Re: [PATCH 1/3] x86/mce/amd: Fix bank lookup in amd_mce_usable_address()

[Yazen Ghannam]

On Thu, Oct 01, 2026 at 10:19:42PM -0700, Borislav Petkov wrote:
> On Thu, Sep 03, 2026 at 09:20:47AM -0500, Yazen Ghannam wrote:
> > amd_mce_usable_address() reads the per-CPU smca_banks array at index
> > m->bank on the running CPU. The index is not checked, and the bank
> > belongs to m->extcpu, not to whichever CPU is decoding.
> > 
> > m->bank is unbounded here. apei_mce_report_mem_error() sets it to -1,
> > which is 255 in the u8 field, and apei_smca_report_x86_error() takes it
> > from a firmware BERT record. Both reach this function through the
> > decoder chain, which runs from a workqueue on any CPU. Bank counts
> > differ per CPU on SMCA systems. So the read can land past the end of
> > the array, or on a CPU where that bank means something else.
> > 
> > Use m->extcpu for both the bound and the lookup.
> >
> > Fixes: 821f5fe4dbcb ("x86/mce: Add support for physical address valid bit")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Assisted-by: Claude:claude-opus-5
> 
> ...: LLM
> 
> For all three pls.
> 

Yes, will do.

> > Signed-off-by: Yazen Ghannam <yazen.ghannam@xxxxxxx>
> > ---
> >  arch/x86/kernel/cpu/mce/amd.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kernel/cpu/mce/amd.c b/arch/x86/kernel/cpu/mce/amd.c
> > index f916fb4c5d13..e6542e00dc5a 100644
> > --- a/arch/x86/kernel/cpu/mce/amd.c
> > +++ b/arch/x86/kernel/cpu/mce/amd.c
> > @@ -837,7 +837,8 @@ bool amd_mce_usable_address(struct mce *m)
> >  			return false;
> >  	}
> >  
> > -	if (this_cpu_ptr(smca_banks)[m->bank].paddrv)
> > +	if (m->bank < per_cpu(mce_num_banks, m->extcpu) &&
> > +	    per_cpu(smca_banks, m->extcpu)[m->bank].paddrv)
> 
> Except that this conditional turns into an unreadable mess now. IOW, split it:
> 
>         if (m->bank >= per_cpu(mce_num_banks, m->extcpu)) 
>                 return false;
> 
>         if (per_cpu(smca_banks, m->extcpu)[m->bank].paddrv)
>                 return m->status & MCI_STATUS_PADDRV;
> 

Okay.

Thanks,
Yazen