Re: [PATCH 5/5] x86/ucode: Drop the match_reg[] field from AMD's microcode_patch

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 28/10/2024 1:18 pm, Jan Beulich wrote:
> On 24.10.2024 15:22, Andrew Cooper wrote:
>> This was true in the K10 days, but even back then the match registers were
>> really payload data rather than header data.
>>
>> But, it's really model specific data, and these days typically part of the
>> signature, so is random data for all intents and purposes.
>>
>> No functional change.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
>> ---
>> CC: Jan Beulich <JBeulich@suse.com>
>> CC: Roger Pau Monné <roger.pau@citrix.com>
>>
>> The single difference from this is:
>>
>>   @@ -207587,7 +207587,7 @@
>>    ffff82d0402ad261:	4c 89 ce             	mov    %r9,%rsi
>>    ffff82d0402ad264:	4c 39 c8             	cmp    %r9,%rax
>>    ffff82d0402ad267:	0f 82 c2 11 f6 ff    	jb     ffff82d04020e42f <amd_ucode_parse.cold+0x55>
>>   -ffff82d0402ad26d:	41 83 f9 3f          	cmp    $0x3f,%r9d
>>   +ffff82d0402ad26d:	41 83 f9 1f          	cmp    $0x1f,%r9d
>>    ffff82d0402ad271:	0f 86 b8 11 f6 ff    	jbe    ffff82d04020e42f <amd_ucode_parse.cold+0x55>
>>    ffff82d0402ad277:	85 ed                	test   %ebp,%ebp
>>    ffff82d0402ad279:	75 55                	jne    ffff82d0402ad2d0 <amd_ucode_parse+0x170>
>>
>> which is "mc->len < sizeof(struct microcode_patch)" expression in
>> amd_ucode_parse().
> Yet is it correct to effectively relax that check, i.e. to accept something
> we previously would have rejected?

Yes.  This is the bounds check about whether it's safe to look at fields
in the header.  It's not part of the other validity checks.

~Andrew