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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20181026152208.GB18848@redhat.com>
Date:   Fri, 26 Oct 2018 17:22:09 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     serge@...lyn.com,
        syzbot <syzbot+a9ac39bf55329e206219@...kaller.appspotmail.com>,
        jmorris@...ei.org, keescook@...omium.org,
        linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        syzkaller-bugs@...glegroups.com
Subject: Re: KASAN: use-after-free Read in task_is_descendant

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.