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

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

[Kees Cook]

On Mon, Apr 25, 2016 at 7:50 AM, Pengfei Wang <wpengfeinudt@gmail.com> 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. :)

virtual report
virtual org

@cfu_twice@
position p;
identifier src;
expression dest1, dest2, size1, size2, offset;
@@

*copy_from_user(dest1, src, size1)
 ... when != src = offset
     when != src += offset
*copy_from_user@p(dest2, src, size2)

@script:python depends on org@
p << cfu_twice.p;
@@

cocci.print_main("potentially dangerous second copy_from_user()",p)

@script:python depends on report@
p << cfu_twice.p;
@@

coccilib.report.print_report(p[0],"potentially dangerous second
copy_from_user()")


It would be great to have some one go through all the reports to see
which are legit. I'll send separate emails with the patch for
coccicheck and the output.

-Kees

>
> In function ioctl_send_fib(), the driver fetches user space data by pointer
> arg via copy_from_user(), and this happens twice at line 81 and line 116
> respectively. The first fetched value (stored in kfib) is used to get the
> header and calculate the size at line 90 so as to copy the whole message
> later at line 116, which means the copy size of the whole message is based
> on the old value that came from the first fetch. Besides, the whole message
> copied in the  second fetch also contains the header.
>
> However, when the function processes the message after the second fetch at
> line 130, it uses kfib->header.Size that came from the second fetch, which
> might be different from the one came from the first fetch as well as
> calculated the size to copy the message from user space to driver.
>
> If the kfib->header.Size is modified by a user thread under race condition
> between the fetch operations, for example changing to a very large value,
> this will lead to over-boundary access or other serious consequences in
> function aac_fib_send().
>
> I also reported this to bugzilla,
> https://bugzilla.kernel.org/show_bug.cgi?id=116751
>
> I am expecting a reply to confirm this, thank you!
>
>
>
>
>
> Kind regards
> Pengfei
>



-- 
Kees Cook
Chrome OS & Brillo Security

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

[Julia Lawall]

On Tue, 26 Apr 2016, Kees Cook wrote:

> On Mon, Apr 25, 2016 at 7:50 AM, Pengfei Wang <wpengfeinudt@gmail.com> 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?

thanks,
julia

> virtual report
> virtual org
> 
> @cfu_twice@
> position p;
> identifier src;
> expression dest1, dest2, size1, size2, offset;
> @@
> 
> *copy_from_user(dest1, src, size1)
>  ... when != src = offset
>      when != src += offset
> *copy_from_user@p(dest2, src, size2)
> 
> @script:python depends on org@
> p << cfu_twice.p;
> @@
> 
> cocci.print_main("potentially dangerous second copy_from_user()",p)
> 
> @script:python depends on report@
> p << cfu_twice.p;
> @@
> 
> coccilib.report.print_report(p[0],"potentially dangerous second
> copy_from_user()")
> 
> 
> It would be great to have some one go through all the reports to see
> which are legit. I'll send separate emails with the patch for
> coccicheck and the output.
> 
> -Kees
> 
> >
> > In function ioctl_send_fib(), the driver fetches user space data by pointer
> > arg via copy_from_user(), and this happens twice at line 81 and line 116
> > respectively. The first fetched value (stored in kfib) is used to get the
> > header and calculate the size at line 90 so as to copy the whole message
> > later at line 116, which means the copy size of the whole message is based
> > on the old value that came from the first fetch. Besides, the whole message
> > copied in the  second fetch also contains the header.
> >
> > However, when the function processes the message after the second fetch at
> > line 130, it uses kfib->header.Size that came from the second fetch, which
> > might be different from the one came from the first fetch as well as
> > calculated the size to copy the message from user space to driver.
> >
> > If the kfib->header.Size is modified by a user thread under race condition
> > between the fetch operations, for example changing to a very large value,
> > this will lead to over-boundary access or other serious consequences in
> > function aac_fib_send().
> >
> > I also reported this to bugzilla,
> > https://bugzilla.kernel.org/show_bug.cgi?id=116751
> >
> > I am expecting a reply to confirm this, thank you!
> >
> >
> >
> >
> >
> > Kind regards
> > Pengfei
> >
> 
> 
> 
> -- 
> Kees Cook
> Chrome OS & Brillo Security
> 

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

[Dan Carpenter]

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@gmail.com> 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.

regards,
dan carpenter

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

[Julia Lawall]

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@gmail.com> 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.

thanks,
julia

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@lip6.fr> 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@gmail.com> 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