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Re: [for-4.22 PATCH] xen/gnttab: Fix TOCTOU race in gnttab_set_version()
- To: Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Fri, 22 May 2026 13:31:37 +0200
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Julien Grall <julien@xxxxxxx>, Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Oleksandr Tyshchenko <Oleksandr_Tyshchenko@xxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Fri, 22 May 2026 11:32:00 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 22.05.2026 12:57, Alejandro Vallejo wrote:
> Move first read of gt->gt_version inside the critical region of the
> rwlock, otherwise concurrent gnttab operations (silly as they would be)
> may get mutually confused as to the actual current version.
>
> Fixes: c1488502c949("grant-tables: do not fail attempts to...")
> Reported-by: Oleksandr Tyshchenko <Oleksandr_Tyshchenko@xxxxxxxx>
> Signed-off-by: Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> There are a number of lockless reads of gt_version (e.g: right after unlock),
> but they aren't very worrying because they are effectively snapshots of the
> instantaneous version. I'd feel better if they were all atomic_read(), but all
> Xen ports guarantee atomic access on aligned 4 octet fields, so I couldn't be
> bothered to go chase them.
Sooner or later we will want to deal with all (latent) problems of this kind.
Jan
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