[PATCH] OvmfPkg/ResetVector: Fix assembler bit test flag check

[PATCH] OvmfPkg/ResetVector: Fix assembler bit test flag check

[Lendacky, Thomas]

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.

Fixes: 63c50d3ff285 ("OvmfPkg/ResetVector: cache the SEV status MSR...")
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
---
 OvmfPkg/ResetVector/Ia32/Flat32ToFlat64.asm | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/OvmfPkg/ResetVector/Ia32/Flat32ToFlat64.asm b/OvmfPkg/ResetVector/Ia32/Flat32ToFlat64.asm
index c5c683ebed3e..429a58c5efd3 100644
--- a/OvmfPkg/ResetVector/Ia32/Flat32ToFlat64.asm
+++ b/OvmfPkg/ResetVector/Ia32/Flat32ToFlat64.asm
@@ -44,7 +44,7 @@ Transition32FlatTo64Flat:
 
     mov     ecx, 1
     bt      [SEV_ES_WORK_AREA_STATUS_MSR], ecx
-    jz      EnablePaging
+    jnc     EnablePaging
 
     ;
     ; SEV-ES is active, perform a quick sanity check against the reported
-- 
2.40.1

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

[Ard Biesheuvel]

On Fri, 14 Jul 2023 at 22:28, Tom Lendacky <thomas.lendacky@amd.com> 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.
>
> Fixes: 63c50d3ff285 ("OvmfPkg/ResetVector: cache the SEV status MSR...")
> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
> ---
>  OvmfPkg/ResetVector/Ia32/Flat32ToFlat64.asm | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/OvmfPkg/ResetVector/Ia32/Flat32ToFlat64.asm b/OvmfPkg/ResetVector/Ia32/Flat32ToFlat64.asm
> index c5c683ebed3e..429a58c5efd3 100644
> --- a/OvmfPkg/ResetVector/Ia32/Flat32ToFlat64.asm
> +++ b/OvmfPkg/ResetVector/Ia32/Flat32ToFlat64.asm
> @@ -44,7 +44,7 @@ Transition32FlatTo64Flat:
>
>      mov     ecx, 1
>      bt      [SEV_ES_WORK_AREA_STATUS_MSR], ecx
> -    jz      EnablePaging
> +    jnc     EnablePaging
>
>      ;
>      ; SEV-ES is active, perform a quick sanity check against the reported

Merged as #4646

Thanks,

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

[Gerd Hoffmann]

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?

take care,
  Gerd



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

[Lendacky, Thomas via 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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