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From: Oleg Nesterov <oleg@redhat.com>
To: Kees Cook <keescook@chromium.org>
Cc: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
	"Serge E. Hallyn" <serge@hallyn.com>,
	syzbot <syzbot+a9ac39bf55329e206219@syzkaller.appspotmail.com>,
	James Morris <jmorris@namei.org>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-security-module <linux-security-module@vger.kernel.org>,
	syzkaller-bugs@googlegroups.com
Subject: Re: KASAN: use-after-free Read in task_is_descendant
Date: Thu, 25 Oct 2018 14:05:20 +0200	[thread overview]
Message-ID: <20181025120520.GC3725@redhat.com> (raw)
In-Reply-To: <CAGXu5jK5c5mf=EarFNBWeq5Gc3LB=Kd8tqDc_a2nc6Xmtu=s6g@mail.gmail.com>

On 10/25, Kees Cook wrote:
>
> On Thu, Oct 25, 2018 at 12:13 PM, Oleg Nesterov <oleg@redhat.com> wrote:
> > So again, suppose that "child" is already dead. Its task_struct can't be freed,
> > but child->real_parent can point to the already freed memory.
>
> I can't find a path for "child" to be released. I see task_lock()
> always called on it before it ends up in Yama.

Are you saying that yama_ptrace_access_check() is always called under
task_lock(child) ? Yes, it seems so

So what? Say, ptrace_attach() can hit a dead task. It should notice this
and fail later, but security_ptrace_access_check() is called before that.


> > This means that the 1st walker = rcu_dereference(walker->real_parent) is fine,
> > this simply reads the child->real_parent pointer, but on the second iteration
> >
> >         walker = rcu_dereference(walker->real_parent);
> >
> > reads the alredy freed memory.
>
> What does rcu_read_lock() protect actually protect here? I thought
> none of the task structs would be freed until after all
> rcu_read_unlock() finished.

See another email I sent you a minute ago.

Oleg.


  reply	other threads:[~2018-10-25 12:05 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 [this message]
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
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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