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: <CAJuCfpGPjMSMqL1p=dMK3NJ_Cytk037Gm63TU5OuY6QK7Vi+4g@mail.gmail.com>
Date: Thu, 24 Jul 2025 16:36:05 +0000
From: Suren Baghdasaryan <surenb@...gle.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>, Lorenzo Stoakes <lorenzo.stoakes@...cle.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 Thu, Jul 24, 2025 at 2:45 PM Jann Horn <jannh@...gle.com> wrote:
>
> On Thu, Jul 24, 2025 at 10:38 AM Vlastimil Babka <vbabka@...e.cz> wrote:
> > On 7/24/25 04:30, Suren Baghdasaryan wrote:
> > > So, I think vma_refcount_put() can mmgrab(vma->mm) before calling
> > > __refcount_dec_and_test(), to stabilize that mm and then mmdrop()
> > > after it calls rcuwait_wake_up(). What do you think about this
> > > approach, folks?
> >
> > Yeah except it would be wasteful to do for all vma_refcount_put(). Should be
> > enough to have this version (as Jann suggested) for inval_end_read: part of
> > lock_vma_under_rcu. I think we need it also for the vma_refcount_put() done
> > in vma_start_read() when we fail the seqcount check? I think in that case
> > the same thing can be happening too, just with different race windows?
> >
> > Also as Jann suggested, maybe it's not great (or even safe) to perform
> > __mmdrop() under rcu? And maybe some vma_start_read() users are even more
> > restricted? Maybe then we'd need to make __mmdrop_delayed() not RT-only, and
> > use that.
>
> FWIW, I think I have been mixing things up in my head - mmdrop_async()
> exists, but this comment in free_signal_struct() explains that it's
> because __mmdrop() is not softirq-safe because x86's pgd_lock spinlock
> does not disable IRQs.
>
> /*
> * __mmdrop is not safe to call from softirq context on x86 due to
> * pgd_dtor so postpone it to the async context
> */
>
> So I guess using mmdrop() here might actually be fine, since we're
> just in atomic context, not in softirq.

Thanks for looking more into this. Even if it's safe, I would still
prefer to make mmdrop() outside of RCU read section. The code might
actually end-up cleaner that way too.