And btw zap_other_threads(may_hang == 0) is racy. Either you need tasklist or
> exit_notify() should set tsk->exit_state under siglock, otherwise zap() can
> return the wrong count.
zap_other_thread(tsk, 0) only gets called in the case where we don't
care about the return value. It does not get called from fs/exec.c
> Finally. This patch creates the nice security hole. Let me modify my test-case
> again:
>
> 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(path-to-setuid-binary, args);
> }
>
> sleep(1);
>
> // Now we can send the signals to setiuid app
> kill(pid+1, ANYSIGNAL);
>
> return 0;
> }
That is a substantive objection, and something that definitely needs
to get fixed. Can you think of anything else?
Eric
next prev parent reply other threads:[~2017-04-02 18:58 UTC|newest]
Thread overview: 63+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-02-13 14:14 [PATCH 0/2] fix the traced mt-exec deadlock Oleg Nesterov
2017-02-13 14:15 ` [PATCH 1/2] exec: don't wait for zombie threads with cred_guard_mutex held Oleg Nesterov
2017-02-13 16:12 ` kbuild test robot
2017-02-13 16:47 ` Oleg Nesterov
2017-02-13 16:39 ` kbuild test robot
2017-02-13 17:27 ` Mika Penttilä
2017-02-13 18:01 ` Oleg Nesterov
2017-02-13 18:04 ` [PATCH V2 " Oleg Nesterov
2017-02-16 11:42 ` Eric W. Biederman
2017-02-20 15:22 ` Oleg Nesterov
2017-02-20 15:36 ` Oleg Nesterov
2017-02-20 22:30 ` Eric W. Biederman
2017-02-21 17:53 ` Oleg Nesterov
2017-02-21 20:20 ` Eric W. Biederman
2017-02-22 17:41 ` Oleg Nesterov
2017-02-17 4:42 ` Eric W. Biederman
2017-02-20 15:50 ` Oleg Nesterov
2017-02-13 14:15 ` [PATCH 2/2] ptrace: ensure PTRACE_EVENT_EXIT won't stop if the tracee is killed by exec Oleg Nesterov
2017-02-24 16:03 ` [PATCH 0/2] fix the traced mt-exec deadlock Oleg Nesterov
2017-03-03 1:05 ` Eric W. Biederman
2017-03-03 17:33 ` Oleg Nesterov
2017-03-03 18:23 ` Eric W. Biederman
2017-03-03 18:59 ` Eric W. Biederman
2017-03-03 20:06 ` Eric W. Biederman
2017-03-03 20:11 ` [RFC][PATCH] exec: Don't wait for ptraced threads to be reaped Eric W. Biederman
2017-03-04 17:03 ` Oleg Nesterov
2017-03-30 8:07 ` Eric W. Biederman
2017-04-01 5:11 ` [RFC][PATCH 0/2] exec: Fixing ptrace'd mulit-threaded hang Eric W. Biederman
2017-04-01 5:14 ` [RFC][PATCH 1/2] sighand: Count each thread group once in sighand_struct Eric W. Biederman
2017-04-01 5:16 ` [RFC][PATCH 2/2] exec: If possible don't wait for ptraced threads to be reaped Eric W. Biederman
2017-04-02 15:35 ` Oleg Nesterov
2017-04-02 18:53 ` Eric W. Biederman [this message]
2017-04-03 18:12 ` Oleg Nesterov
2017-04-03 21:04 ` Eric W. Biederman
2017-04-05 16:44 ` Oleg Nesterov
2017-04-02 15:38 ` [RFC][PATCH 0/2] exec: Fixing ptrace'd mulit-threaded hang Oleg Nesterov
2017-04-02 22:50 ` [RFC][PATCH v2 0/5] " Eric W. Biederman
2017-04-02 22:51 ` [RFC][PATCH v2 1/5] ptrace: Don't wait in PTRACE_O_TRACEEXIT for exec or coredump Eric W. Biederman
2017-04-05 16:19 ` Oleg Nesterov
2017-04-02 22:51 ` [RFC][PATCH v2 2/5] sighand: Count each thread group once in sighand_struct Eric W. Biederman
2017-04-02 22:52 ` [RFC][PATCH v2 3/5] clone: Disallown CLONE_THREAD with a shared sighand_struct Eric W. Biederman
2017-04-05 16:24 ` Oleg Nesterov
2017-04-05 17:34 ` Eric W. Biederman
2017-04-05 18:11 ` Oleg Nesterov
2017-04-02 22:53 ` [RFC][PATCH v2 4/5] exec: If possible don't wait for ptraced threads to be reaped Eric W. Biederman
2017-04-05 16:15 ` Oleg Nesterov
2017-04-02 22:57 ` [RFC][PATCH v2 5/5] signal: Don't allow accessing signal_struct by old threads after exec Eric W. Biederman
2017-04-05 16:18 ` Oleg Nesterov
2017-04-05 18:16 ` Eric W. Biederman
2017-04-06 15:48 ` Oleg Nesterov
2017-04-02 16:15 ` [RFC][PATCH] exec: Don't wait for ptraced threads to be reaped Oleg Nesterov
2017-04-02 21:07 ` Eric W. Biederman
2017-04-03 18:37 ` Oleg Nesterov
2017-04-03 22:49 ` Eric W. Biederman
2017-04-03 22:49 ` scope of cred_guard_mutex Eric W. Biederman
2017-04-05 16:08 ` Oleg Nesterov
2017-04-05 16:11 ` Kees Cook
2017-04-05 17:53 ` Eric W. Biederman
2017-04-05 18:15 ` Oleg Nesterov
2017-04-06 15:55 ` Oleg Nesterov
2017-04-07 22:07 ` Kees Cook
2017-09-04 3:19 ` [RFC][PATCH] exec: Don't wait for ptraced threads to be reaped Robert O'Callahan
2017-03-04 16:54 ` [PATCH 0/2] fix the traced mt-exec deadlock Oleg Nesterov
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