Re: [edk2-devel] [PATCH] OvmfPkg: Harden #VC instruction emulation somewhat (CVE-2024-25742)

["Ard Biesheuvel" <ardb@kernel.org>]

(cc Jiewen)

Please cc the OVMF maintainers when you send edk2 patches. (There is a
Maintainers file in the root of the repo)


On Wed, 17 Apr 2024 at 18:54, Adam Dunlap via groups.io
<acdunlap=google.com@groups.io> wrote:
>
> Ensure that when a #VC exception happens, the instruction at the
> instruction pointer matches the instruction that is expected given the
> error code. This is to mitigate the ahoi WeSee attack [1] that could
> allow hypervisors to breach integrity and confidentiality of the
> firmware by maliciously injecting interrupts. This change is a
> translated version of a linux patch e3ef461af35a ("x86/sev: Harden #VC
> instruction emulation somewhat")
>
> [1] https://ahoi-attacks.github.io/wesee/
>
> Cc: Borislav Petkov (AMD) <bp@alien8.de>
> Cc: Tom Lendacky <thomas.lendacky@amd.com>
> Signed-off-by: Adam Dunlap <acdunlap@google.com>
> ---
>  OvmfPkg/Library/CcExitLib/CcExitVcHandler.c | 171 ++++++++++++++++++--
>  1 file changed, 160 insertions(+), 11 deletions(-)
>
> diff --git a/OvmfPkg/Library/CcExitLib/CcExitVcHandler.c b/OvmfPkg/Library/CcExitLib/CcExitVcHandler.c
> index 0fc30f7bc4..bd3e9f304a 100644
> --- a/OvmfPkg/Library/CcExitLib/CcExitVcHandler.c
> +++ b/OvmfPkg/Library/CcExitLib/CcExitVcHandler.c
> @@ -532,8 +532,6 @@ MwaitExit (
>    IN     CC_INSTRUCTION_DATA     *InstructionData
>    )
>  {
> -  CcDecodeModRm (Regs, InstructionData);
> -
>    Ghcb->SaveArea.Rax = Regs->Rax;
>    CcExitVmgSetOffsetValid (Ghcb, GhcbRax);
>    Ghcb->SaveArea.Rcx = Regs->Rcx;
> @@ -564,8 +562,6 @@ MonitorExit (
>    IN     CC_INSTRUCTION_DATA     *InstructionData
>    )
>  {
> -  CcDecodeModRm (Regs, InstructionData);
> -
>    Ghcb->SaveArea.Rax = Regs->Rax;  // Identity mapped, so VA = PA
>    CcExitVmgSetOffsetValid (Ghcb, GhcbRax);
>    Ghcb->SaveArea.Rcx = Regs->Rcx;
> @@ -670,8 +666,6 @@ VmmCallExit (
>  {
>    UINT64  Status;
>
> -  CcDecodeModRm (Regs, InstructionData);
> -
>    Ghcb->SaveArea.Rax = Regs->Rax;
>    CcExitVmgSetOffsetValid (Ghcb, GhcbRax);
>    Ghcb->SaveArea.Cpl = (UINT8)(Regs->Cs & 0x3);
> @@ -1603,8 +1597,6 @@ Dr7WriteExit (
>    Ext       = &InstructionData->Ext;
>    SevEsData = (SEV_ES_PER_CPU_DATA *)(Ghcb + 1);
>
> -  CcDecodeModRm (Regs, InstructionData);
> -
>    //
>    // MOV DRn always treats MOD == 3 no matter how encoded
>    //
> @@ -1655,8 +1647,6 @@ Dr7ReadExit (
>    Ext       = &InstructionData->Ext;
>    SevEsData = (SEV_ES_PER_CPU_DATA *)(Ghcb + 1);
>
> -  CcDecodeModRm (Regs, InstructionData);
> -
>    //
>    // MOV DRn always treats MOD == 3 no matter how encoded
>    //
> @@ -1671,6 +1661,160 @@ Dr7ReadExit (
>    return 0;
>  }
>
> +/**
> +  Check that the opcode matches the exit code for a #VC.
> +
> +  Each exit code should only be raised while executing certain instructions.
> +  Verify that rIP points to a correct instruction based on the exit code to
> +  protect against maliciously injected interrupts via the hypervisor. If it does
> +  not, report an unsupported event to the hypervisor.
> +
> +  Decodes the ModRm byte into InstructionData if necessary.
> +
> +  @param[in, out] Ghcb             Pointer to the Guest-Hypervisor Communication
> +                                   Block
> +  @param[in, out] Regs             x64 processor context
> +  @param[in, out] InstructionData  Instruction parsing context
> +  @param[in]      ExitCode         Exit code given by #VC.
> +
> +  @retval 0                        No problems detected.
> +  @return                          New exception value to propagate
> +
> +
> +**/
> +STATIC
