linux-kernel - [PATCH] KVM: x86: Assert that the emulator doesn't load CS with garbage in !RM

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Message-ID: <20230216202254.671772-1-seanjc@google.com>
Date:   Thu, 16 Feb 2023 12:22:54 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Hou Wenlong <houwenlong.hwl@...group.com>
Subject: [PATCH] KVM: x86: Assert that the emulator doesn't load CS with
 garbage in !RM

Yell loudly if KVM attempts to load CS outside of Real Mode without an
accompanying control transfer type, i.e. on X86_TRANSFER_NONE.  KVM uses
X86_TRANSFER_NONE when emulating IRET and exceptions/interrupts for Real
Mode, but IRET emulation for Protected Mode is non-existent.  WARN instead
of trying to pass in a less-wrong type, e.g. X86_TRANSFER_RET, as
emulating IRET goes even beyond emulating FAR RET (which KVM also doesn't
fully support).

Reported-by: Hou Wenlong <houwenlong.hwl@...group.com>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/emulate.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index baf97c56aefa..07e5f473f6fe 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -1640,6 +1640,14 @@ static int __load_segment_descriptor(struct x86_emulate_ctxt *ctxt,
 			goto exception;
 		break;
 	case VCPU_SREG_CS:
+		/*
+		 * KVM uses "none" when loading CS as part of emulating Real
+		 * Mode exceptions and IRET (handled above).  In all other
+		 * cases, loading CS without a control transfer is a KVM bug.
+		 */
+		if (WARN_ON_ONCE(transfer == X86_TRANSFER_NONE))
+			goto exception;
+
 		if (!(seg_desc.type & 8))
 			goto exception;
 

base-commit: 62ef199250cd46fb66fe98267137b7f64e0b41b4
-- 
2.39.2.637.g21b0678d19-goog