Re: [PATCH] x86/mce: Initialize "bank" when we find a fatal error in mce_no_way_out()

From: Borislav Petkov
Date: Fri Feb 01 2019 - 04:56:06 EST

On Thu, Jan 31, 2019 at 04:33:41PM -0800, Tony Luck wrote:
> Internal injection testing crashed with a console log that said:
> mce: [Hardware Error]: CPU 7: Machine Check Exception: f Bank 0: bd80000000100134
> This caused a lot of head scratching because the MCACOD (bits 15:0) of that
> status is a signature from an L1 data cache error. But Linux says that it found
> it in "Bank 0", which on this model CPU only reports L1 instruction cache errors.
> The answer was that Linux doesn't initialize "m->bank" in the case that it finds
> a fatal error in the mce_no_way_out() pre-scan of banks. If this was a local machine
> check, then we pass this partially initialized "struct mce" to mce_panic().
> Fix is simple. Just initialize m->bank in the case that we found a fatal error.
> Fixes: 40c36e2741d7 ("x86/mce: Fix incorrect "Machine check from unknown source" message")
> Cc: stable@xxxxxxxxxxxxxxx # v4.18 Note pre-v5.0 arch/x86/kernel/cpu/mce/core.c was called arch/x86/kernel/cpu/mcheck/mce.c
> Signed-off-by: Tony Luck <tony.luck@xxxxxxxxx>
> ---
> arch/x86/kernel/cpu/mce/core.c | 1 +
> 1 file changed, 1 insertion(+)
> diff --git a/arch/x86/kernel/cpu/mce/core.c b/arch/x86/kernel/cpu/mce/core.c
> index 672c7225cb1b..6ce290c506d9 100644
> --- a/arch/x86/kernel/cpu/mce/core.c
> +++ b/arch/x86/kernel/cpu/mce/core.c
> @@ -784,6 +784,7 @@ static int mce_no_way_out(struct mce *m, char **msg, unsigned long *validp,
> quirk_no_way_out(i, m, regs);
> if (mce_severity(m, mca_cfg.tolerant, &tmp, true) >= MCE_PANIC_SEVERITY) {
> + m->bank = i;

So conceptually this write belongs in...

> mce_read_aux(m, i);

... this function, i.e., in mce_read_aux() because it gets the bank
number passed in already. And our calling pattern when populating struct
mce is:


so it'll be more robust if we moved it there.

Also, that argument "i" of mce_read_aux() is not very telling and it
should be "bank" but that would complicate the stable backporting so if
you feel like it, you could do a second, cleanup patch ontop to fix that



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