KVM: x86: Emulate split-lock access as a write in emulator

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 at [2][3]. [1] https://lkml.kernel.org/r/[email protected] [2] https://lkml.kernel.org/r/[email protected] [3] https://lkml.kernel.org/r/[email protected] Suggested-by: Sean Christopherson <[email protected]> Signed-off-by: Xiaoyao Li <[email protected]> Signed-off-by: Sean Christopherson <[email protected]> Signed-off-by: Thomas Gleixner <[email protected]> Signed-off-by: Borislav Petkov <[email protected]> Acked-by: Paolo Bonzini <[email protected]> Link: https://lkml.kernel.org/r/[email protected]
mrcsosa · Apr 11, 2020 · 9de6fe3 · 9de6fe3
1 parent d7e94db
commit 9de6fe3
Showing 1 changed file with 11 additions and 1 deletion.
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
@@ -5839,6 +5839,7 @@ static int emulator_cmpxchg_emulated(struct x86_emulate_ctxt *ctxt,
 {
 	struct kvm_host_map map;
 	struct kvm_vcpu *vcpu = emul_to_vcpu(ctxt);
+	u64 page_line_mask;
 	gpa_t gpa;
 	char *kaddr;
 	bool exchanged;
@@ -5853,7 +5854,16 @@ static int emulator_cmpxchg_emulated(struct x86_emulate_ctxt *ctxt,
 	    (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))