With Commit 7917f9cdb503 ("acpi/nfit: rely on mce->misc to determine
poison granularity") that changed nfit_handle_mce() callback to report
badrange according to 1ULL << MCI_MISC_ADDR_LSB(mce->misc), it's been
discovered that the mce->misc LSB field is 0x1000 bytes, hence injecting
2 back-to-back poisons and the driver ends up logging 8 badblocks,
because 0x1000 bytes is 8 512-byte.

Dan Williams noticed that apei_mce_report_mem_error() hardcode
the LSB field to PAGE_SHIFT instead of consulting the input
struct cper_sec_mem_err record.  So change to rely on hardware whenever
support is available.

Link: https://lore.kernel.org/r/7ed50fd8-521e-cade-77b1-738b8bfb8502@oracle.com

Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Jane Chu <jane.chu@oracle.com>
---
 arch/x86/kernel/cpu/mce/apei.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/mce/apei.c b/arch/x86/kernel/cpu/mce/apei.c
index 717192915f28..a8274fd57add 100644
--- a/arch/x86/kernel/cpu/mce/apei.c
+++ b/arch/x86/kernel/cpu/mce/apei.c
@@ -37,7 +37,7 @@ void apei_mce_report_mem_error(int severity, struct cper_sec_mem_err *mem_err)
 	m.bank = -1;
 	/* Fake a memory read error with unknown channel */
 	m.status = MCI_STATUS_VAL | MCI_STATUS_EN | MCI_STATUS_ADDRV | MCI_STATUS_MISCV | 0x9f;
-	m.misc = (MCI_MISC_ADDR_PHYS << 6) | PAGE_SHIFT;
+	m.misc = (MCI_MISC_ADDR_PHYS << 6) | __ffs64(mem_err->physical_addr_mask);
 
 	if (severity >= GHES_SEV_RECOVERABLE)
 		m.status |= MCI_STATUS_UC;
-- 
2.18.4

+	m.misc = (MCI_MISC_ADDR_PHYS << 6) | __ffs64(mem_err->physical_addr_mask);

Do we want to unconditionally trust the sanity of the BIOS provided physical_address_mask?

There's a warning comment on the kernel __ffs64() function:

 * The result is not defined if no bits are set, so check that @word
 * is non-zero before calling this.

Otherwise, this looks like a good idea.

-Tony

Luck, Tony wrote:
> +m.misc = (MCI_MISC_ADDR_PHYS << 6) | __ffs64(mem_err->physical_addr_mask);
> 
> Do we want to unconditionally trust the sanity of the BIOS provided physical_address_mask?
> 
> There's a warning comment on the kernel __ffs64() function:
> 
>  * The result is not defined if no bits are set, so check that @word
>  * is non-zero before calling this.
> 
> Otherwise, this looks like a good idea.

It appears the kernel is trusting that ->physical_addr_mask is non-zero
in other paths. So this is at least equally broken in the presence of a
broken BIOS. The impact is potentially larger though with this change,
so it might be a good follow-on patch to make sure that
->physical_addr_mask gets fixed up to a minimum mask value.

> It appears the kernel is trusting that ->physical_addr_mask is non-zero
> in other paths. So this is at least equally broken in the presence of a
> broken BIOS. The impact is potentially larger though with this change,
> so it might be a good follow-on patch to make sure that
> ->physical_addr_mask gets fixed up to a minimum mask value.

Agreed. Separate patch to sanitize early, so other kernel code can just use it.

-Tony

On 7/18/2022 12:22 PM, Luck, Tony wrote:
>> It appears the kernel is trusting that ->physical_addr_mask is non-zero
>> in other paths. So this is at least equally broken in the presence of a
>> broken BIOS. The impact is potentially larger though with this change,
>> so it might be a good follow-on patch to make sure that
>> ->physical_addr_mask gets fixed up to a minimum mask value.
> 
> Agreed. Separate patch to sanitize early, so other kernel code can just use it.
> 

Is it possible that with
   if (mem->validation_bits & CPER_MEM_VALID_PA_MASK)
the ->physical_addr_mask is still untrustworthy?

include/ras/ras_event.h has this
       if (mem->validation_bits & CPER_MEM_VALID_PA_MASK)
                         __entry->pa_mask_lsb = 
(u8)__ffs64(mem->physical_addr_mask);
                 else
                         __entry->pa_mask_lsb = ~0;
which hints otherwise.

apei_mce_report_mem_error() already checks mem->validation_bits
up front.

thanks!
-jane


> -Tony

On Mon, Jul 18, 2022 at 09:11:33PM +0000, Jane Chu wrote:
> On 7/18/2022 12:22 PM, Luck, Tony wrote:
> >> It appears the kernel is trusting that ->physical_addr_mask is non-zero
> >> in other paths. So this is at least equally broken in the presence of a
> >> broken BIOS. The impact is potentially larger though with this change,
> >> so it might be a good follow-on patch to make sure that
> >> ->physical_addr_mask gets fixed up to a minimum mask value.
> > 
> > Agreed. Separate patch to sanitize early, so other kernel code can just use it.
> > 
> 
> Is it possible that with
>    if (mem->validation_bits & CPER_MEM_VALID_PA_MASK)
> the ->physical_addr_mask is still untrustworthy?

The validation_bits just show which fields the BIOS *says* it filled in.
If a validation bit isn't set, then Linux should certainly ignore that
field. But if it is set, then Linux needs to decide whether to use the
value, or do a sanity check first.

-Tony