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

From: Thomas Gleixner
Date: Fri Apr 10 2020 - 07:56:17 EST


From: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>

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@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>

---
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))