linux-kernel - [PATCH 1/2] KVM: x86/emulator: Defer not-present segment check in __load_segment

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Message-Id: <117283244eab58e94d589af58a5f2b245b8c0025.1642669684.git.houwenlong.hwl@antgroup.com>
Date:   Thu, 20 Jan 2022 17:33:29 +0800
From:   "Hou Wenlong" <houwenlong.hwl@...group.com>
To:     kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Gleb Natapov <gleb@...hat.com>, linux-kernel@...r.kernel.org
Subject: [PATCH 1/2] KVM: x86/emulator: Defer not-present segment check in __load_segment_descriptor()

Per Intel's SDM on the "Instruction Set Reference", when
loading segment descriptor, not-present segment check should
be after all type and privilege checks. But the emulator checks
it first, then #NP is triggered instead of #GP if privilege fails
and segment is not present. Put not-present segment check after
type and privilege checks in __load_segment_descriptor().

Fixes: 38ba30ba51a00 (KVM: x86 emulator: Emulate task switch in emulator.c)
Signed-off-by: Hou Wenlong <houwenlong.hwl@...group.com>
---
 arch/x86/kvm/emulate.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 166a145fc1e6..864db6fbe8db 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -1616,11 +1616,6 @@ static int __load_segment_descriptor(struct x86_emulate_ctxt *ctxt,
 		goto exception;
 	}

-	if (!seg_desc.p) {
-		err_vec = (seg == VCPU_SREG_SS) ? SS_VECTOR : NP_VECTOR;
-		goto exception;
-	}
-
 	dpl = seg_desc.dpl;

 	switch (seg) {
@@ -1660,6 +1655,10 @@ static int __load_segment_descriptor(struct x86_emulate_ctxt *ctxt,
 	case VCPU_SREG_TR:
 		if (seg_desc.s || (seg_desc.type != 1 && seg_desc.type != 9))
 			goto exception;
+		if (!seg_desc.p) {
+			err_vec = NP_VECTOR;
+			goto exception;
+		}
 		old_desc = seg_desc;
 		seg_desc.type |= 2; /* busy */
 		ret = ctxt->ops->cmpxchg_emulated(ctxt, desc_addr, &old_desc, &seg_desc,
@@ -1684,6 +1683,11 @@ static int __load_segment_descriptor(struct x86_emulate_ctxt *ctxt,
 		break;
 	}

+	if (!seg_desc.p) {
+		err_vec = (seg == VCPU_SREG_SS) ? SS_VECTOR : NP_VECTOR;
+		goto exception;
+	}
+
 	if (seg_desc.s) {
 		/* mark segment as accessed */
 		if (!(seg_desc.type & 1)) {
--
2.31.1