linux-kernel - Re: [PATCH v2 1/2] KVM: X86: Fix operand size during instruction decoding

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Message-ID: <f8d11c1f-bd7f-2549-d2f6-a2e7b7a37e2b@cs.washington.edu>
Date:   Tue, 31 Oct 2017 19:35:24 -0700
From:   Pedro Fonseca <pfonseca@...washington.edu>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        Nadav Amit <nadav.amit@...il.com>
Subject: Re: [PATCH v2 1/2] KVM: X86: Fix operand size during instruction
 decoding

This patch fixes the problem and passes my tests on the CS.DB field, 
this includes the "push es" and the "high 16-bits of ESP" test cases.

Tested-by: Pedro Fonseca <pfonseca@...washington.edu>

On 10/27/17 1:36 AM, 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>
> 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>
> ---
> 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_REAL || mode == X86EMUL_MODE_VM86) {
> +			ctxt->ops->get_segment(ctxt, &dummy, &desc, NULL, VCPU_SREG_CS);
> +			if (desc.d)
> +				def_op_bytes = 4;
> +		}
>   		break;
>   	case X86EMUL_MODE_PROT32:
>   		def_op_bytes = def_ad_bytes = 4;