Re: [PATCH] lib/firmware_table: validate subtable bounds

[Dave Jiang <dave.jiang@intel.com>]

On 10/21/26 8:04 AM, jiale yao wrote:
> At 2026-10-19 23:29:57, "Dave Jiang" <dave.jiang@intel.com> wrote:
>>
>>
>> On 10/16/26 4:15 PM, Andrew Morton wrote:
>>> On Fri, 16 Oct 2026 22:41:54 +0800 Jiale Yao <yaojiale02@163.com> wrote:
>>>
>>>> The generic firmware table walker dispatches matching entries after only
>>>> checking that a generic subtable header is present. A record whose
>>>> declared length is shorter than its header or extends past the table can
>>>> therefore reach a handler before its bounds have been established.
>>>>
>>>> Validate the complete generic header and the declared record range
>>>> before looking up or invoking a handler. This also rejects zero-length
>>>> records and prevents malformed lengths from advancing the cursor outside
>>>> the table.
>>>
>>> Thanks.
>>>
>>> get_maintainer is quite unreliable with lib/ files.  Please always use
>>> git-blame when determining who should be cc'ed.
>>>
>>> Dave, can you please take a look?
>>>
>>>> --- a/lib/fw_table.c
>>>> +++ b/lib/fw_table.c
>>>> @@ -172,7 +172,20 @@ acpi_parse_entries_array(char *id, unsigned long table_size,
>>>>  	    ((unsigned long)table_header + table_size);
>>>>  	subtable_len = acpi_get_subtable_header_length(&entry);
>>>>  
>>>> -	while (((unsigned long)entry.hdr) + subtable_len < table_end) {
>>>> +	while ((unsigned long)entry.hdr < table_end) {
>>>> +		if (table_end - (unsigned long)entry.hdr < subtable_len) {
>>>> +			pr_err("[%4.4s] Invalid subtable header\n", id);
>>>> +			return -EINVAL;
>>>> +		}
>>>> +
>>>> +		entry_len = acpi_get_entry_length(&entry);
>>>> +		if (entry_len < subtable_len ||
>>>> +		    entry_len > table_end - (unsigned long)entry.hdr) {
>>>> +			pr_err("[%4.4s:0x%02lx] Invalid length %lu\n", id,
>>>> +			       acpi_get_entry_type(&entry), entry_len);
>>>> +			return -EINVAL;
>>>> +		}
>>>> +
>>>>  		for (i = 0; i < proc_num; i++) {
>>>>  			if (acpi_get_entry_type(&entry) != proc[i].id)
>>>>  				continue;
>>>> @@ -186,16 +199,6 @@ acpi_parse_entries_array(char *id, unsigned long table_size,
>>>>  			break;
>>>>  		}
>>>>  
>>>> -		/*
>>>> -		 * If entry->length is 0, break from this loop to avoid
>>>> -		 * infinite loop.
>>>> -		 */
>>>> -		entry_len = acpi_get_entry_length(&entry);
>>>> -		if (entry_len == 0) {
>>>> -			pr_err("[%4.4s:0x%02x] Invalid zero length\n", id, proc->id);
>>>> -			return -EINVAL;
>>>> -		}
>>>> -
>>>>  		entry.hdr = (union acpi_subtable_headers *)
>>>>  		    ((unsigned long)entry.hdr + entry_len);
>>>>  	}
>>>> -- 
>>>> 2.34.1
>>
>> Can this turn a previously accepted table into a fatal parse error? My
>> main concern is breaking previously tolerated behavior and we end up
>> breaking deployed systems. Given this code has been around a long time.
>> Maybe Rafael or Len can comment.
>>
>> The old loop stopped quietly when subtable_len or fewer bytes remained
>> after the last entry. Here subtable_len is sizeof() the subtable header
>> structure, 2 bytes for MADT or 4 for CDAT, not the entry's length field.
>>
>> Now fewer than subtable_len leftover bytes return -EINVAL. Exactly
>> subtable_len leftover bytes are read as a header, and fail the entry_len
>> check below unless they happen to hold a valid one. A table with a few
>> bytes of padding after the last entry now fails the whole parse.
>>
>> On x86 that error is not contained to one entry type:
>>
>> early_acpi_process_madt()
>>  early_acpi_parse_madt_lapic_addr_ovr()
>>    acpi_table_parse_madt(ACPI_MADT_TYPE_LOCAL_APIC_OVERRIDE, ...)
>>      -> -EINVAL
>>
>> 	if (error == -EINVAL) {
>> 		...
>> 		pr_err("Invalid BIOS MADT, disabling ACPI\n");
>> 		disable_acpi();
>> 	}
>>
>> Would it be safer to keep tolerating leftover bytes too small to hold a
>> subtable header, and reject only entries whose declared length is
>> invalid? The loop condition can end the walk in that case without a
>> separate check:
>>
>> 	while ((unsigned long)entry.hdr + subtable_len <= table_end) {
>> 		entry_len = acpi_get_entry_length(&entry);
>> 		if (entry_len < subtable_len ||
>> 		    entry_len > table_end - (unsigned long)entry.hdr) {
>> 			...
>> 			return -EINVAL;
>> 		}
> Thanks for your review,
> how about this one, https://lore.kernel.org/all/20261021150159.399043-1-yaojiale02@163.com

LGTM
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
> 
> Jiale