linux-kernel - [PATCH] x86/mce: Do not overwrite MCi_STATUS in mce_no_way

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Date:   Thu, 21 Jun 2018 14:40:00 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Tony Luck <tony.luck@...el.com>
Cc:     LKML <linux-kernel@...r.kernel.org>
Subject: [PATCH] x86/mce: Do not overwrite MCi_STATUS in mce_no_way_out()

From: Borislav Petkov <bp@...e.de>

mce_no_way_out() does a quick check during #MC to see whether some of
the MCEs logged would require the kernel to panic immediately. And it
passes a struct mce where MCi_STATUS gets written.

However, after having saved a valid status value, the next iteration
of the loop which goes over the MCA banks on the CPU, overwrites the
valid status value because we're using struct mce as storage instead of
a temporary variable.

Which leads to MCE records with an empty status value:

  mce: [Hardware Error]: CPU 0: Machine Check Exception: 6 Bank 0: 0000000000000000
  mce: [Hardware Error]: RIP 10:<ffffffffbd42fbd7> {trigger_mce+0x7/0x10}

Change it to use a temporary variable and save to mce.status only when
valid. It will still overwrite the old status value if more than one
MCEs are logged, which warrants the second change in this patch:

Return immediately when severity is a panic one so that we can panic
immediately with the first fatal MCE logged.

Cc: <stable@...r.kernel.org>
Signed-off-by: Borislav Petkov <bp@...e.de>
---
 arch/x86/kernel/cpu/mcheck/mce.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/cpu/mcheck/mce.c b/arch/x86/kernel/cpu/mcheck/mce.c
index 5c38d1f861f2..5c0a97b2bfce 100644
--- a/arch/x86/kernel/cpu/mcheck/mce.c
+++ b/arch/x86/kernel/cpu/mcheck/mce.c
@@ -772,12 +772,15 @@ EXPORT_SYMBOL_GPL(machine_check_poll);
 static int mce_no_way_out(struct mce *m, char **msg, unsigned long *validp,
 			  struct pt_regs *regs)
 {
+	u64 mci_status;
 	int i, ret = 0;
 	char *tmp;

 	for (i = 0; i < mca_cfg.banks; i++) {
-		m->status = mce_rdmsrl(msr_ops.status(i));
-		if (m->status & MCI_STATUS_VAL) {
+		mci_status = mce_rdmsrl(msr_ops.status(i));
+		if (mci_status & MCI_STATUS_VAL) {
+			m->status = mci_status;
+
 			__set_bit(i, validp);
 			if (quirk_no_way_out)
 				quirk_no_way_out(i, m, regs);
@@ -785,7 +788,7 @@ static int mce_no_way_out(struct mce *m, char **msg, unsigned long *validp,

 		if (mce_severity(m, mca_cfg.tolerant, &tmp, true) >= MCE_PANIC_SEVERITY) {
 			*msg = tmp;
-			ret = 1;
+			return 1;
 		}
 	}
 	return ret;
-- 
2.17.0.582.gccdcbd54c