File ovmf-OvmfPkg-CcExitLib-Use-the-proper-register-when-filte.patch of Package ovmf-edk2-stable202408

From 856bdc8eec0fd450ffb582808ad9649a5d02b480 Mon Sep 17 00:00:00 2001
From: Tom Lendacky <thomas.lendacky@amd.com>
Date: Fri, 25 Apr 2025 12:49:52 -0500
Subject: [PATCH] OvmfPkg/CcExitLib: Use the proper register when filtering
 MSRs

The MsrExit() routine uses an incorrect register to check for the CAA MSR.
Instead of checking RCX, the input register for RDMSR/WRMSR that holds the
MSR value, it is checking RAX, which results in failure to detect the CAA
MSR request. The check should only be checking the lower 32-bits of the
register (ECX), too.

Change the check in MsrExit() to check the MSR value contained in ECX.

Fixes: 47001ab98914 ("Ovmfpkg/CcExitLib: Provide SVSM discovery support")
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
---
 OvmfPkg/Library/CcExitLib/CcExitVcHandler.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/OvmfPkg/Library/CcExitLib/CcExitVcHandler.c b/OvmfPkg/Library/CcExitLib/CcExitVcHandler.c
index 2031fa9e22..2c12f78f8d 100644
--- a/OvmfPkg/Library/CcExitLib/CcExitVcHandler.c
+++ b/OvmfPkg/Library/CcExitLib/CcExitVcHandler.c
@@ -701,6 +701,7 @@ MsrExit (
   MSR_SVSM_CAA_REGISTER  Msr;
   UINT64                 ExitInfo1;
   UINT64                 Status;
+  UINT32                 EcxIn;
 
   ExitInfo1 = 0;
 
@@ -708,7 +709,8 @@ MsrExit (
   // The SVSM CAA MSR is a software implemented MSR and not supported
   // by the hardware, handle it directly.
   //
-  if (Regs->Rax == MSR_SVSM_CAA) {
+  EcxIn = (UINT32)(UINTN)Regs->Rcx;
+  if (EcxIn == MSR_SVSM_CAA) {
     // Writes to the SVSM CAA MSR are ignored
     if (*(InstructionData->OpCodes + 1) == 0x30) {
       return 0;
-- 
2.43.0