Re: [PATCH v2] mm/rmap: Add anon_vma lifetime debug check

[Jann Horn <jannh@google.com>]

On Mon, Jul 28, 2025 at 6:06 AM Harry Yoo <harry.yoo@oracle.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@oracle.com>

Thanks!