> +UINT64
> +VcCheckOpcodeBytes (
> +  IN OUT GHCB                    *Ghcb,
> +  IN OUT EFI_SYSTEM_CONTEXT_X64  *Regs,
> +  IN OUT CC_INSTRUCTION_DATA     *InstructionData,
> +  IN     UINT64                  ExitCode
> +  )
> +{
> +  UINT8  OpCode;
> +
> +  //
> +  // Expected opcodes are either 1 or 2 bytes. If they are 2 bytes, they always
> +  // start with TWO_BYTE_OPCODE_ESCAPE (0x0f), so skip over that.
> +  //
> +  OpCode = *(InstructionData->OpCodes);
> +  if (OpCode == TWO_BYTE_OPCODE_ESCAPE) {
> +    OpCode = *(InstructionData->OpCodes + 1);
> +  }
> +
> +  switch (ExitCode) {
> +    case SVM_EXIT_IOIO_PROT:
> +    case SVM_EXIT_NPF:
> +      /* handled separately */
> +      return 0;
> +
> +    case SVM_EXIT_CPUID:
> +      if (OpCode == 0xa2) {
> +        return 0;
> +      }
> +
> +      break;
> +
> +    case SVM_EXIT_INVD:
> +      break;
> +
> +    case SVM_EXIT_MONITOR:
> +      CcDecodeModRm (Regs, InstructionData);
> +
> +      if ((OpCode == 0x01) && (InstructionData->ModRm.Uint8 == 0xc8)) {
> +        return 0;
> +      }
> +
> +      break;
> +
> +    case SVM_EXIT_MWAIT:
> +      CcDecodeModRm (Regs, InstructionData);
> +
> +      if ((OpCode == 0x01) && (InstructionData->ModRm.Uint8 == 0xc9)) {
> +        return 0;
> +      }
> +
> +      break;
> +
> +    case SVM_EXIT_MSR:
> +      /* RDMSR */
> +      if ((OpCode == 0x32) ||
> +          /* WRMSR */
> +          (OpCode == 0x30))
> +      {
> +        return 0;
> +      }
> +
> +      break;
> +
> +    case SVM_EXIT_RDPMC:
> +      if (OpCode == 0x33) {
> +        return 0;
> +      }
> +
> +      break;
> +
> +    case SVM_EXIT_RDTSC:
> +      if (OpCode == 0x31) {
> +        return 0;
> +      }
> +
> +      break;
> +
> +    case SVM_EXIT_RDTSCP:
> +      CcDecodeModRm (Regs, InstructionData);
> +
> +      if ((OpCode == 0x01) && (InstructionData->ModRm.Uint8 == 0xf9)) {
> +        return 0;
> +      }
> +
> +      break;
> +
> +    case SVM_EXIT_DR7_READ:
> +      CcDecodeModRm (Regs, InstructionData);
> +
> +      if ((OpCode == 0x21) &&
> +          (InstructionData->Ext.ModRm.Reg == 7))
> +      {
> +        return 0;
> +      }
> +
> +      break;
> +
> +    case SVM_EXIT_VMMCALL:
> +      CcDecodeModRm (Regs, InstructionData);
> +
> +      if ((OpCode == 0x01) && (InstructionData->ModRm.Uint8 == 0xd9)) {
> +        return 0;
> +      }
> +
> +      break;
> +
> +    case SVM_EXIT_DR7_WRITE:
> +      CcDecodeModRm (Regs, InstructionData);
> +
> +      if ((OpCode == 0x23) &&
> +          (InstructionData->Ext.ModRm.Reg == 7))
> +      {
> +        return 0;
> +      }
> +
> +      break;
> +
> +    case SVM_EXIT_WBINVD:
> +      if (OpCode == 0x9) {
> +        return 0;
> +      }
> +
> +      break;
> +
> +    default:
> +      break;
> +  }
> +
> +  return UnsupportedExit (Ghcb, Regs, InstructionData);
> +}
> +
>  /**
>    Handle a #VC exception.
>
> @@ -1773,7 +1917,12 @@ InternalVmgExitHandleVc (
>
>    CcInitInstructionData (&InstructionData, Ghcb, Regs);
>
> -  Status = NaeExit (Ghcb, Regs, &InstructionData);
> +  Status = VcCheckOpcodeBytes (Ghcb, Regs, &InstructionData, ExitCode);
> +
> +  if (Status == 0) {
> +    Status = NaeExit (Ghcb, Regs, &InstructionData);
> +  }
> +

This looks a bit dodgy. First of all, I have a personal dislike of
this 'success handling' anti-pattern, but more importantly, it seems
like we are relying here on VcCheckOpcodeBytes() never returning on
failure, right? If so, that at least needs a comment.

>    if (Status == 0) {
>      Regs->Rip += CcInstructionLength (&InstructionData);
>    } else {
> --
> 2.44.0.683.g7961c838ac-goog
>
>
>
> 
>
>


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