linux-kernel - Re: [syzbot] [mm?] WARNING in __folio_rmap_sanity

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Message-ID: <Z4FKV8cPJJZjDhyz@casper.infradead.org>
Date: Fri, 10 Jan 2025 16:27:03 +0000
From: Matthew Wilcox <willy@...radead.org>
To: David Hildenbrand <david@...hat.com>
Cc: syzbot <syzbot+c0673e1f1f054fac28c2@...kaller.appspotmail.com>,
	akpm@...ux-foundation.org, hdanton@...a.com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	syzkaller-bugs@...glegroups.com,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Subject: Re: [syzbot] [mm?] WARNING in __folio_rmap_sanity_checks (2)

On Fri, Jan 10, 2025 at 05:19:54PM +0100, David Hildenbrand wrote:
> On 10.01.25 17:14, Matthew Wilcox wrote:
> > On Fri, Jan 10, 2025 at 04:48:03PM +0100, David Hildenbrand wrote:
> > > If I would have to guess, I would assume that we have a refcount issue such
> > > that we succeed in splitting a folio while concurrently mapping it.
> > 
> > That would seem hard to accomplish, because both hold the folio lock,
> > so it wouldn't be just a refcount bug but also a locking bug.  Not sure
> > what this is though.
> 
> Yeah, but we also have
> 
> https://lkml.kernel.org/r/6774bf44.050a0220.25abdd.098a.GAE@google.com

That one is a UAF on the vma, so it's either a different issue, or the
problem is with the VMA refcount/lookup/..., not the folio refcount.
cc'ing the relevant maintainers.