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

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

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.




>   
> -	while ((unsigned long)entry + proc_sz < table_end) {
> +	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)
> @@ -273,7 +273,7 @@ static struct acpi_pptt_processor *acpi_find_processor_node(struct acpi_table_he
>   	proc_sz = sizeof(struct acpi_pptt_processor);
>   
>   	/* find the processor structure associated with this cpuid */
> -	while ((unsigned long)entry + proc_sz < table_end) {
> +	while ((unsigned long)entry + proc_sz <= table_end) {
>   		cpu_node = (struct acpi_pptt_processor *)entry;
>   
>   		if (entry->length == 0) {