OvmfPkg/ResetVector/Ia32/Flat32ToFlat64.asm | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
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
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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 -=-=-=-=-=-=-=-=-=-=-=- Groups.io Links: You receive all messages sent to this group. View/Reply Online (#108817): https://edk2.groups.io/g/devel/message/108817 Mute This Topic: https://groups.io/mt/100149226/1787277 Group Owner: devel+owner@edk2.groups.io Unsubscribe: https://edk2.groups.io/g/devel/unsub [importer@patchew.org] -=-=-=-=-=-=-=-=-=-=-=-
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 > -=-=-=-=-=-=-=-=-=-=-=- Groups.io Links: You receive all messages sent to this group. View/Reply Online (#108830): https://edk2.groups.io/g/devel/message/108830 Mute This Topic: https://groups.io/mt/100149226/1787277 Group Owner: devel+owner@edk2.groups.io Unsubscribe: https://edk2.groups.io/g/devel/unsub [importer@patchew.org] -=-=-=-=-=-=-=-=-=-=-=-
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, -=-=-=-=-=-=-=-=-=-=-=- Groups.io Links: You receive all messages sent to this group. View/Reply Online (#106947): https://edk2.groups.io/g/devel/message/106947 Mute This Topic: https://groups.io/mt/100149226/1787277 Group Owner: devel+owner@edk2.groups.io Unsubscribe: https://edk2.groups.io/g/devel/unsub [importer@patchew.org] -=-=-=-=-=-=-=-=-=-=-=-
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