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

Ben Hutchings
2017-02-24T13:38:56Z

[-- Attachment #1: Type: text/plain, Size: 1339 bytes --]

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.

> + opcode = buf[i];
> + line = buf[i + 1];
> + status = buf[i + 2];
> + val = buf[i + 3];
[...]

Ben.

--
Ben Hutchings
All the simple programs have been written, and all the good names
taken.

[-- Attachment #2: This is a digitally signed message part --]
[-- Type: application/pgp-signature, Size: 833 bytes --]