Re: [PATCH 6/8] x86, microcode, intel: total_size is valid only when data_size != 0

From: Borislav Petkov
Date: Fri Jul 25 2014 - 12:46:29 EST

On Wed, Jul 23, 2014 at 05:10:49PM -0300, Henrique de Moraes Holschuh wrote:
> According to the Intel SDM vol 3A (order code 253668-051US, June 2014),
> on section 9.11.1, page 9-28:
> "For microcode updates with a data size field equal to 00000000H, the
> size of the microcode update is 2048 bytes. The first 48 bytes contain
> the microcode update header. The remaining 2000 bytes contain encrypted
> data."
> "For microcode updates with a data size not equal to 00000000H, the total
> size field specifies the size of the microcode update"
> We were incorrectly assuming that total_size is valid when it is
> non-zero, instead of checking data_size to be non-zero. IOW, we were
> trusting a reserved field to be zero in a situation where it was, in
> fact, undefined.

I'm perfectly fine with the patch except this statement above. I can't
parse it. What reserved field?

I'd guess the packages they distribute always have total_size
initialized properly, i.e. in the case where data_size is 0, total_size
is 2048 anyway. This is maybe the reason why nobody hit this bug as
get_totalsize() returning always the correct number.



Sent from a fat crate under my desk. Formatting is fine.
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