[patch 0/3] x86/kvm: Basic split lock #AC handling

[patch 0/3] x86/kvm: Basic split lock #AC handling

[Thomas Gleixner]

This is a reworked version of the patches posted by Sean:

 https://lore.kernel.org/r/20200402155554.27705-1-sean.j.christopherson@intel.com

The changes vs. this are:

    1) Use a separate function for guest split lock handling

    2) Force SIGBUS when SLD mode fatal

    3) Rename the misnomed helper function which decides whether
       #AC is injected into the guest or not and move the feature
       check and the comments into that helper.

Thanks,

	tglx

[patch 1/3] x86/split_lock: Provide handle_guest_split_lock()

[Thomas Gleixner]

From: Thomas Gleixner <tglx@linutronix.de>

Without at least minimal handling for split lock detection induced #AC, VMX
will just run into the same problem as the VMWare hypervisor, which was
reported by Kenneth.

It will inject the #AC blindly into the guest whether the guest is prepared
or not.

Provide a function for guest mode which acts depending on the host SLD
mode.  If mode == sld_warn, treat it like user space, i.e. emit a warning
disable SLD and mark the task accordingly. Otherwise force SIGBUS.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: "Kenneth R. Crudup" <kenny@panix.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: Xiaoyao Li <xiaoyao.li@intel.com>
Cc: Nadav Amit <namit@vmware.com>
Cc: Thomas Hellstrom <thellstrom@vmware.com>
Cc: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: Tony Luck <tony.luck@intel.com>

---
 arch/x86/include/asm/cpu.h  |    1 +
 arch/x86/kernel/cpu/intel.c |   33 ++++++++++++++++++++++++++++-----
 2 files changed, 29 insertions(+), 5 deletions(-)

--- a/arch/x86/include/asm/cpu.h
+++ b/arch/x86/include/asm/cpu.h
@@ -44,6 +44,7 @@ unsigned int x86_stepping(unsigned int s
 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);
+extern bool handle_guest_split_lock(unsigned long ip);
 #else
 static inline void __init cpu_set_core_cap_bits(struct cpuinfo_x86 *c) {}
 static inline void switch_to_sld(unsigned long tifn) {}
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -21,6 +21,7 @@
 #include <asm/elf.h>
 #include <asm/cpu_device_id.h>
 #include <asm/cmdline.h>
+#include <asm/traps.h>
 
 #ifdef CONFIG_X86_64
 #include <linux/topology.h>
@@ -1066,13 +1067,10 @@ static void split_lock_init(void)
 	split_lock_verify_msr(sld_state != sld_off);
 }
 
-bool handle_user_split_lock(struct pt_regs *regs, long error_code)
+static void split_lock_warn(unsigned long ip)
 {
-	if ((regs->flags & X86_EFLAGS_AC) || sld_state == sld_fatal)
-		return false;
-
 	pr_warn_ratelimited("#AC: %s/%d took a split_lock trap at address: 0x%lx\n",
-			    current->comm, current->pid, regs->ip);
+			    current->comm, current->pid, ip);
 
 	/*
 	 * Disable the split lock detection for this task so it can make
@@ -1081,6 +1079,31 @@ bool handle_user_split_lock(struct pt_re
 	 */
 	sld_update_msr(false);
 	set_tsk_thread_flag(current, TIF_SLD);
+}
+
+bool handle_guest_split_lock(unsigned long ip)
+{
+	if (sld_state == sld_warn) {
+		split_lock_warn(ip);
+		return true;
+	}
+
+	pr_warn_once("#AC: %s/%d %s split_lock trap at address: 0x%lx\n",
+		     current->comm, current->pid,
+		     sld_state == sld_fatal ? "fatal" : "bogus", ip);
+
+	current->thread.error_code = 0;
+	current->thread.trap_nr = X86_TRAP_AC;
+	force_sig_fault(SIGBUS, BUS_ADRALN, NULL);
+	return false;
+}
+EXPORT_SYMBOL_GPL(handle_guest_split_lock);
+
+bool handle_user_split_lock(struct pt_regs *regs, long error_code)
+{
+	if ((regs->flags & X86_EFLAGS_AC) || sld_state == sld_fatal)
+		return false;
+	split_lock_warn(regs->ip);
 	return true;
 }
 

[patch 2/3] KVM: x86: Emulate split-lock access as a write in emulator

[Thomas Gleixner]

From: Xiaoyao Li <xiaoyao.li@intel.com>

Emulate split-lock accesses as writes if split lock detection is on to
avoid #AC during emulation, which will result in a panic().  This should
never occur for a well behaved guest, but a malicious guest can
manipulate the TLB to trigger emulation of a locked instruction[1].

