Re: [CHECKER] copy_*_user length bugs?

[Chris Evans]

On Wed, 18 Apr 2001, David Schleef wrote:

> On Tue, Apr 17, 2001 at 09:39:15PM -0700, Dawson Engler wrote:
> > Hi All,
> >
> > at the suggestion of Chris ([e-mail]) I wrote a simple
> > checker to warn when the length parameter to copy_*_user was (1) an
> > integer and (2) not checked < 0.
> >
> > As an example, the ipv6 routine rawv6_geticmpfilter gets an integer 'len'
> > from user space, checks that it is smaller than a struct size and then
> > uses length as an argument to copy_to_user:
> >
> >                 if (get_user(len, optlen))
> >                         return -EFAULT;
> >                 if (len > sizeof(struct icmp6_filter))
> >                         len = sizeof(struct icmp6_filter);
> >                 if (put_user(len, optlen))
> >                         return -EFAULT;
> >                 if (copy_to_user(optval, &sk->tp_pinfo.tp_raw.filter, len))
> >                         return -EFAULT;
> >
> > Is this a real bug?  Or is the checked rule only applicable to
> > __copy_*_user routines rather than copy_*_user routines?  (If its a real
> > bug, theres about 8 others that we found).
>
> The len parameter is an unsigned value, so this code is ok as
> long as access_ok() correctly checks that the range to copy
> doesn't stray outside of the userspace range, including the
> possible wraparound for a very large len.  access_ok() on i386
> checks for the wraparound.  m68k doesn't use it.  PowerPC
> is correct, but only because TASK_SIZE is 0x80000000.  If it
> is ever changed, there could be a problem.  I didn't check
> other architectures, because I don't understand the asm.

Incorrect - if the "len" variable is a signed integer, this is a nasty
bug.

To justify this, consider if len were set to minus 2 billion. This will
pass the sanity check, and pass the value straight on to copy_to_user. The
copy_to_user parameter is unsigned, so this value because approximately
+2Gb.

Now, providing the malicious user passes a low user space pointer (e.g.
just above 0), the kernel's virtual address space wrap check will not
trigger because ~0 + ~2Gb does not exceed 4G. And the result is the user
being able to read kernel memory.

Cheers
Chris