Re: [edk2-devel] [PATCH] OvmfPkg/ResetVector: Fix assembler bit test flag check

["Lendacky, Thomas via groups.io" <thomas.lendacky=amd.com@groups.io>]

On 9/19/23 02:59, Gerd Hoffmann wrote:
> On Fri, Jul 14, 2023 at 03:28:26PM -0500, Tom Lendacky wrote:
>> Commit 63c50d3ff2854a76432b752af4f2a76f33ff1974 changed the check that is
>> used to determine if SEV-ES is active. Originally, a CMP instruction with
>> a supporting JZ instruction was used for the check. It was changed to use
>> the BT instruction but not JZ instruction. The result of a BT instruction
>> changes the the carry flag (CF) and not the zero flag (ZF). As a result,
>> the wrong condition is being checked. Update the JZ to a JNC to properly
>> detect if SEV-ES is active.
> 
> What is the effect of this bug?  Is it just the encryption sanity check
> being skipped?

Yes, it is just causing the mitigation check to be skipped. Because of the 
previous xor instruction, the JZ instruction will always be taken.

Thanks,
Tom

> 
> take care,
>    Gerd
> 


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