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Re: [for-4.22 PATCH] xen/gnttab: Fix TOCTOU race in gnttab_set_version()
- To: Jan Beulich <jbeulich@xxxxxxxx>, Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>
- From: Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>
- Date: Fri, 22 May 2026 14:28:53 +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 12:29:09 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On Fri May 22, 2026 at 1:31 PM CEST, Jan Beulich wrote:
> 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>
Thanks
>
>> 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.
Quite. Concurrency is hard enough without making assumptions about the
underlying ISAs :/
Cheers,
Alejandro
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