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    SubjectRe: [CHECKER] copy_*_user length bugs?

    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 (chris@ferret.lmh.ox.ac.uk) 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

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