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: <0e057095-e4c9-4869-b147-c48c9f66e500@lucifer.local>
Date: Thu, 24 Jul 2025 06:24:09 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Jann Horn <jannh@...gle.com>
Cc: Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Liam R. Howlett" <Liam.Howlett@...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 10:00:40PM +0200, Jann Horn wrote:
> On Wed, Jul 23, 2025 at 9:52 PM Jann Horn <jannh@...gle.com> wrote:
> > I'm not sure if I'm understanding you correctly; but yes,
> > __vma_enter_locked() waits for all the waiters to drop their
> > "refcounts". (It's not really a refcount, you can also think of it as
> > a sleepable read-write lock where the low bits are the number of
> > readers.)
>
> Sorry, that's not entirely true, since an attached VMA has a refcount
> elevated by one. It's kind of a refcount, and kind of forms part of a
> sleepable read-write lock, it's complicated.

And needs internal impl detail documentation IMO. Which I'll provide...