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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Tue, 15 Feb 2022 11:43:14 -0800
Message-ID: <CAJuCfpGixCqZDAqokxCrQ1YjMb+32Y77U5jHkULKM0htnR19MQ@mail.gmail.com>
Subject: Re: [syzbot] KASAN: use-after-free Read in __oom_reap_task_mm
To:     Michal Hocko <mhocko@suse.com>
Cc:     Yang Shi <shy828301@gmail.com>,
        syzbot <syzbot+2ccf63a4bd07cf39cab0@syzkaller.appspotmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Christian Brauner <brauner@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>
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On Tue, Feb 15, 2022 at 11:36 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Tue 15-02-22 10:10:53, Suren Baghdasaryan wrote:
> > On Tue, Feb 15, 2022 at 9:53 AM Yang Shi <shy828301@gmail.com> wrote:
> [...]
> > > Isn't the below race possible?
> > >
> > >           CPU A                                     CPU B
> > > exiting:
> > >   mmap_write_lock
> > >   remove_vma()
> > >   mmap_write_unlock
> > >                                                 process_mrelease:
> > >                                                   mmap_read_lock
> > >                                                   __oom_reap_task_mm
> > >                                                   mmap_read_unlock
> > >
> >
> > Sure, that sequence (would not call it a race) is possible but in this
> > case __oom_reap_task_mm will find no vmas in the mm because exit_mmap
> > freed and removed all of them.
>
> I didn't really have chance to have a closer look but I do not see
> exit_mmap doing mm->mmap = NULL so the pointer can be a freed vma unless
> I am missing something. I thought we've had it in your patches? Has this
> got lost somewhere in the process?

Doh! Yes, it looks like I completely missed the actual pointer. That
must be it since I don't see any other possibility. Will post a patch
shortly.
Thanks!

> --
> Michal Hocko
> SUSE Labs