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Date:   Wed, 16 Feb 2022 08:54:54 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Suren Baghdasaryan <surenb@google.com>
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Subject: Re: [PATCH 1/1] mm: fix use-after-free bug when mm->mmap is reused
 after being freed
Message-ID: <YgytzntIfx6Toom2@dhcp22.suse.cz>
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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")
"

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.

> 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!
> ---
>  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