linux-kernel - Re: [PATCH v1 05/12] mm/memory: detect writability in restore_exclusive_pte() through can_change_pte

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <Z5t4rrkRiOsRY2jH@phenom.ffwll.local>
Date: Thu, 30 Jan 2025 14:03:42 +0100
From: Simona Vetter <simona.vetter@...ll.ch>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
	dri-devel@...ts.freedesktop.org, linux-mm@...ck.org,
	nouveau@...ts.freedesktop.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jérôme Glisse <jglisse@...hat.com>,
	Jonathan Corbet <corbet@....net>, Alex Shi <alexs@...nel.org>,
	Yanteng Si <si.yanteng@...ux.dev>,
	Karol Herbst <kherbst@...hat.com>, Lyude Paul <lyude@...hat.com>,
	Danilo Krummrich <dakr@...nel.org>,
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
	Pasha Tatashin <pasha.tatashin@...een.com>,
	Peter Xu <peterx@...hat.com>, Alistair Popple <apopple@...dia.com>,
	Jason Gunthorpe <jgg@...dia.com>
Subject: Re: [PATCH v1 05/12] mm/memory: detect writability in
 restore_exclusive_pte() through can_change_pte_writable()

On Thu, Jan 30, 2025 at 10:58:51AM +0100, David Hildenbrand wrote:
> On 30.01.25 10:51, Simona Vetter wrote:
> > On Wed, Jan 29, 2025 at 12:54:03PM +0100, David Hildenbrand wrote:
> > > Let's do it just like mprotect write-upgrade or during NUMA-hinting
> > > faults on PROT_NONE PTEs: detect if the PTE can be writable by using
> > > can_change_pte_writable().
> > > 
> > > Set the PTE only dirty if the folio is dirty: we might not
> > > necessarily have a write access, and setting the PTE writable doesn't
> > > require setting the PTE dirty.
> > 
> > Not sure whether there's much difference in practice, since a device
> > exclusive access means a write, so the folio better be dirty (unless we
> > aborted halfway through). But then I couldn't find the code in nouveau to
> > do that, so now I'm confused.
> 
> That confused me as well. Requiring the PTE to be writable does not imply
> that it is dirty.
> 
> So something must either set the PTE or the folio dirty.

Yeah I'm not finding that something.

> ( In practice, most anonymous folios are dirty most of the time ... )

And yup that's why I think it hasn't blown up yet.

> If we assume that "device-exclusive entries" are always dirty, then it
> doesn't make sense to set the folio dirty when creating device-exclusive
> entries. We'd always have to set the PTE dirty when restoring the exclusive
> pte.

I do agree with your change, I think it's correct to put this
responsibility onto drivers. It's just that nouveau seems to not be
entirely correct.

And thinking about this I have vague memories that I've discussed the case
of the missing folio_set_dirty in noveau hmm code before, maybe with
Alistair. But quick search in archives didn't turn up anything.
-Sima
-- 
Simona Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch