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: <CAG48ez0782x2dqa5LrK2cf9M0_Q=Gvo5tXsUFQDtyK00LpcvyQ@mail.gmail.com>
Date: Wed, 23 Jul 2025 22:00:40 +0200
From: Jann Horn <jannh@...gle.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.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 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.