(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....@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) <b...@alien8.de> > Cc: Tom Lendacky <thomas.lenda...@amd.com> > Signed-off-by: Adam Dunlap <acdun...@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 > > > > > > -=-=-=-=-=-=-=-=-=-=-=- Groups.io Links: You receive all messages sent to this group. View/Reply Online (#117927): https://edk2.groups.io/g/devel/message/117927 Mute This Topic: https://groups.io/mt/105581633/21656 Mute #vc:https://edk2.groups.io/g/devel/mutehashtag/vc Group Owner: devel+ow...@edk2.groups.io Unsubscribe: https://edk2.groups.io/g/devel/unsub [arch...@mail-archive.com] -=-=-=-=-=-=-=-=-=-=-=-