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: <16c97e30-19c9-41e8-b73b-c0b3c8eceff3@suse.cz>
Date: Wed, 23 Jul 2025 19:32:24 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Jann Horn <jannh@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Suren Baghdasaryan <surenb@...gle.com>
Cc: 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 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()?