x86/mce: Fixup exception only for the correct MCEs

The severity grading code returns IN_KERNEL_RECOV error context for
errors which have happened in kernel space but from which the kernel can
recover. Whether the recovery can happen is determined by the exception
table entry having as handler ex_handler_fault() and which has been
declared at build time using _ASM_EXTABLE_FAULT().

IN_KERNEL_RECOV is used in mce_severity_intel() to lookup the
corresponding error severity in the severities table.

However, the mapping back from error severity to whether the error is
IN_KERNEL_RECOV is ambiguous and in the very paranoid case - which
might not be possible right now - but be better safe than sorry later,
an exception fixup could be attempted for another MCE whose address
is in the exception table and has the proper severity. Which would be
unfortunate, to say the least.

Therefore, mark such MCEs explicitly as MCE_IN_KERNEL_RECOV so that the
recovery attempt is done only for them.

Document the whole handling, while at it, as it is not trivial.
Reported-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Tested-by: Tony Luck <tony.luck@intel.com>
Link: https://lkml.kernel.org/r/20200407163414.18058-10-bp@alien8.de

arch/x86/include/asm/mce.h

@@ -136,6 +136,7 @@
 #define	MCE_HANDLED_NFIT	BIT_ULL(3)
 #define	MCE_HANDLED_EDAC	BIT_ULL(4)
 #define	MCE_HANDLED_MCELOG	BIT_ULL(5)
+#define MCE_IN_KERNEL_RECOV	BIT_ULL(6)
 
 /*
  * This structure contains all data related to the MCE log.  Also

arch/x86/kernel/cpu/mce/core.c

@@ -1331,8 +1331,19 @@ void notrace do_machine_check(struct pt_regs *regs, long error_code)
 		local_irq_disable();
 		ist_end_non_atomic();
 	} else {
-		if (!fixup_exception(regs, X86_TRAP_MC, error_code, 0))
-			mce_panic("Failed kernel mode recovery", &m, msg);
+		/*
+		 * Handle an MCE which has happened in kernel space but from
+		 * which the kernel can recover: ex_has_fault_handler() has
+		 * already verified that the rIP at which the error happened is
+		 * a rIP from which the kernel can recover (by jumping to
+		 * recovery code specified in _ASM_EXTABLE_FAULT()) and the
+		 * corresponding exception handler which would do that is the
+		 * proper one.
+		 */
+		if (m.kflags & MCE_IN_KERNEL_RECOV) {
+			if (!fixup_exception(regs, X86_TRAP_MC, error_code, 0))
+				mce_panic("Failed kernel mode recovery", &m, msg);
+		}
 	}
 
 out_ist:

arch/x86/kernel/cpu/mce/severity.c

@@ -213,8 +213,12 @@ static int error_context(struct mce *m)
 {
 	if ((m->cs & 3) == 3)
 		return IN_USER;
-	if (mc_recoverable(m->mcgstatus) && ex_has_fault_handler(m->ip))
+
+	if (mc_recoverable(m->mcgstatus) && ex_has_fault_handler(m->ip)) {
+		m->kflags |= MCE_IN_KERNEL_RECOV;
 		return IN_KERNEL_RECOV;
+	}
+
 	return IN_KERNEL;
 }