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