Re: Double-Fetch bug in Linux-4.5/drivers/scsi/aacraid/commctrl.c

[Kees Cook]

On Wed, Apr 27, 2016 at 1:07 AM, Julia Lawall <julia.lawall@xxxxxxx> wrote:
>
>
> On Wed, 27 Apr 2016, Dan Carpenter wrote:
>
>> On Wed, Apr 27, 2016 at 07:42:04AM +0200, Julia Lawall wrote:
>> >
>> >
>> > On Tue, 26 Apr 2016, Kees Cook wrote:
>> >
>> > > On Mon, Apr 25, 2016 at 7:50 AM, Pengfei Wang <wpengfeinudt@xxxxxxxxx> wrote:
>> > > > Hello,
>> > > >
>> > > > I found this Double-Fetch bug in Linux-4.5/drivers/scsi/aacraid/commctrl.c
>> > > > when I was examining the source code.
>> > >
>> > > Thanks for these reports! I wrote a coccinelle script to find these,
>> > > but it requires some manual checking. For what it's worth, it found
>> > > your report as well:
>> > >
>> > > ./drivers/scsi/aacraid/commctrl.c:116:5-19: potentially dangerous
>> > > second copy_from_user()
>> > >
>> > > So I should probably get this added to the coccicheck run... Maybe it
>> > > can get some clean up from Julia. :)
>> >
>> > I looked a bit at the results, and didn't see anything obvious.  What is
>> > the problem, exactly, and what would be a characteristic of a false
>> > positive?
>> >
>>
>>
>>       copy_from_user(dest, src, sizeof(dest));
>>
>>       if (dest.extra > MAX_SIZE)
>>               return -EINVAL;
>>
>>       copy_from_user(dest, src, sizeof(dest) + dest.extra);
>>
>>       for (i = 0; i < dest.extra; i++) {
>>               dest.foo[i] = xxx;
>>
>>
>> We get dest.extra from the user, we verify the size, then we copy more
>> data from the user but that over writes dest.extra again.  We use
>> dest.extra a second time without checking that it's still <= MAX_SIZE.
>
> OK, so the problem is when data that was checked on the first copy is used
> after the second copy?  It would probably be possible to get rid of a lot
> of false positives with that.

Yeah, though sometimes it's not into the same structure/variable:

copy_from_user(&header, src, sizeof(header));
full_structure = kmalloc(header.size);
copy_from_user(full_structure, src, header.size);
do_things(full_structure);
copy_to_user(dest, full_structure, full_structure->size);

Dan's example is the worst-case, but my above example can lead to
under-reads, or otherwise confusing actions taken when examining
full_structures's "size" field vs what has actually be written, etc.
(In my example, do_things may operate on uninitialize fields in
full_structure, and will leak heap contents on the copy_to_user.)

As a result of these variations, I was just detecting a double read
from the same location, which is usually an indication of some kind of
confusion in the code.

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security