Re: [PATCH 4.4 17/25] USB: serial: digi_acceleport: fix OOB data sanity check

Johan Hovold
2017-03-13T17:14:58Z

On Fri, Feb 24, 2017 at 06:55:02PM +0100, Greg Kroah-Hartman wrote:
> On Fri, Feb 24, 2017 at 06:33:04PM +0100, Johan Hovold wrote:
> > On Fri, Feb 24, 2017 at 01:38:25PM +0000, Ben Hutchings wrote: > On Fri, 2017-02-24 at 09:25 +0100, Greg Kroah-Hartman wrote:
> > > > 4.4-stable review patch.  If anyone has any objections, please let me know.
> > > >
> > > > ------------------
> > > >
> > > > From: Johan Hovold <johan@kernel.org>
> > > >
> > > > commit 2d380889215fe20b8523345649dee0579821800c upstream.
> > > >
> > > > Make sure to check for short transfers to avoid underflow in a loop
> > > > condition when parsing the receive buffer.
> > > >
> > > > Also fix an off-by-one error in the incomplete sanity check which could
> > > > lead to invalid data being parsed.
> > >
> > > This appears to *introduce* an off-by-one. Which is not as serious as
> > > the underflow, but is still a regression.
> > >
> > > Suppose we have urb->actual_length == 4:
> > >
> > > [...]
> > > > - for (i = 0; i < urb->actual_length - 3;) {
> > >
> > > i < 1 is true, so we would run the loop once.
> > >
> > > > - opcode = ((unsigned char *)urb->transfer_buffer)[i++];
> > > > - line = ((unsigned char *)urb->transfer_buffer)[i++];
> > > > - status = ((unsigned char *)urb->transfer_buffer)[i++];
> > > > - val = ((unsigned char *)urb->transfer_buffer)[i++];
> > > > + for (i = 0; i < urb->actual_length - 4; i += 4) {
> > >
> > > i < 0 is false, so we now skip the loop.
> >
> > Good catch, thanks! The original loop condition was indeed correct
> > (modulo the missing underflow check), and I'll post a follow-up fix to
> > address this.
> >
> > > > + opcode = buf[i];
> > > > + line = buf[i + 1];
> > > > + status = buf[i + 2];
> > > > + val = buf[i + 3];
> >
> > You should probably not apply this one until after the follow-up is in
> > Linus' tree as this patch breaks TIOCMGET.
>
> Ok, I'll drop this one from the stable tree now. Remind me to pick this
> one up when the fixup hits Linus's tree.

The follow-up fix is now in Linus's tree so both this one:

2d380889215f ("USB: serial: digi_acceleport: fix OOB data sanity check")

and the follow-up:

2e46565cf622 ("USB: serial: digi_acceleport: fix OOB-event processing")

can now be applied to the stable trees.

Thanks,
Johan