Re: [PATCH 1/1] mm: fix use-after-free bug when mm->mmap is reused after being freed

[Suren Baghdasaryan <surenb@google.com>]

On Tue, Feb 15, 2022 at 11:54 PM Michal Hocko <mhocko@suse.com> wrote:
>
> On Tue 15-02-22 12:19:22, Suren Baghdasaryan wrote:
> > After exit_mmap frees all vmas in the mm, mm->mmap needs to be reset,
> > otherwise it points to a vma that was freed and when reused leads to
> > a use-after-free bug.
>
> OK, so I have dived into this again. exit_mmap doesn't reset mmap
> indeed.  That doesn't really matter for _oom victims_. Both the oom reaper and
> mrelease do check for MMF_OOM_SKIP before calling __oom_reap_task_mm.
> exit_mmap still sets MMF_OOM_SKIP before taking the mmap_lock for oom
> victims so those paths should be still properly synchronized.  I have
> proposed to get rid of this
> http://lkml.kernel.org/r/YbHIaq9a0CtqRulE@dhcp22.suse.cz but we haven't
> agreed on that.
>
> mrelease path is broken because it doesn't mark the process oom_victim
> and so the MMF_OOM_SKIP synchronization doesn't work. So we really need
> this.
>
> I would propose to rephrase the changelog to be more specific because I
> do not want to remember all those details later on.
> What about
> "
> oom reaping (__oom_reap_task_mm) relies on a 2 way synchronization with
> exit_mmap. First it relies on the mmap_lock to exclude from unlock
> path[1], page tables tear down (free_pgtables) and vma destruction.
> This alone is not sufficient because mm->mmap is never reset. For
> historical reasons[2] the lock is taken there is also MMF_OOM_SKIP set
> for oom victims before.
>
> The oom reaper only ever looks at oom victims so the whole scheme works
> properly but process_mrelease can opearate on any task (with fatal
> signals pending) which doesn't really imply oom victims. That means that
> the MMF_OOM_SKIP part of the synchronization doesn't work and it can
> see a task after the whole address space has been demolished and
> traverse an already released mm->mmap list. This leads to use after free
> as properly caught up by KASAN report.
>
> Fix the issue by reseting mm->mmap so that MMF_OOM_SKIP synchronization
> is not needed anymore. The MMF_OOM_SKIP is not removed from exit_mmap
> yet but it acts mostly as an optimization now.
>
> [1] 27ae357fa82b ("mm, oom: fix concurrent munlock and oom reaper unmap,
> v3")
> [2] 212925802454 ("mm: oom: let oom_reap_task and exit_mmap run
> concurrently")
> "

This changelog is very detailed and I support switching to it. Andrew,
please let me know if I should re-post the patch with this description
or you will just amend the one in your tree.

>
> I really have to say that I hate how complex this has grown in the name
> of optimizations. This has backfired several times already resulting in
> 2 security issues. I really hope to get read any note of the oom reaper
> from exit_mmap.

Agree. I want to take another stab at removing __oom_reap_task_mm from
exit_mmap. Now that Hugh made changes to mlock mechanisms and
__oom_reap_task_mm does not skip locked vmas I think that should be
possible. Planning to look into that sometimes next week.

>
> > Reported-by: syzbot+2ccf63a4bd07cf39cab0@syzkaller.appspotmail.com
> > Suggested-by: Michal Hocko <mhocko@suse.com>
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
>
> Acked-by: Michal Hocko <mhocko@suse.com>

Thanks!

>
> Thanks!
> > ---
> >  mm/mmap.c | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index 1e8fdb0b51ed..d445c1b9d606 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -3186,6 +3186,7 @@ void exit_mmap(struct mm_struct *mm)
> >               vma = remove_vma(vma);
> >               cond_resched();
> >       }
> > +     mm->mmap = NULL;
> >       mmap_write_unlock(mm);
> >       vm_unacct_memory(nr_accounted);
> >  }
> > --
> > 2.35.1.265.g69c8d7142f-goog
>
> --
> Michal Hocko
> SUSE Labs