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

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Message-ID: <CAG48ez1qhjQNHC+3572udqVWHTANFpQ0ngxn_4ZDC9F8NCXsFA@mail.gmail.com>
Date: Wed, 23 Jul 2025 19:49:29 +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 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...