linux-kernel - Re: [RFC PATCH] mm, oom: fix use-after-free in oom_kill

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Message-ID: <CALvZod6zRy69bHoXvEWED28OFZ8u4o8JBAL7nyjKMmUjBb5n4w@mail.gmail.com>
Date:   Sun, 20 Jan 2019 12:20:57 -0800
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Roman Gushchin <guro@...com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] mm, oom: fix use-after-free in oom_kill_process

On Fri, Jan 18, 2019 at 5:58 PM Roman Gushchin <guro@...com> wrote:
>
> Hi Shakeel!
>
> >
> > On looking further it seems like the process selected to be oom-killed
> > has exited even before reaching read_lock(&tasklist_lock) in
> > oom_kill_process(). More specifically the tsk->usage is 1 which is due
> > to get_task_struct() in oom_evaluate_task() and the put_task_struct
> > within for_each_thread() frees the tsk and for_each_thread() tries to
> > access the tsk. The easiest fix is to do get/put across the
> > for_each_thread() on the selected task.
>
> Please, feel free to add
> Reviewed-by: Roman Gushchin <guro@...com>
> for this part.
>

Thanks.

> >
> > Now the next question is should we continue with the oom-kill as the
> > previously selected task has exited? However before adding more
> > complexity and heuristics, let's answer why we even look at the
> > children of oom-kill selected task? The select_bad_process() has already
> > selected the worst process in the system/memcg. Due to race, the
> > selected process might not be the worst at the kill time but does that
> > matter matter? The userspace can play with oom_score_adj to prefer
> > children to be killed before the parent. I looked at the history but it
> > seems like this is there before git history.
>
> I'd totally support you in an attempt to remove this logic,
> unless someone has a good example of its usefulness.
>
> I believe it's a very old hack to select children over parents
> in case they have the same oom badness (e.g. share most of the memory).
>
> Maybe we can prefer older processes in case of equal oom badness,
> and it will be enough.
>
> Thanks!

I am thinking of removing the whole logic of selecting children.

Shakeel