linux-kernel - Re: KASAN: use-after-free Read in task_is

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Message-ID: <20181025120520.GC3725@redhat.com>
Date:   Thu, 25 Oct 2018 14:05:20 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        syzbot <syzbot+a9ac39bf55329e206219@...kaller.appspotmail.com>,
        James Morris <jmorris@...ei.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        syzkaller-bugs@...glegroups.com
Subject: Re: KASAN: use-after-free Read in task_is_descendant

On 10/25, Kees Cook wrote:
>
> On Thu, Oct 25, 2018 at 12:13 PM, Oleg Nesterov <oleg@...hat.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.