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

Rafael J. Wysocki
2025-05-07T17:01:32Z

On Wed, May 7, 2025 at 6:41 PM Jeremy Linton <jeremy.linton@arm.com> wrote:
>
> On 5/7/25 11:38 AM, Jeremy Linton wrote:
> > On 5/7/25 11:31 AM, Jeremy Linton wrote:
> >> 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);
> >
> > Although, maybe I just wrote code that justifies using
> > acpi_pptt_processor here because the entry->num_of_priv_resources length
> > check isn't being made without it. So ok, use proc_sz = sizeof(struct
> > acpi_subtable_type) and assume that we don't care if the subtable type
> > is less than proc_sz.
>
> Sorry for the noise, scratch that, a better solution is just to swap the
> length checking in the 'if' statement. Then its clear its iterating
> subtable types not processor nodes.

Do you mean something like this (modulo GMail-induced whitespace damage):

--- a/drivers/acpi/pptt.c
+++ b/drivers/acpi/pptt.c
@@ -231,16 +231,22 @@
sizeof(struct acpi_table_pptt));
proc_sz = sizeof(struct acpi_pptt_processor);

- while ((unsigned long)entry + proc_sz < table_end) {
- cpu_node = (struct acpi_pptt_processor *)entry;
- if (entry->type == ACPI_PPTT_TYPE_PROCESSOR &&
- cpu_node->parent == node_entry)
- return 0;
+ while ((unsigned long)entry + proc_sz <= table_end) {
+ if ((unsigned long)entry + entry->length <= table_end &&
+ entry->type == ACPI_PPTT_TYPE_PROCESSOR &&
+ entry->length == proc_sz +
+ entry->number_of_priv_resources * sizeof(u32)) {
+ cpu_node = (struct acpi_pptt_processor *)entry;
+
+ if (cpu_node->parent == node_entry)
+ return 0;
+ }
+
if (entry->length == 0)
return 0;
+
entry = ACPI_ADD_PTR(struct acpi_subtable_header, entry,
entry->length);
-
}
return 1;
}