Re: [PATCH] ACPI/PPTT: fix off-by-one error

[Jeremy Linton <jeremy.linton@arm.com>]

On 5/7/25 11:12 AM, Rafael J. Wysocki wrote:
> On Wed, May 7, 2025 at 5:51 PM Jeremy Linton <jeremy.linton@arm.com> wrote:
>>
>> On 5/7/25 10:42 AM, Rafael J. Wysocki wrote:
>>> On Wed, May 7, 2025 at 5:25 PM Jeremy Linton <jeremy.linton@arm.com> wrote:
>>>>
>>>> Hi,
>>>>
>>>> On 5/6/25 8:13 AM, Heyne, Maximilian wrote:
>>>>> Commit 7ab4f0e37a0f ("ACPI PPTT: Fix coding mistakes in a couple of
>>>>> sizeof() calls") corrects the processer entry size but unmasked a longer
>>>>> standing bug where the last entry in the structure can get skipped due
>>>>> to an off-by-one mistake if the last entry ends exactly at the end of
>>>>> the ACPI subtable.
>>>>>
>>>>> The error manifests for instance on EC2 Graviton Metal instances with
>>>>>
>>>>>      ACPI PPTT: PPTT table found, but unable to locate core 63 (63)
>>>>>      [...]
>>>>>      ACPI: SPE must be homogeneous
>>>>>
>>>>> Fixes: 2bd00bcd73e5 ("ACPI/PPTT: Add Processor Properties Topology Table parsing")
>>>>> Cc: stable@vger.kernel.org
>>>>> Signed-off-by: Maximilian Heyne <mheyne@amazon.de>
>>>>> ---
>>>>>     drivers/acpi/pptt.c | 4 ++--
>>>>>     1 file changed, 2 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/drivers/acpi/pptt.c b/drivers/acpi/pptt.c
>>>>> index f73ce6e13065d..4364da90902e5 100644
>>>>> --- a/drivers/acpi/pptt.c
>>>>> +++ b/drivers/acpi/pptt.c
>>>>> @@ -231,7 +231,7 @@ static int acpi_pptt_leaf_node(struct acpi_table_header *table_hdr,
>>>>>                              sizeof(struct acpi_table_pptt));
>>>>>         proc_sz = sizeof(struct acpi_pptt_processor);
>>>>
>>>> This isn't really right, it should be struct acpi_subtable_header, then
>>>> once the header is safe, pull the length from it.
>>>>
>>>> But then, really if we are trying to fix the original bug that the table
>>>> could be shorter than the data in it suggests, the struct
>>>> acpi_pptt_processor length plus its resources needs to be checked once
>>>> the subtype is known to be a processor node.
>>>>
>>>> Otherwise the original sizeof * change isn't really fixing anything.
>>>
>>> Sorry, what sense did it make to do
>>>
>>> proc_sz = sizeof(struct acpi_pptt_processor *);
>>>
>>> here?  As much as proc_sz = 0 I suppose?
>>
>> No, I agree, I think the original checks were simplified along the way
>> to that. It wasn't 'right' either.
>>
>> The problem is that there are three subtypes of which processor is only
>> one, and that struct acpi_pptt_processor doesn't necessarily reflect the
>> actual size of the processor structure in the table because it has
>> optional private resources tagged onto the end.
> 
> Right.
> 
>> So if the bug being fixed is that the length check is validating that
>> the table length is less than the data in the table, that's still a
>> problem because its only validating the processor node without resources.
> 
> Admittedly, it is not my code, but I understand this check as a
> termination condition for the loop: If there's not enough space in the
> table to hold a thing that I'm looking for, I may as well bail out.
> 
>> AKA the return is still potentially returning a pointer to a structure
>> which may not be entirely contained in the table.
> 
> Right, but this check should be made anyway before comparing
> cpu_node->parent to node_entry, when it is known to be a CPU entry
> because otherwise why bother.

Right, but then there is a clarity because really its walking the 
table+subtypes looking for the cpu node. Exiting early because its not 
big enough for a cpu node makes sense but you still need the cpu node 
check to avoid a variation on the original bug.



> 
> Roughly something like this:
> 
> proc_sz = sizeof(struct acpi_pptt_processor);
> 
> while ((unsigned long)entry + entry->length <= table_end) {

Here your reading the entry, without knowing its long enough. For the 
leaf check just using struct acpi_pptt_processor is fine, but for the 
acpi_find_processor_node():

proc_sz = sizeof(struct acpi_subtable_type);

while ((unsigned long)entry + proc_sz <= table_end) {
  if (entry->type == ACPI_PPTT_TYPE_PROCESSOR &&
  entry->length == sizeof(struct acpi_pptt_processor) +
	entry->number_of_priv_resources * sizeof(u32) &&
  entry + entry->length <= table_end &&
  acpi_pptt_leaf_node(...))
	return (...)entry;


Although at this point the while loops entry + proc_sz could just be < 
table_end under the assumption that entry->length will be > 0 but 
whichever makes more sense.