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

[Ben Hutchings]

On Fri, 2017-02-24 at 09:25 +0100, Greg Kroah-Hartman wrote:
> 4.4-stable review patch.ÂÂIf anyone has any objections, please let me know.
> 
> ------------------
> 
> From: Johan Hovold <johan@xxxxxxxxxx>
> 
> 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.

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