[PATCH v10 5/6] x86/split_lock: Handle #AC exception for split lock

[Fenghua Yu]

Currently Linux does not expect to see an alignment check exception in
kernel mode (since it does not set CR4.AC). The existing #AC handlers
will just return from exception to the faulting instruction which will
trigger another exception.

Add a new handler for #AC exceptions that will force a panic on split
lock for kernel mode.

Signed-off-by: Fenghua Yu <fenghua.yu@xxxxxxxxx>
Reviewed-by: Tony Luck <tony.luck@xxxxxxxxx>
---
 arch/x86/include/asm/traps.h |  3 +++
 arch/x86/kernel/cpu/intel.c  |  2 ++
 arch/x86/kernel/traps.c      | 22 +++++++++++++++++++++-
 3 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/traps.h b/arch/x86/include/asm/traps.h
index b25e633033c3..0fa4eef83057 100644
--- a/arch/x86/include/asm/traps.h
+++ b/arch/x86/include/asm/traps.h
@@ -172,4 +172,7 @@ enum x86_pf_error_code {
 	X86_PF_INSTR	=		1 << 4,
 	X86_PF_PK	=		1 << 5,
 };
+
+extern bool split_lock_detect_enabled;
+
 #endif /* _ASM_X86_TRAPS_H */
diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index 2614616fb6d3..bc0c2f288509 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -32,6 +32,8 @@
 #include <asm/apic.h>
 #endif
 
+bool split_lock_detect_enabled;
+
 /*
  * Just in case our CPU detection goes bad, or you have a weird system,
  * allow a way to override the automatic disabling of MPX.
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 4bb0f8447112..044033ff4326 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -293,9 +293,29 @@ DO_ERROR(X86_TRAP_OLD_MF, SIGFPE,           0, NULL, "coprocessor segment overru
 DO_ERROR(X86_TRAP_TS,     SIGSEGV,          0, NULL, "invalid TSS",         invalid_TSS)
 DO_ERROR(X86_TRAP_NP,     SIGBUS,           0, NULL, "segment not present", segment_not_present)
 DO_ERROR(X86_TRAP_SS,     SIGBUS,           0, NULL, "stack segment",       stack_segment)
-DO_ERROR(X86_TRAP_AC,     SIGBUS,  BUS_ADRALN, NULL, "alignment check",     alignment_check)
 #undef IP
 
+dotraplinkage void do_alignment_check(struct pt_regs *regs, long error_code)
+{
+	unsigned int trapnr = X86_TRAP_AC;
+	char str[] = "alignment check";
+	int signr = SIGBUS;
+
+	RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
+
+	if (notify_die(DIE_TRAP, str, regs, error_code, trapnr, signr) == NOTIFY_STOP)
+		return;
+
+	if (!user_mode(regs) && split_lock_detect_enabled)
+		panic("Split lock detected\n");
+
+	cond_local_irq_enable(regs);
+
+	/* Handle #AC generated in any other cases. */
+	do_trap(X86_TRAP_AC, SIGBUS, "alignment check", regs,
+		error_code, BUS_ADRALN, NULL);
+}
+
 #ifdef CONFIG_VMAP_STACK
 __visible void __noreturn handle_stack_overflow(const char *message,
 						struct pt_regs *regs,
-- 
2.19.1