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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: [PATCH 0/2] fix the traced mt-exec deadlock
Date: Fri, 03 Mar 2017 12:59:19 -0600	[thread overview]
Message-ID: <87d1dyw5iw.fsf@xmission.com> (raw)
In-Reply-To: <87tw7axlr0.fsf@xmission.com> (Eric W. Biederman's message of "Fri, 03 Mar 2017 12:23:31 -0600")

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

> The big lesson for me, and what was not obvious from your change
> description is that we are changing the user space visible semantics
> of exec+ptrace and that cred_guard_mutex is not at all the problem (as
> we always take cred_guard_mutex in a killable or interruptible way).

Just to follow up.

Because the cred_guard_mutex is fine as is we don't need to move
de_thread out from under cred_guard_mutex.  We just need to change
de_thread to wait until all of the other threads are zombies.
Which should remove about half your proposed patch.

The other key thing is that knowning it isn't cred_guard_mutex let's us
know that this kind of deadlock goes all of the way back to when
CLONE_THREAD was merged into the kernel.

Insteresingly enough looking at zap_other_threads and notify_count I
have found a second bug.  When a multi-threaded processes becomes a
zombie we don't send the notification to the parent process until the
non-leader threads have been reaped.  Which means ptrace can mess up
sending SIGCHLD to the parent.

Now arguably that might be what is desirable but I don't think so.  If
we aren't ptracing a thread then I don't think we want to delay sending
SIGCHLD to the parent.

So this whole area of the semantics of a ptrace'd multi-threaded process
exiting/exec'ing looks like it needs a thorough going over.

Eric

  reply	other threads:[~2017-03-04  0:02 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 [this message]
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
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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