linux-kernel - Re: [BUG] page table UAF, Re: [PATCH v8 14/21] mm/mmap: Avoid zeroing vma tree in mmap

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Message-ID: <CAG48ez0afDQ94ZDYvU1-MdYC2n5AO3yZLXW93JkQMtJ7oh0vaA@mail.gmail.com>
Date: Tue, 8 Oct 2024 20:06:33 +0200
From: Jann Horn <jannh@...gle.com>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: "Liam R. Howlett" <Liam.Howlett@...cle.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Linux-MM <linux-mm@...ck.org>, 
	kernel list <linux-kernel@...r.kernel.org>, Matthew Wilcox <willy@...radead.org>, 
	Vlastimil Babka <vbabka@...e.cz>, Sidhartha Kumar <sidhartha.kumar@...cle.com>, 
	Bert Karwatzki <spasswolf@....de>, Jiri Olsa <olsajiri@...il.com>, Kees Cook <kees@...nel.org>, 
	"Paul E . McKenney" <paulmck@...nel.org>, Jeff Xu <jeffxu@...omium.org>, 
	Seth Jenkins <sethjenkins@...gle.com>
Subject: Re: [BUG] page table UAF, Re: [PATCH v8 14/21] mm/mmap: Avoid zeroing
 vma tree in mmap_region()

On Tue, Oct 8, 2024 at 7:52 PM Suren Baghdasaryan <surenb@...gle.com> wrote:
> On Tue, Oct 8, 2024 at 10:16 AM Jann Horn <jannh@...gle.com> wrote:
> > Is that code in a tree somewhere?
> >
> > What locking will those RCU walkers use when accessing VMAs? I guess
> > they probably anyway have to take the VMA locks to ensure they see
> > consistent state, though I guess with enough effort you could avoid it
> > (seqlock-style) on some fastpaths when the vma is not concurrently
> > modified and the fastpath doesn't need access to the VMA's file?
>
> Sorry, it's not posted upstream yet but yes, the idea is to walk the
> tree under RCU and detect concurrent changes using seq-counters. A
> prototype was posted here:
> https://lore.kernel.org/all/20240123231014.3801041-3-surenb@google.com/
> but it had some issues I'm yet to resolve.

Ah, thanks for the pointer.