linux-kernel - Re: [BUG] hard-to-hit mm_struct UAF due to insufficiently careful vma_refcount_put() wrt SLAB_TYPESAFE_BY

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAG48ez2cGX3e5bqgcw0OEnho29+YhwG2eOrHRgNwGAA_Nb65nw@mail.gmail.com>
Date: Wed, 23 Jul 2025 20:19:09 +0200
From: Jann Horn <jannh@...gle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Andrew Morton <akpm@...ux-foundation.org>, "Liam R. Howlett" <Liam.Howlett@...cle.com>, 
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Suren Baghdasaryan <surenb@...gle.com>, 
	Pedro Falcato <pfalcato@...e.de>, Linux-MM <linux-mm@...ck.org>, 
	kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [BUG] hard-to-hit mm_struct UAF due to insufficiently careful
 vma_refcount_put() wrt SLAB_TYPESAFE_BY_RCU

On Wed, Jul 23, 2025 at 8:10 PM Vlastimil Babka <vbabka@...e.cz> wrote:
> On 7/23/25 19:49, Jann Horn wrote:
> > On Wed, Jul 23, 2025 at 7:32 PM Vlastimil Babka <vbabka@...e.cz> wrote:
> >> On 7/23/25 18:26, Jann Horn wrote:
> >> > There's a racy UAF in `vma_refcount_put()` when called on the
> >> > `lock_vma_under_rcu()` path because `SLAB_TYPESAFE_BY_RCU` is used
> >> > without sufficient protection against concurrent object reuse:
> >>
> >> Oof.
> >>
> >> > I'm not sure what the right fix is; I guess one approach would be to
> >> > have a special version of vma_refcount_put() for cases where the VMA
> >> > has been recycled by another MM that grabs an extra reference to the
> >> > MM? But then dropping a reference to the MM afterwards might be a bit
> >> > annoying and might require something like mmdrop_async()...
> >>
> >> Would we need mmdrop_async()? Isn't this the case for mmget_not_zero() and
> >> mmput_async()?
> >
> > Now I'm not sure anymore if either of those approaches would work,
> > because they rely on the task that's removing the VMA to wait until we
> > do __refcount_dec_and_test() before deleting the MM... but I don't
> > think we have any such guarantee...
>
> I think it would be waiting in exit_mmap->vma_mark_detached(), but then
> AFAIU you're right and we'd really need to work with mmgrab/mmdrop because
> at that point the  mmget_not_zero() would already be failing...

Ah, I see! vma_mark_detached() drops its reference, then does
__vma_enter_locked() to bump the refcount by VMA_LOCK_OFFSET again
(after which the reader path can't acquire it anymore), then waits
until the refcount drops to VMA_LOCK_OFFSET, and then decrements it
down to 0 from there. Makes sense.