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Re: [Xen-devel] [PATCH L1TF MDS GT v1 3/3] common/grant_table: harden version dependent accesses


  • To: Jan Beulich <JBeulich@xxxxxxxx>
  • From: Norbert Manthey <nmanthey@xxxxxxxxx>
  • Date: Wed, 10 Jul 2019 13:48:56 +0200
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  • Cc: Juergen Gross <JGross@xxxxxxxx>, Tim Deegan <tim@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, IanJackson <Ian.Jackson@xxxxxxxxxxxxx>, Dario Faggioli <dfaggioli@xxxxxxxx>, Martin Pohlack <mpohlack@xxxxxxxxx>, "wipawel@xxxxxxxxx" <wipawel@xxxxxxxxx>, Julien Grall <julien.grall@xxxxxxx>, David Woodhouse <dwmw@xxxxxxxxxxxx>, "Martin Mazein\(amazein\)" <amazein@xxxxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Bjoern Doebel <doebel@xxxxxxxxx>
  • Delivery-date: Wed, 10 Jul 2019 11:49:35 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
  • Openpgp: preference=signencrypt

On 7/10/19 05:12, Jan Beulich wrote:
> On 08.07.2019 15:53, Norbert Manthey wrote:
>> On 5/23/19 17:01, Jan Beulich wrote:
>>>>>> On 21.05.19 at 09:45, <nmanthey@xxxxxxxxx> wrote:
>>>>   * gnttab_set_version: all accessible data is allocated for both versions
>>> This is not enough for my taste: The very first loop is safe only
>>> because nr_grant_entries() is. And speculating into
>>> gnttab_unpopulate_status_frames() doesn't look safe at all, as
>>> gt->status[i] may be NULL.
>> So, you basically want to see a block_speculation() at the beginning of
>> the function gnttab_populate_status_frames and
>> gnttab_unpopulate_status_frames? I do not claim to protect against
>> speculative out-of-bound accesses that are caused by the for loop in
>> gnttab_set_version.
> The point isn't the loop, but the fact that by mis-speculating through
> the two conditions before the function call a NULL gt->status[0] may
> get accessed, entirely independent of this being a loop or just a
> singular access.

I understand. To prevent this kind of access during speculative
execution, I will add a block_speculation() at the top of the function
to make sure the code is reached only when the correct version number is
used.

Best,
Norbert





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