[PATCH 1/2] 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.

A sane guest should never tigger emulation on a split-lock access, but
it cannot prevent malicous guest from doing this. So just emulating the
access as a write if it's a split-lock access to avoid malicous guest
polluting the kernel log.

Suggested-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Signed-off-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
---
 arch/x86/include/asm/cpu.h  | 12 ++++++++++++
 arch/x86/kernel/cpu/intel.c | 12 ++++++------
 arch/x86/kvm/x86.c          | 11 +++++++++++
 3 files changed, 29 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/cpu.h b/arch/x86/include/asm/cpu.h
index ff6f3ca649b3..167d0539e0ad 100644
--- a/arch/x86/include/asm/cpu.h
+++ b/arch/x86/include/asm/cpu.h
@@ -40,11 +40,23 @@ int mwait_usable(const struct cpuinfo_x86 *);
 unsigned int x86_family(unsigned int sig);
 unsigned int x86_model(unsigned int sig);
 unsigned int x86_stepping(unsigned int sig);
+
+enum split_lock_detect_state {
+	sld_off = 0,
+	sld_warn,
+	sld_fatal,
+};
+
 #ifdef CONFIG_CPU_SUP_INTEL
+extern enum split_lock_detect_state get_split_lock_detect_state(void);
 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(struct pt_regs *regs, long error_code);
 #else
+static inline enum split_lock_detect_state get_split_lock_detect_state(void)
+{
+	return sld_off;
+}
 static inline void __init cpu_set_core_cap_bits(struct cpuinfo_x86 *c) {}
 static inline void switch_to_sld(unsigned long tifn) {}
 static inline bool handle_user_split_lock(struct pt_regs *regs, long error_code)
diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index 5d92e381fd91..2f9c48e91caf 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -33,12 +33,6 @@
 #include <asm/apic.h>
 #endif
 
-enum split_lock_detect_state {
-	sld_off = 0,
-	sld_warn,
-	sld_fatal,
-};
-
 /*
  * Default to sld_off because most systems do not support split lock detection
  * split_lock_setup() will switch this to sld_warn on systems that support
@@ -1004,6 +998,12 @@ cpu_dev_register(intel_cpu_dev);
 #undef pr_fmt
 #define pr_fmt(fmt) "x86/split lock detection: " fmt
 
+enum split_lock_detect_state get_split_lock_detect_state(void)
+{
+	return sld_state;
+}
+EXPORT_SYMBOL_GPL(get_split_lock_detect_state);
+
 static const struct {
 	const char			*option;
 	enum split_lock_detect_state	state;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index e6d4e4dcd11c..7d9303c303d9 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5800,6 +5800,13 @@ static int emulator_write_emulated(struct x86_emulate_ctxt *ctxt,
 	(cmpxchg64((u64 *)(ptr), *(u64 *)(old), *(u64 *)(new)) == *(u64 *)(old))
 #endif
 
+static inline bool is_split_lock_access(gpa_t gpa, unsigned int bytes)
+{
+	unsigned int cache_line_size = cache_line_size();
+
+	return (gpa & (cache_line_size - 1)) + bytes > cache_line_size;
+}
+
 static int emulator_cmpxchg_emulated(struct x86_emulate_ctxt *ctxt,
 				     unsigned long addr,
 				     const void *old,
@@ -5826,6 +5833,10 @@ static int emulator_cmpxchg_emulated(struct x86_emulate_ctxt *ctxt,
 	if (((gpa + bytes - 1) & PAGE_MASK) != (gpa & PAGE_MASK))
 		goto emul_write;
 
+	if (get_split_lock_detect_state() != sld_off &&
+	    is_split_lock_access(gpa, bytes))
+		goto emul_write;
+
 	if (kvm_vcpu_map(vcpu, gpa_to_gfn(gpa), &map))
 		goto emul_write;
 
-- 
2.23.0