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

[1] https://lkml.kernel.org/r/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>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

---
 arch/x86/kvm/x86.c |   12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5838,6 +5838,7 @@ static int emulator_cmpxchg_emulated(str
 {
 	struct kvm_host_map map;
 	struct kvm_vcpu *vcpu = emul_to_vcpu(ctxt);
+	u64 page_line_mask;
 	gpa_t gpa;
 	char *kaddr;
 	bool exchanged;
@@ -5852,7 +5853,16 @@ static int emulator_cmpxchg_emulated(str
 	    (gpa & PAGE_MASK) == APIC_DEFAULT_PHYS_BASE)
 		goto emul_write;
 
-	if (((gpa + bytes - 1) & PAGE_MASK) != (gpa & PAGE_MASK))
+	/*
+	 * Emulate the atomic as a straight write to avoid #AC if SLD is
+	 * enabled in the host and the access splits a cache line.
+	 */
+	if (boot_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT))
+		page_line_mask = ~(cache_line_size() - 1);
+	else
+		page_line_mask = PAGE_MASK;
+
+	if (((gpa + bytes - 1) & page_line_mask) != (gpa & page_line_mask))
 		goto emul_write;
 
 	if (kvm_vcpu_map(vcpu, gpa_to_gfn(gpa), &map))

[patch 3/3] KVM: VMX: Extend VMXs #AC interceptor to handle split lock #AC in guest

[Thomas Gleixner]

From: Xiaoyao Li <xiaoyao.li@intel.com>

Two types of #AC can be generated in Intel CPUs:
 1. legacy alignment check #AC
 2. split lock #AC

Reflect #AC back into the guest if the guest has legacy alignment checks
enabled or if split lock detection is disabled.

If the #AC is not a legacy one and split lock detection is enabled, then
invoke handle_guest_split_lock() which will either warn and disable split
lock detection for this task or force SIGBUS on it.

[ tglx: Switch it to handle_guest_split_lock() and renamed the misnomed
  	helper function ]

Suggested-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
 arch/x86/kvm/vmx/vmx.c |   37 ++++++++++++++++++++++++++++++++++---
 1 file changed, 34 insertions(+), 3 deletions(-)

--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -4592,6 +4592,26 @@ static int handle_machine_check(struct k
 	return 1;
 }
 
+/*
+ * If the host has split lock detection disabled, then #AC is
+ * unconditionally injected into the guest, which is the pre split lock
+ * detection behaviour.
+ *
+ * If the host has split lock detection enabled then #AC is
+ * only injected into the guest when:
+ *  - Guest CPL == 3 (user mode)
+ *  - Guest has #AC detection enabled in CR0
+ *  - Guest EFLAGS has AC bit set
+ */
+static inline bool guest_inject_ac(struct kvm_vcpu *vcpu)
+{
+	if (!boot_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT))
+		return true;
+
+	return vmx_get_cpl(vcpu) == 3 && kvm_read_cr0_bits(vcpu, X86_CR0_AM) &&
+	       (kvm_get_rflags(vcpu) & X86_EFLAGS_AC);
+}
+
 static int handle_exception_nmi(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
@@ -4657,9 +4677,6 @@ static int handle_exception_nmi(struct k
 		return handle_rmode_exception(vcpu, ex_no, error_code);
 
 	switch (ex_no) {
-	case AC_VECTOR:
-		kvm_queue_exception_e(vcpu, AC_VECTOR, error_code);
-		return 1;
 	case DB_VECTOR:
 		dr6 = vmcs_readl(EXIT_QUALIFICATION);
 		if (!(vcpu->guest_debug &
@@ -4688,6 +4705,20 @@ static int handle_exception_nmi(struct k
 		kvm_run->debug.arch.pc = vmcs_readl(GUEST_CS_BASE) + rip;
 		kvm_run->debug.arch.exception = ex_no;
 		break;
+	case AC_VECTOR:
+		if (guest_inject_ac(vcpu)) {
+			kvm_queue_exception_e(vcpu, AC_VECTOR, error_code);
+			return 1;
+		}
+
+		/*
+		 * Handle split lock. Depending on detection mode this will
+		 * either warn and disable split lock detection for this
+		 * task or force SIGBUS on it.
+		 */
+		if (handle_guest_split_lock(kvm_rip_read(vcpu)))
+			return 1;
+		fallthrough;
 	default:
 		kvm_run->exit_reason = KVM_EXIT_EXCEPTION;
 		kvm_run->ex.exception = ex_no;

