linux-kernel - [PATCH AUTOSEL for 3.18 03/16] KVM: X86: Fix operand/address-size during instruction decoding

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Message-ID: <20180124041716.32520-3-alexander.levin@microsoft.com>
Date:   Wed, 24 Jan 2018 04:17:23 +0000
From:   Sasha Levin <Alexander.Levin@...rosoft.com>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
CC:     Wanpeng Li <wanpeng.li@...mail.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Pedro Fonseca <pfonseca@...washington.edu>,
        "Sasha Levin" <Alexander.Levin@...rosoft.com>
Subject: [PATCH AUTOSEL for 3.18 03/16] KVM: X86: Fix operand/address-size
 during instruction decoding

From: Wanpeng Li <wanpeng.li@...mail.com>

[ Upstream commit 3853be2603191829b442b64dac6ae8ba0c027bf9 ]

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/address-size/66H
prefix/67H prefix during instruction decoding. This patch fixes it by also
adjusting operand/address-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>
Reviewed-by: Paolo Bonzini <pbonzini@...hat.com>
Signed-off-by: Radim Krčmář <rkrcmar@...hat.com>
Signed-off-by: Sasha Levin <alexander.levin@...rosoft.com>
---
 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 852572c971c4..08751c554d35 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -4385,6 +4385,8 @@ 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;
@@ -4403,6 +4405,11 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len)
 	switch (mode) {
 	case X86EMUL_MODE_REAL:
 	case X86EMUL_MODE_VM86:
+		def_op_bytes = def_ad_bytes = 2;
+		ctxt->ops->get_segment(ctxt, &dummy, &desc, NULL, VCPU_SREG_CS);
+		if (desc.d)
+			def_op_bytes = def_ad_bytes = 4;
+		break;
 	case X86EMUL_MODE_PROT16:
 		def_op_bytes = def_ad_bytes = 2;
 		break;
-- 
2.11.0