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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: Mon, 8 Jul 2019 15:53:38 +0200
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  • Cc: Juergen Gross <jgross@xxxxxxxx>, Tim Deegan <tim@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>, Dario Faggioli <dfaggioli@xxxxxxxx>, Martin Pohlack <mpohlack@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: Mon, 08 Jul 2019 13:54:01 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
  • Openpgp: preference=signencrypt

On 5/23/19 17:01, Jan Beulich wrote:
>>>> On 21.05.19 at 09:45, <nmanthey@xxxxxxxxx> wrote:
>> Guests can issue grant table operations and provide guest controlled
>> data to them. This data is used as index for memory loads after bound
>> checks have been done. Depending on the grant table version, the
>> size of elements in containers differ. As the base data structure is
>> a page, the number of elements per page also differs. Consequently,
>> bound checks are version dependent, so that speculative execution can
>> happen in several stages, the bound check as well as the version check.
>>
>> This commit mitigates cases where out-of-bound accesses could happen
>> due to the version comparison. In cases, where no different memory
>> locations are accessed on the code path that follow an if statement,
>> no protection is required. No different memory locations are accessed
>> in the following functions after a version check:
>>
>>  * gnttab_setup_table: only calculated numbersi are used, and then
>>         function gnttab_grow_table is called, which is version protected
>>
>>  * gnttab_transfer: the case that depends on the version check just gets
>>         into copying a page or not
> Well, this is a little lax, but I'm willing to accept it. There could, after
> all, still be speculation into alloc_domheap_page().
>
>>  * acquire_grant_for_copy: the not fixed comparison is on the abort path
>>         and does not access other structures, and on the else branch only
>>         accesses structures that are properly allocated
> As said in my recent reply to v10 of the original series, in particular
> for fixup_status_for_copy_pin() this isn't immediately obvious. In
> no case is "does not access other structures" true, though. How
> about saying "accesses only structures that have been validated
> before" or some such instead (I don't particularly like "validated"
> here, but I can't currently think of anything better)?
I will rephrase accordingly.
>
>>  * 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.
>
>>  * gnttab_release_mappings: this function is called only during domain
>>        destruction and control is not returned to the guest
>>
>>  * mem_sharing_gref_to_gfn: speculation will be stoped by the second if
>>        statement, as that places a barrier on any path to be executed.
>>
>>  * gnttab_get_status_frame_mfn: no version dependent check, because all
>>        accesses, except the gt->status[idx], do not perform actual
>>        out-of-bound accesses, including the gnttab_grow_table function
>>        call.
> Nit: I very much hope no code anywhere performs _actual_ out of
> bound accesses. I'm sure you mean speculative ones here.
Yes, I do not mean actual out-of-bound accesses. What I actually meant:
all other accesses in this function are to variables, and not based on
an index.
>
>>  * gnttab_get_shared_frame: block_speculation in
>>        gnttab_get_status_frame_mfn blocks accesses
> The former doesn't call the latter, and as per my patch 2 comments
> gnttab_get_shared_frame_mfn() looks to remain unprotected after
> patch 2.

True, I will add a block_speculation() below the assert statement, so
that the condition is true even when executing speculatively.

Best,
Norbert





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