linux-kernel - Re: [PATCH v9 27/27] KVM: x86: Stop emulating for CET protected branch instructions

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Date: Fri, 26 Jan 2024 20:56:10 +0800
From: "Yang, Weijiang" <weijiang.yang@...el.com>
To: Chao Gao <chao.gao@...el.com>
CC: <seanjc@...gle.com>, <pbonzini@...hat.com>, <dave.hansen@...el.com>,
	<kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>, <x86@...nel.org>,
	<yuan.yao@...ux.intel.com>, <peterz@...radead.org>,
	<rick.p.edgecombe@...el.com>, <mlevitsk@...hat.com>, <john.allen@....com>
Subject: Re: [PATCH v9 27/27] KVM: x86: Stop emulating for CET protected
 branch instructions

On 1/26/2024 4:53 PM, Chao Gao wrote:
> On Tue, Jan 23, 2024 at 06:42:00PM -0800, Yang Weijiang wrote:
>> Don't emulate the branch instructions, e.g., CALL/RET/JMP etc., when CET
>> is active in guest, return KVM_INTERNAL_ERROR_EMULATION to userspace to
>> handle it.
>>
>> KVM doesn't emulate CPU behaviors to check CET protected stuffs while
>> emulating guest instructions, instead it stops emulation on detecting
>> the instructions in process are CET protected. By doing so, it can avoid
>> generating bogus #CP in guest and preventing CET protected execution flow
>> subversion from guest side.
>>
>> Suggested-by: Chao Gao <chao.gao@...el.com>
>> Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
>> ---
>> arch/x86/kvm/emulate.c | 27 ++++++++++++++++-----------
>> 1 file changed, 16 insertions(+), 11 deletions(-)
>>
>> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
>> index e223043ef5b2..ad15ce055a1d 100644
>> --- a/arch/x86/kvm/emulate.c
>> +++ b/arch/x86/kvm/emulate.c
>> @@ -178,6 +178,7 @@
>> #define IncSP       ((u64)1 << 54)  /* SP is incremented before ModRM calc */
>> #define TwoMemOp    ((u64)1 << 55)  /* Instruction has two memory operand */
>> #define IsBranch    ((u64)1 << 56)  /* Instruction is considered a branch. */
>> +#define IsProtected ((u64)1 << 57)  /* Instruction is protected by CET. */
> the name IsProtected doesn't seem clear to me. Its meaning isn't obvious from
> the name and may be confused with protected mode. Maybe we can add two flags:
> "IndirectBranch" and "ShadowStack".

Hmm, maybe it's worth to distinguish specific instruction protection type against current CET
enabling status. Let me double check.

>> #define DstXacc     (DstAccLo | SrcAccHi | SrcWrite)
>>
>> @@ -4098,9 +4099,9 @@ static const struct opcode group4[] = {
>> static const struct opcode group5[] = {
>> 	F(DstMem | SrcNone | Lock,		em_inc),
>> 	F(DstMem | SrcNone | Lock,		em_dec),
>> -	I(SrcMem | NearBranch | IsBranch,       em_call_near_abs),
>> -	I(SrcMemFAddr | ImplicitOps | IsBranch, em_call_far),
>> -	I(SrcMem | NearBranch | IsBranch,       em_jmp_abs),
>> +	I(SrcMem | NearBranch | IsBranch | IsProtected, em_call_near_abs),
>> +	I(SrcMemFAddr | ImplicitOps | IsBranch | IsProtected, em_call_far),
>> +	I(SrcMem | NearBranch | IsBranch | IsProtected, em_jmp_abs),
> In SDM, I don't see a list of instructions that are affected by CET. how do you
> get the list.

In SDM Vol. 1/17.2 and 17.3, and Vol.2 instruction references on branch instructions.