[tip: x86/urgent] KVM: x86: Emulate split-lock access as a write in emulator

[tip-bot2 for Xiaoyao Li]

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     9de6fe3c28d6d8feadfad907961f1f31b85c6985
Gitweb:        https://git.kernel.org/tip/9de6fe3c28d6d8feadfad907961f1f31b85c6985
Author:        Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
AuthorDate:    Fri, 10 Apr 2020 13:54:01 +02:00
Committer:     Borislav Petkov <bp@xxxxxxx>
CommitterDate: Sat, 11 Apr 2020 16:40:55 +02:00

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/8c5b11c9-58df-38e7-a514-dc12d687b198@xxxxxxxxxx
[2] https://lkml.kernel.org/r/20200131200134.GD18946@xxxxxxxxxxxxxxx
[3] https://lkml.kernel.org/r/20200227001117.GX9940@xxxxxxxxxxxxxxx

Suggested-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Signed-off-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Acked-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/20200410115517.084300242@xxxxxxxxxxxxx
---
 arch/x86/kvm/x86.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 027dfd2..3bf2eca 100644
--- 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))