Re: [patch 0/3] x86/kvm: Basic split lock #AC handling

[Paolo Bonzini]

On 10/04/20 13:53, Thomas Gleixner wrote:
> This is a reworked version of the patches posted by Sean:
> 
>  https://lore.kernel.org/r/20200402155554.27705-1-sean.j.christopherson@intel.com
> 
> The changes vs. this are:
> 
>     1) Use a separate function for guest split lock handling
> 
>     2) Force SIGBUS when SLD mode fatal
> 
>     3) Rename the misnomed helper function which decides whether
>        #AC is injected into the guest or not and move the feature
>        check and the comments into that helper.
> 
> Thanks,
> 
> 	tglx

Acked-by: Paolo Bonzini <pbonzini@redhat.com>

Or just tell me if you want me to send it Linus's way.

Paolo

Re: [patch 0/3] x86/kvm: Basic split lock #AC handling

[Sean Christopherson]

On Fri, Apr 10, 2020 at 01:53:59PM +0200, Thomas Gleixner wrote:
> This is a reworked version of the patches posted by Sean:
> 
>  https://lore.kernel.org/r/20200402155554.27705-1-sean.j.christopherson@intel.com
> 
> The changes vs. this are:
> 
>     1) Use a separate function for guest split lock handling
> 
>     2) Force SIGBUS when SLD mode fatal

Not that it matters as the code is correct, but I think you meant
"when SLD mode off" here.

>     3) Rename the misnomed helper function which decides whether
>        #AC is injected into the guest or not and move the feature
>        check and the comments into that helper.

Thanks a bunch for helping push this along!

[tip: x86/urgent] KVM: VMX: Extend VMXs #AC interceptor to handle split lock #AC in guest

[tip-bot2 for Xiaoyao Li]

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     e6f8b6c12f03818baacc5f504fe83fa5e20771d6
Gitweb:        https://git.kernel.org/tip/e6f8b6c12f03818baacc5f504fe83fa5e20771d6
Author:        Xiaoyao Li <xiaoyao.li@intel.com>
AuthorDate:    Fri, 10 Apr 2020 13:54:02 +02:00
Committer:     Borislav Petkov <bp@suse.de>
CommitterDate: Sat, 11 Apr 2020 16:42:41 +02:00

KVM: VMX: Extend VMXs #AC interceptor to handle split lock #AC in guest

Two types of #AC can be generated in Intel CPUs:
 1. legacy alignment check #AC
 2. split lock #AC

Reflect #AC back into the guest if the guest has legacy alignment checks
enabled or if split lock detection is disabled.

If the #AC is not a legacy one and split lock detection is enabled, then
invoke handle_guest_split_lock() which will either warn and disable split
lock detection for this task or force SIGBUS on it.

[ tglx: Switch it to handle_guest_split_lock() and rename the misnamed
  helper function. ]

Suggested-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Link: https://lkml.kernel.org/r/20200410115517.176308876@linutronix.de
---
 arch/x86/kvm/vmx/vmx.c | 37 ++++++++++++++++++++++++++++++++++---
 1 file changed, 34 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 8959514..8305097 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -4588,6 +4588,26 @@ static int handle_machine_check(struct kvm_vcpu *vcpu)
 	return 1;
 }
 
