Re: [PATCH SpectreV1+L1TF v4 05/11] common/grant_table: block speculative out-of-bound accesses

[Norbert Manthey <nmanthey@amazon.de>]

On 1/29/19 10:46, Jan Beulich wrote:
>>>> Norbert Manthey <nmanthey@amazon.de> 01/29/19 9:35 AM >>>
>> I am aware that both version use the same base array, and access it via
>> different macros, which essentially partition the array based on the
>> size of the respective struct. The underlying raw array has the same
>> size for both version.
> And this is the problem afaics: If a guest has requested its grant table to
> be sized as a single page, this page can fit twice as many entries for
> v1 than it can fit for v2. Hence the v1 grant reference pointing at the last
> entry would point at the last entry in the (not mapped) second page for v2.

I might understand the code wrong, but a guest would ask to get at most
N grant frames, and this number cannot be increased afterwards, i.e. the
field gt->max_grant_frames is written exactly once. Furthermore, the
void** shared_raw array is allocated and written exactly once with
sufficient pointers for, namely gt->max_grant_frames many in function
grant_table_init. Hence, independently of the version being used, at
least the shared_raw array cannot be used for out-of-bound accesses
during speculation with my above evaluate_nospec.

That being said, let's assume we have a v1 grant table, and speculation
uses the v2 accesses. In that case, an existing and zero-initialized
entry of shared_raw might be used in the first part of the
shared_entry_v2 macro, and even if that pointer would be non-NULL, the
page it would point to would have been cleared when growing the grant
table in function gnttab_grow_table.

That being said, I believe it is fine to let the above speculative
happen without extra hardening.

Best,
Norbert

PS: I just noticed that the shared_raw array might be allocated with a
smaller size, as long as more than 1 grant_entry fits into a page.

>
>
> Jan
>




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