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Reply-To: ahs3@redhat.com
Subject: Re: [PATCH v3 2/3] ACPI: ensure acpi_parse_entries_array() does not
 access non-existent table data
From:   Al Stone <ahs3@redhat.com>
To:     "Rafael J. Wysocki" <rafael@kernel.org>
Cc:     ACPI Devel Maling List <linux-acpi@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "Rafael J . Wysocki" <rjw@rjwysocki.net>,
        Len Brown <lenb@kernel.org>
References: <20180501003907.4322-1-ahs3@redhat.com>
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 <04afae30-7012-ebb9-cd0e-3ec39bfd955c@redhat.com>
Organization: Red Hat, Inc.
Message-ID: <944148ef-e8aa-72fc-069a-64ca793d1eeb@redhat.com>
Date:   Wed, 16 May 2018 09:09:55 -0600
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On 05/15/2018 03:53 PM, Al Stone wrote:
> On 05/15/2018 11:19 AM, Rafael J. Wysocki wrote:
>> On Tue, May 1, 2018 at 2:39 AM, Al Stone <ahs3@redhat.com> 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@redhat.com>
>>> Cc: Rafael J. Wysocki <rjw@rjwysocki.net>
>>> Cc: Len Brown <lenb@kernel.org>
>>> ---
>>>  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

Let's just drop this completely -- but please do send the acpidump.  It's
going to take me a while to figure why this innocuous little loop does not
behave the way I expect it to.  I'll send a separate patch, if needed.

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