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

[Johan Hovold <johan@kernel.org>]

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.

Thanks,
Johan