linux-kernel - [patch 2/3] KVM: x86: Emulate split-lock access as a write in emulator

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Date:   Fri, 10 Apr 2020 13:54:01 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     x86@...nel.org,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Xiaoyao Li <xiaoyao.li@...el.com>,
        "Kenneth R. Crudup" <kenny@...ix.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Nadav Amit <namit@...are.com>,
        Thomas Hellstrom <thellstrom@...are.com>,
        Tony Luck <tony.luck@...el.com>
Subject: [patch 2/3] KVM: x86: Emulate split-lock access as a write in emulator

From: Xiaoyao Li <xiaoyao.li@...el.com>

Emulate split-lock accesses as writes if split lock detection is on to
avoid #AC during emulation, which will result in a panic().  This should
never occur for a well behaved guest, but a malicious guest can
manipulate the TLB to trigger emulation of a locked instruction[1].

More discussion can be found [2][3].

[1] https://lkml.kernel.org/r/8c5b11c9-58df-38e7-a514-dc12d687b198@redhat.com
[2] https://lkml.kernel.org/r/20200131200134.GD18946@linux.intel.com
[3] https://lkml.kernel.org/r/20200227001117.GX9940@linux.intel.com

Suggested-by: Sean Christopherson <sean.j.christopherson@...el.com>
Signed-off-by: Xiaoyao Li <xiaoyao.li@...el.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>

---
 arch/x86/kvm/x86.c |   12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5838,6 +5838,7 @@ static int emulator_cmpxchg_emulated(str
 {
 	struct kvm_host_map map;
 	struct kvm_vcpu *vcpu = emul_to_vcpu(ctxt);
+	u64 page_line_mask;
 	gpa_t gpa;
 	char *kaddr;
 	bool exchanged;
@@ -5852,7 +5853,16 @@ static int emulator_cmpxchg_emulated(str
 	    (gpa & PAGE_MASK) == APIC_DEFAULT_PHYS_BASE)
 		goto emul_write;
 
-	if (((gpa + bytes - 1) & PAGE_MASK) != (gpa & PAGE_MASK))
+	/*
+	 * Emulate the atomic as a straight write to avoid #AC if SLD is
+	 * enabled in the host and the access splits a cache line.
+	 */
+	if (boot_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT))
+		page_line_mask = ~(cache_line_size() - 1);
+	else
+		page_line_mask = PAGE_MASK;
+
+	if (((gpa + bytes - 1) & page_line_mask) != (gpa & page_line_mask))
 		goto emul_write;
 
 	if (kvm_vcpu_map(vcpu, gpa_to_gfn(gpa), &map))