x86/mce: Fix MCE severity messages

commit 17fea54b upstream.

Derek noticed that a critical MCE gets reported with the wrong
error type description:

  [Hardware Error]: CPU 34: Machine Check Exception: 5 Bank 9: f200003f000100b0
  [Hardware Error]: RIP !INEXACT! 10:<ffffffff812e14c1> {intel_idle+0xb1/0x170}
  [Hardware Error]: TSC 49587b8e321cb
  [Hardware Error]: PROCESSOR 0:306e4 TIME 1431561296 SOCKET 1 APIC 29
  [Hardware Error]: Some CPUs didn't answer in synchronization
  [Hardware Error]: Machine check: Invalid
				   ^^^^^^^

The last line with 'Invalid' should have printed the high level
MCE error type description we get from mce_severity, i.e.
something like:

  [Hardware Error]: Machine check: Action required: data load error in a user process

this happens due to the fact that mce_no_way_out() iterates over
all MCA banks and possibly overwrites the @msg argument which is
used in the panic printing later.

Change behavior to take the message of only and the (last)
critical MCE it detects.
Reported-by: Derek <denc716@gmail.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tony Luck <tony.luck@intel.com>
Link: http://lkml.kernel.org/r/1431936437-25286-3-git-send-email-bp@alien8.deSigned-off-by: Ingo Molnar <mingo@kernel.org>
[ kamal: backport to 3.13-stable: context ]
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

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

@@ -660,6 +660,7 @@ static int mce_no_way_out(struct mce *m, char **msg, unsigned long *validp,
 			  struct pt_regs *regs)
 {
 	int i, ret = 0;
+	char *tmp;
 
 	for (i = 0; i < mca_cfg.banks; i++) {
 		m->status = mce_rdmsrl(MSR_IA32_MCx_STATUS(i));
@@ -668,9 +669,12 @@ static int mce_no_way_out(struct mce *m, char **msg, unsigned long *validp,
 			if (quirk_no_way_out)
 				quirk_no_way_out(i, m, regs);
 		}
-		if (mce_severity(m, mca_cfg.tolerant, msg) >= MCE_PANIC_SEVERITY)
+
+		if (mce_severity(m, mca_cfg.tolerant, &tmp) >= MCE_PANIC_SEVERITY) {
+			*msg = tmp;
 			ret = 1;
 		}
+	}
 	return ret;
 }