[PATCH v5 5/9] kvm: x86: Emulate split-lock access as a write

From: Xiaoyao Li <xiaoyao.li@intel.com>
To: Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	hpa@zytor.com, Paolo Bonzini <pbonzini@redhat.com>,
	Sean Christopherson <sean.j.christopherson@intel.com>,
	kvm@vger.kernel.org, x86@kernel.org,
	linux-kernel@vger.kernel.org
Cc: Andy Lutomirski <luto@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Arvind Sankar <nivedita@alum.mit.edu>,
	Fenghua Yu <fenghua.yu@intel.com>,
	Tony Luck <tony.luck@intel.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Jim Mattson <jmattson@google.com>,
	Xiaoyao Li <xiaoyao.li@intel.com>
Subject: [PATCH v5 5/9] kvm: x86: Emulate split-lock access as a write
Date: Sun, 15 Mar 2020 13:05:13 +0800	[thread overview]
Message-ID: <20200315050517.127446-6-xiaoyao.li@intel.com> (raw)
In-Reply-To: <20200315050517.127446-1-xiaoyao.li@intel.com>

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
attacking kernel.

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

[1] https://lore.kernel.org/lkml/8c5b11c9-58df-38e7-a514-dc12d687b198@redhat.com/
[2] https://lkml.kernel.org/r/20200131200134.GD18946@linux.intel.com
[3] https://lkml.kernel.org/r/20200227001117.GX9940@linux.intel.com

Suggested-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
---
 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 c401d174c8db..de94957a11a4 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -1102,6 +1102,12 @@ static void split_lock_init(struct cpuinfo_x86 *c)
 	sld_state = sld_disable;
 }
 
+bool split_lock_detect_on(void)
+{
+	return sld_state == sld_warn || sld_state == sld_fatal;
+}
+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 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