+/*
+ * If the host has split lock detection disabled, then #AC is
+ * unconditionally injected into the guest, which is the pre split lock
+ * detection behaviour.
+ *
+ * If the host has split lock detection enabled then #AC is
+ * only injected into the guest when:
+ *  - Guest CPL == 3 (user mode)
+ *  - Guest has #AC detection enabled in CR0
+ *  - Guest EFLAGS has AC bit set
+ */
+static inline bool guest_inject_ac(struct kvm_vcpu *vcpu)
+{
+	if (!boot_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT))
+		return true;
+
+	return vmx_get_cpl(vcpu) == 3 && kvm_read_cr0_bits(vcpu, X86_CR0_AM) &&
+	       (kvm_get_rflags(vcpu) & X86_EFLAGS_AC);
+}
+
 static int handle_exception_nmi(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
@@ -4653,9 +4673,6 @@ static int handle_exception_nmi(struct kvm_vcpu *vcpu)
 		return handle_rmode_exception(vcpu, ex_no, error_code);
 
 	switch (ex_no) {
-	case AC_VECTOR:
-		kvm_queue_exception_e(vcpu, AC_VECTOR, error_code);
-		return 1;
 	case DB_VECTOR:
 		dr6 = vmcs_readl(EXIT_QUALIFICATION);
 		if (!(vcpu->guest_debug &
@@ -4684,6 +4701,20 @@ static int handle_exception_nmi(struct kvm_vcpu *vcpu)
 		kvm_run->debug.arch.pc = vmcs_readl(GUEST_CS_BASE) + rip;
 		kvm_run->debug.arch.exception = ex_no;
 		break;
+	case AC_VECTOR:
+		if (guest_inject_ac(vcpu)) {
+			kvm_queue_exception_e(vcpu, AC_VECTOR, error_code);
+			return 1;
+		}
+
+		/*
+		 * Handle split lock. Depending on detection mode this will
+		 * either warn and disable split lock detection for this
+		 * task or force SIGBUS on it.
+		 */
+		if (handle_guest_split_lock(kvm_rip_read(vcpu)))
+			return 1;
+		fallthrough;
 	default:
 		kvm_run->exit_reason = KVM_EXIT_EXCEPTION;
 		kvm_run->ex.exception = ex_no;

[tip: x86/urgent] x86/split_lock: Provide handle_guest_split_lock()

[tip-bot2 for Thomas Gleixner]

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     d7e94dbdac1a40924626b0efc7ff530c8baf5e4a
Gitweb:        https://git.kernel.org/tip/d7e94dbdac1a40924626b0efc7ff530c8baf5e4a
Author:        Thomas Gleixner <tglx@linutronix.de>
AuthorDate:    Fri, 10 Apr 2020 13:54:00 +02:00
Committer:     Borislav Petkov <bp@suse.de>
CommitterDate: Sat, 11 Apr 2020 16:39:30 +02:00

x86/split_lock: Provide handle_guest_split_lock()

Without at least minimal handling for split lock detection induced #AC,
VMX will just run into the same problem as the VMWare hypervisor, which
was reported by Kenneth.

It will inject the #AC blindly into the guest whether the guest is
prepared or not.

Provide a function for guest mode which acts depending on the host
SLD mode. If mode == sld_warn, treat it like user space, i.e. emit a
warning, disable SLD and mark the task accordingly. Otherwise force
SIGBUS.

 [ bp: Add a !CPU_SUP_INTEL stub for handle_guest_split_lock(). ]

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Link: https://lkml.kernel.org/r/20200410115516.978037132@linutronix.de
Link: https://lkml.kernel.org/r/20200402123258.895628824@linutronix.de
---
 arch/x86/include/asm/cpu.h  |  6 ++++++
 arch/x86/kernel/cpu/intel.c | 33 ++++++++++++++++++++++++++++-----
 2 files changed, 34 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/cpu.h b/arch/x86/include/asm/cpu.h
index ff6f3ca..dd17c2d 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(struct pt_regs *regs, long error_code);
+extern bool handle_guest_split_lock(unsigned long ip);
 #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 +52,10 @@ static inline bool handle_user_split_lock(struct pt_regs *regs, long error_code)
 {
 	return false;
 }
+
+static inline bool handle_guest_split_lock(unsigned long ip)
+{
+	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 9a26e97..bf08d45 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -21,6 +21,7 @@
 #include <asm/elf.h>
 #include <asm/cpu_device_id.h>
 #include <asm/cmdline.h>
+#include <asm/traps.h>
 
 #ifdef CONFIG_X86_64
 #include <linux/topology.h>
@@ -1066,13 +1067,10 @@ static void split_lock_init(void)
 	split_lock_verify_msr(sld_state != sld_off);
 }
 
