[PATCH v4 06/10] kvm: x86: Emulate split-lock access as a write

[Xiaoyao Li]

If split lock detect is enabled (warn/fatal), #AC handler calls die()
when split lock happens in kernel.

Malicous guest can exploit the KVM emulator to trigger split lock #AC
in kernel[1]. So just emulating the access as a write if it's a
split-lock access (the same as access spans page) to avoid malicious
attacking kernel.

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

[1] https://lore.kernel.org/lkml/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>
---
 arch/x86/kvm/x86.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 5de200663f51..1a0e6c0b1b39 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5873,6 +5873,7 @@ 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 = PAGE_MASK;
 	gpa_t gpa;
 	char *kaddr;
 	bool exchanged;
@@ -5887,7 +5888,11 @@ 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))
+	if (split_lock_detect_on())
+		page_line_mask = ~(cache_line_size() - 1);
+
+	/* when write spans page or spans cache when SLD enabled */
+	if (((gpa + bytes - 1) & page_line_mask) != (gpa & page_line_mask))
 		goto emul_write;
 
 	if (kvm_vcpu_map(vcpu, gpa_to_gfn(gpa), &map))
-- 
2.20.1