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From: Oleg Nesterov <oleg@redhat.com>
To: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: serge@hallyn.com,
	syzbot <syzbot+a9ac39bf55329e206219@syzkaller.appspotmail.com>,
	jmorris@namei.org, keescook@chromium.org,
	linux-kernel@vger.kernel.org,
	linux-security-module@vger.kernel.org,
	syzkaller-bugs@googlegroups.com
Subject: Re: KASAN: use-after-free Read in task_is_descendant
Date: Fri, 26 Oct 2018 17:22:09 +0200	[thread overview]
Message-ID: <20181026152208.GB18848@redhat.com> (raw)
In-Reply-To: <4264173d-4947-9fad-97e6-5269defb6eb4@i-love.sakura.ne.jp>

On 10/27, Tetsuo Handa wrote:
>
> On 2018/10/26 23:39, Oleg Nesterov wrote:
> > On 10/26, Tetsuo Handa wrote:
> >> Suppose p1 == p2->real_parent and p2 == p3->real_parent, and p1 exited
> >> when someone tried to attach on p2, p2->real_parent was pointing to already
> >> (or about to be) freed p1.
> >
> > I don't see a difference.
> >
> > If p1 exits it will re-parent p2, p2->real_parent will be updated.
> >
> >> So, the puzzle part is why p2->real_parent was still pointing p1 even after
> >> p1 was freed...
> >
> > I don't understand the question.
> >
> > Once again. TASK->real_parent can point to the freed mem only if a) TASK exits,
> > and b) _after_ that its parent TASK->real_parent exits too.
>
> Oh, p2 exited and then p1 also exited when someone tried to attach on p2.
> Then, p2->real_parent can point to already (or about to be) freed p1.

Then we must see pid_alive(p2) == F as I already explained you in my
yesterday's email.

Because if p1 exits _after_ p2, then pid_alive(p2) == F must be already
completed, p1 can't exit (I mean, release_task(p1) can't be called) until
release_task(p2) does detach_pid() and drops tasklist_lock.

> (By the way, if p->real_parent were updated to point to init_task when p exits,
> we could omit pid_alive() check?)

Sorry, I don't understand the question...

Ignoring the PR_SET_CHILD_SUBREAPER parents, ignoring the exiting sub-threads
which reparent to group leader, ->real_parent is alwasy updated to point to
init_task. But I don't see why we could omit pid_alive() check.

Oleg.


  reply	other threads:[~2018-10-26 15:22 UTC|newest]

Thread overview: 31+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-10-21  7:10 KASAN: use-after-free Read in task_is_descendant syzbot
2018-10-21  7:12 ` Tetsuo Handa
2018-10-22  9:54   ` Oleg Nesterov
2018-10-22 10:06     ` Tetsuo Handa
2018-10-22 13:46       ` Oleg Nesterov
2018-10-25  2:15         ` Tetsuo Handa
2018-10-25 11:13           ` Oleg Nesterov
2018-10-25 11:36             ` Kees Cook
2018-10-25 12:05               ` Oleg Nesterov
2018-10-25 11:47             ` Tetsuo Handa
2018-10-25 12:17               ` Oleg Nesterov
2018-10-25 13:01                 ` Oleg Nesterov
2018-10-26 16:09                   ` Kees Cook
2018-10-29 12:23                     ` Oleg Nesterov
2018-10-29 15:05                       ` yama: unsafe usage of ptrace_relation->tracer Oleg Nesterov
2019-01-10 11:05                         ` Tetsuo Handa
2019-01-10 18:47                           ` Kees Cook
2019-01-16 17:40                           ` Oleg Nesterov
2018-10-25 13:14                 ` KASAN: use-after-free Read in task_is_descendant Tetsuo Handa
2018-10-25 15:55                   ` Oleg Nesterov
2018-10-25 16:25                     ` Oleg Nesterov
2018-10-26 12:23                     ` Tetsuo Handa
2018-10-26 13:04                       ` Oleg Nesterov
2018-10-26 13:51                         ` Tetsuo Handa
2018-10-26 14:39                           ` Oleg Nesterov
2018-10-26 15:04                             ` Tetsuo Handa
2018-10-26 15:22                               ` Oleg Nesterov [this message]
2018-10-25  8:19     ` Kees Cook
2018-10-25 11:52       ` Oleg Nesterov
2018-11-10  3:25 ` syzbot
2018-11-10 11:46 ` syzbot

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