linux-kernel - Re: [PATCH v3 1/3] KVM: X86: Fix operand size during instruction decoding

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Date:   Thu, 2 Nov 2017 18:45:42 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Pedro Fonseca <pfonseca@...washington.edu>
Subject: Re: [PATCH v3 1/3] KVM: X86: Fix operand size during instruction
 decoding

On 02/11/2017 09:31, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@...mail.com>
> 
> Pedro reported:
>   During tests that we conducted on KVM, we noticed that executing a "PUSH %ES"
>   instruction under KVM produces different results on both memory and the SP
>   register depending on whether EPT support is enabled. With EPT the SP is
>   reduced by 4 bytes (and the written value is 0-padded) but without EPT support
>   it is only reduced by 2 bytes. The difference can be observed when the CS.DB
>   field is 1 (32-bit) but not when it's 0 (16-bit).
> 
> The internal segment descriptor cache exist even in real/vm8096 mode. The CS.D
> also should be respected instead of just default operand-size/66H prefix during
> instruction decoding. This patch fixes it by also adjusting operand-size according
> to CS.D.
> 
> Reported-by: Pedro Fonseca <pfonseca@...washington.edu>
> Tested-by: Pedro Fonseca <pfonseca@...washington.edu>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Cc: Nadav Amit <nadav.amit@...il.com>
> Cc: Pedro Fonseca <pfonseca@...washington.edu>
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> ---
> v2 -> v3:
>  * cleanup the codes 
> v1 -> v2:
>  * respect cs.d for real/vm8096, other modes have already 
>    been considered in init_emulate_ctxt().
> 
>  arch/x86/kvm/emulate.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 8079d14..6ebc4cb 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -5000,6 +5000,8 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len)
>  	bool op_prefix = false;
>  	bool has_seg_override = false;
>  	struct opcode opcode;
> +	u16 dummy;
> +	struct desc_struct desc;
>  
>  	ctxt->memop.type = OP_NONE;
>  	ctxt->memopp = NULL;
> @@ -5020,6 +5022,11 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len)
>  	case X86EMUL_MODE_VM86:
>  	case X86EMUL_MODE_PROT16:
>  		def_op_bytes = def_ad_bytes = 2;
> +		if (mode < X86EMUL_MODE_PROT16) {
> +			ctxt->ops->get_segment(ctxt, &dummy, &desc, NULL, VCPU_SREG_CS);
> +			if (desc.d)
> +				def_op_bytes = 4;

def_ad_bytes must be changed to 4 as well.  With that change, you should
probably separate X86EMUL_MODE_PROT16 altogether from the others.

Paolo

> +		}
>  		break;
>  	case X86EMUL_MODE_PROT32:
>  		def_op_bytes = def_ad_bytes = 4;
>