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

["Yao, Jiewen" <jiewen.yao@intel.com>]

Thanks Adam and Ard.

Since this #VC specific hardening, I would rely on AMD people's expertise to fix it.
I have no objection for the patch.

Thank you
Yao, Jiewen

> -----Original Message-----
> From: Adam Dunlap <acdunlap@google.com>
> Sent: Thursday, April 18, 2024 1:45 AM
> To: Ard Biesheuvel <ardb@kernel.org>
> Cc: devel@edk2.groups.io; Yao, Jiewen <jiewen.yao@intel.com>; Borislav Petkov
> <bp@alien8.de>; Peter Gonda <pgonda@google.com>; Tom Lendacky
> <thomas.lendacky@amd.com>; Aktas, Erdem <erdemaktas@google.com>; Gerd
> Hoffmann <kraxel@redhat.com>; Michael Roth <michael.roth@amd.com>; Xu,
> Min M <min.m.xu@intel.com>
> Subject: Re: [edk2-devel] [PATCH] OvmfPkg: Harden #VC instruction emulation
> somewhat (CVE-2024-25742)
> 
> On Wed, Apr 17, 2024 at 10:08 AM Ard Biesheuvel <ardb@kernel.org> wrote:
> >
> > (cc Jiewen)
> >
> > Please cc the OVMF maintainers when you send edk2 patches. (There is a
> > Maintainers file in the root of the repo)
> 
> Thanks, I added everyone returned from the GetMaintainer.py script.
> 
> > 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 VcCheckOpcodeBytes() returns failure, that means it thinks that the #VC was
> invalid/injected maliciously and that the guest should abort. From reading the
> code in this file, it looks like calling UnsupportedExit() and returning its
> return value is the standard way of doing this. If UnsupportedExit() doesn't
> abort and instead returns normally for whatever reason, it will just ignore the
> exception which seems like acceptable behavior. Maybe add a comment like
> 
> /* If the opcode does not match the exit code, do not process the exception */
> 
> If we could ensure that UnsupportedExit() always diverged (i.e. never returned)
> then the code could be a bit simpler since it wouldn't need to handle error
> cases.
> 
> > >    if (Status == 0) {
> > >      Regs->Rip += CcInstructionLength (&InstructionData);
> > >    } else {
> > > --
> > > 2.44.0.683.g7961c838ac-goog
> > >
> > >
> > >
> > > 
> > >
> > >


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