linux-kernel - Re: [PATCH v2] mm/rmap: Add anon

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Message-ID: <CAG48ez0sfeF73sBK0V2TiYMc=grhM5fA-673xXNeCqQMYo2dug@mail.gmail.com>
Date: Mon, 28 Jul 2025 16:12:08 +0200
From: Jann Horn <jannh@...gle.com>
To: Harry Yoo <harry.yoo@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, David Hildenbrand <david@...hat.com>, 
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Rik van Riel <riel@...riel.com>, 
	"Liam R. Howlett" <Liam.Howlett@...cle.com>, Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm/rmap: Add anon_vma lifetime debug check

On Mon, Jul 28, 2025 at 6:06 AM Harry Yoo <harry.yoo@...cle.com> wrote:
> On Fri, Jul 25, 2025 at 02:16:24PM +0200, Jann Horn wrote:
> > If an anon folio is mapped into userspace, its anon_vma must be alive,
> > otherwise rmap walks can hit UAF.
> >
> > There have been syzkaller reports a few months ago[1][2] of UAF in rmap
> > walks that seems to indicate that there can be pages with elevated mapcount
> > whose anon_vma has already been freed, but I think we never figured out
> > what the cause is; and syzkaller only hit these UAFs when memory pressure
> > randomly caused reclaim to rmap-walk the affected pages, so it of course
> > didn't manage to create a reproducer.
> >
> > Add a VM_WARN_ON_FOLIO() when we add/remove mappings of anonymous folios to
> > hopefully catch such issues more reliably.
[...]
> Oops, I'm late to the party.
>
> A question; does it make sense to disable reuse of anon_vmas during
> anon_vma_clone() to increase chances of detecting this? (of course,
> for debugging-purpose only)

As Lorenzo said, I think making such a change would risk making it
impossible to hit some bugs in debug builds even though they can
happen in normal builds, which would be bad.

> Regardless of that:
> Acked-by: Harry Yoo <harry.yoo@...cle.com>

Thanks!