Re: [PATCH v5 13/20] KVM: x86: Reject EVEX-prefixed instructions

["Chang S. Bae" <chang.seok.bae@intel.com>]

On 7/20/2026 11:11 AM, sashiko-bot@kernel.org wrote:
>>   
>> +	/* EVEX-prefixed instructions are not implemented */
>> +	if (ctxt->opcode_len == 1 && ctxt->b == 0x62 &&
>> +	    (mode == X86EMUL_MODE_PROT64 || (ctxt->modrm & 0xc0) == 0xc0))
>> +		opcode.flags = NotImpl;
> 
> [Severity: Low]
> Does this evaluate stale data in ctxt->modrm?
> 
> When a 32-bit guest executes an instruction starting with 0x62 in
> x86_decode_insn(), opcode_table[0x62] yields NotImpl, which lacks the ModRM
> flag. Because the ModRM flag is missing, insn_fetch() is bypassed.
> 
> Since the EVEX check doesn't short-circuit in 32-bit mode, it will evaluate
> whatever stale data remains in ctxt->modrm from a prior emulation on that
> context.
> 
> Because opcode.flags is already initialized to NotImpl by the table lookup,
> is this check entirely redundant?
Even if it is not be the most optimized form, I still think it keeps the 
logic straightforward.

Thank,
Chang