Re: [PATCH v3 2/3] ACPI: ensure acpi_parse_entries_array() does not access non-existent table data

[Al Stone]

On 05/15/2018 11:19 AM, Rafael J. Wysocki wrote:
> On Tue, May 1, 2018 at 2:39 AM, Al Stone <ahs3@xxxxxxxxxx> wrote:
>> For ACPI tables that have subtables, acpi_parse_entries_array() gets used
>> to step through each of the subtables in memory.  The primary loop for this
>> was checking that the beginning location of the subtable being examined
>> plus the length of struct acpi_subtable_header was not beyond the end of
>> the complete ACPI table; if it wasn't, the subtable could be examined, but
>> if it was the loop would terminate as it should.
>>
>> In the middle of this subtable loop, a callback is used to examine the
>> subtable in detail.
>>
>> Should the callback function try to examine elements of the subtable that
>> are located past the subtable header, and the ACPI table containing this
>> subtable has an incorrect length, it is possible to access either invalid
>> or protected memory and cause a fault.  And, the length of struct
>> acpi_subtable_header will always be smaller than the length of the actual
>> subtable.
>>
>> To fix this, we make the main loop check that the beginning of the
>> subtable being examined plus the actual length of the subtable does
>> not go past the end of the enclosing ACPI table.  While this cannot
>> protect us from malicious callback functions, it can prevent us from
>> failing because of some poorly constructed ACPI tables.
>>
>> Found by inspection.  There is no functional change to existing code
>> that is known to work when calling acpi_parse_entries_array().
>>
>> Signed-off-by: Al Stone <ahs3@xxxxxxxxxx>
>> Cc: Rafael J. Wysocki <rjw@xxxxxxxxxxxxx>
>> Cc: Len Brown <lenb@xxxxxxxxxx>
>> ---
>>  drivers/acpi/tables.c | 3 +--
>>  1 file changed, 1 insertion(+), 2 deletions(-)
>>
>> diff --git a/drivers/acpi/tables.c b/drivers/acpi/tables.c
>> index 4a3410aa6540..82c3e2c52dd9 100644
>> --- a/drivers/acpi/tables.c
>> +++ b/drivers/acpi/tables.c
>> @@ -274,8 +274,7 @@ acpi_parse_entries_array(char *id, unsigned long table_size,
>>         entry = (struct acpi_subtable_header *)
>>             ((unsigned long)table_header + table_size);
>>
>> -       while (((unsigned long)entry) + sizeof(struct acpi_subtable_header) <
>> -              table_end) {
>> +       while ((unsigned long)entry + entry->length <= table_end) {
>>                 if (max_entries && count >= max_entries)
>>                         break;
>>
>> --
> 
> This breaks the CPU enumeration on my Dell XPS13 9360 (possibly among
> other things), so I'm dropping it.  I can send you acpidump output
> from that machine if need be.
> 
> Thanks,
> Rafael
> 

Yes, please.  My fear is that there are a bunch of MADT tables in the real world
that aren't quite right so that while this may be theoretically correct, it may
be wrong as a practical matter.

-- 
ciao,
al
-----------------------------------
Al Stone
Software Engineer
Red Hat, Inc.
ahs3@xxxxxxxxxx
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