-bool handle_user_split_lock(struct pt_regs *regs, long error_code)
+static void split_lock_warn(unsigned long ip)
 {
-	if ((regs->flags & X86_EFLAGS_AC) || sld_state == sld_fatal)
-		return false;
-
 	pr_warn_ratelimited("#AC: %s/%d took a split_lock trap at address: 0x%lx\n",
-			    current->comm, current->pid, regs->ip);
+			    current->comm, current->pid, ip);
 
 	/*
 	 * Disable the split lock detection for this task so it can make
@@ -1081,6 +1079,31 @@ bool handle_user_split_lock(struct pt_regs *regs, long error_code)
 	 */
 	sld_update_msr(false);
 	set_tsk_thread_flag(current, TIF_SLD);
+}
+
+bool handle_guest_split_lock(unsigned long ip)
+{
+	if (sld_state == sld_warn) {
+		split_lock_warn(ip);
+		return true;
+	}
+
+	pr_warn_once("#AC: %s/%d %s split_lock trap at address: 0x%lx\n",
+		     current->comm, current->pid,
+		     sld_state == sld_fatal ? "fatal" : "bogus", ip);
+
+	current->thread.error_code = 0;
+	current->thread.trap_nr = X86_TRAP_AC;
+	force_sig_fault(SIGBUS, BUS_ADRALN, NULL);
+	return false;
+}
+EXPORT_SYMBOL_GPL(handle_guest_split_lock);
+
+bool handle_user_split_lock(struct pt_regs *regs, long error_code)
+{
+	if ((regs->flags & X86_EFLAGS_AC) || sld_state == sld_fatal)
+		return false;
+	split_lock_warn(regs->ip);
 	return true;
 }
 

[tip: x86/urgent] KVM: x86: Emulate split-lock access as a write in emulator

[tip-bot2 for Xiaoyao Li]

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     9de6fe3c28d6d8feadfad907961f1f31b85c6985
Gitweb:        https://git.kernel.org/tip/9de6fe3c28d6d8feadfad907961f1f31b85c6985
Author:        Xiaoyao Li <xiaoyao.li@intel.com>
AuthorDate:    Fri, 10 Apr 2020 13:54:01 +02:00
Committer:     Borislav Petkov <bp@suse.de>
CommitterDate: Sat, 11 Apr 2020 16:40:55 +02:00

KVM: x86: Emulate split-lock access as a write in emulator

Emulate split-lock accesses as writes if split lock detection is on
to avoid #AC during emulation, which will result in a panic(). This
should never occur for a well-behaved guest, but a malicious guest can
manipulate the TLB to trigger emulation of a locked instruction[1].

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

[1] https://lkml.kernel.org/r/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>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Link: https://lkml.kernel.org/r/20200410115517.084300242@linutronix.de
---
 arch/x86/kvm/x86.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 027dfd2..3bf2eca 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5839,6 +5839,7 @@ 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;
 	gpa_t gpa;
 	char *kaddr;
 	bool exchanged;
@@ -5853,7 +5854,16 @@ 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))
+	/*
+	 * Emulate the atomic as a straight write to avoid #AC if SLD is
+	 * enabled in the host and the access splits a cache line.
+	 */
+	if (boot_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT))
+		page_line_mask = ~(cache_line_size() - 1);
+	else
+		page_line_mask = PAGE_MASK;
+
+	if (((gpa + bytes - 1) & page_line_mask) != (gpa & page_line_mask))
 		goto emul_write;
 
 	if (kvm_vcpu_map(vcpu, gpa_to_gfn(gpa), &map))

Re: [patch 0/3] x86/kvm: Basic split lock #AC handling

[Thomas Gleixner]

Sean,

Sean Christopherson <sean.j.christopherson@intel.com> writes:
> On Fri, Apr 10, 2020 at 01:53:59PM +0200, Thomas Gleixner wrote:
>> This is a reworked version of the patches posted by Sean:
>> 
>>  https://lore.kernel.org/r/20200402155554.27705-1-sean.j.christopherson@intel.com
>> 
>> The changes vs. this are:
>> 
>>     1) Use a separate function for guest split lock handling
>> 
>>     2) Force SIGBUS when SLD mode fatal
>
> Not that it matters as the code is correct, but I think you meant
> "when SLD mode off" here.

Actually for both fatal and off. The latter should never happen :)

Thanks,

        tglx