[PATCH v6 4/8] kvm: x86: Emulate split-lock access as a write in emulator

[Xiaoyao Li]

If split lock detect is on (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
guest 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/include/asm/cpu.h  | 2 ++
 arch/x86/kernel/cpu/intel.c | 6 ++++++
 arch/x86/kvm/x86.c          | 7 ++++++-
 3 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/cpu.h b/arch/x86/include/asm/cpu.h
index ff567afa6ee1..d2071f6a35ac 100644
--- a/arch/x86/include/asm/cpu.h
+++ b/arch/x86/include/asm/cpu.h
@@ -44,6 +44,7 @@ unsigned int x86_stepping(unsigned int sig);
 extern void __init cpu_set_core_cap_bits(struct cpuinfo_x86 *c);
 extern void switch_to_sld(unsigned long tifn);
 extern bool handle_user_split_lock(unsigned long ip);
+extern bool split_lock_detect_on(void);
 #else
 static inline void __init cpu_set_core_cap_bits(struct cpuinfo_x86 *c) {}
 static inline void switch_to_sld(unsigned long tifn) {}
@@ -51,5 +52,6 @@ static inline bool handle_user_split_lock(unsigned long ip)
 {
 	return false;
 }
+static inline bool split_lock_detect_on(void) { return false; }
 #endif
 #endif /* _ASM_X86_CPU_H */
diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index aed2b477e2ad..fd67be719284 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -1070,6 +1070,12 @@ static void split_lock_init(void)
 		sld_update_msr(sld_state != sld_off);
 }
 
+bool split_lock_detect_on(void)
+{
+	return sld_state != sld_off;
+}
+EXPORT_SYMBOL_GPL(split_lock_detect_on);
+
 bool handle_user_split_lock(unsigned long ip)
 {
 	if (sld_state == sld_fatal)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index ebd56aa10d9f..5ef57e3a315f 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5831,6 +5831,7 @@ 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;
@@ -5845,7 +5846,11 @@ 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