ACK w/cmt: [Trusty/Utopic/Vivid][PATCH] cdc-acm: prevent infinite loop when parsing CDC headers.

[Chris J Arges]

Looks to be a clean cherry-pick that fixes patches we already have in
our trees. The patch is being included in stable.

Do you have a way of testing the fix?

--chris

On 06/01/2015 08:18 AM, Adam Lee wrote:
> From: Quentin Casasnovas <quentin.casasnovas at oracle.com>
> 
> BugLink: https://bugs.launchpad.net/1460657
> 
> Phil and I found out a problem with commit:
> 
>   7e860a6e7aa6 ("cdc-acm: add sanity checks")
> 
> It added some sanity checks to ignore potential garbage in CDC headers but
> also introduced a potential infinite loop.  This can happen at the first
> loop iteration (elength = 0 in that case) if the description isn't a
> DT_CS_INTERFACE or later if 'buffer[0]' is zero.
> 
> It should also be noted that the wrong length was being added to 'buffer'
> in case 'buffer[1]' was not a DT_CS_INTERFACE descriptor, since elength was
> assigned after that check in the loop.
> 
> A specially crafted USB device could be used to trigger this infinite loop.
> 
> Fixes: 7e860a6e7aa6 ("cdc-acm: add sanity checks")
> Signed-off-by: Phil Turnbull <phil.turnbull at oracle.com>
> Signed-off-by: Quentin Casasnovas <quentin.casasnovas at oracle.com>
> CC: Sergei Shtylyov <sergei.shtylyov at cogentembedded.com>
> CC: Oliver Neukum <oneukum at suse.de>
> CC: Adam Lee <adam8157 at gmail.com>
> CC: <stable at vger.kernel.org>
> Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
> (cherry picked from commit 0d3bba0287d4e284c3ec7d3397e81eec920d5e7e)
> Signed-off-by: Adam Lee <adam.lee at canonical.com>
> ---
>  drivers/usb/class/cdc-acm.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/usb/class/cdc-acm.c b/drivers/usb/class/cdc-acm.c
> index 82dea40..46321cd 100644
> --- a/drivers/usb/class/cdc-acm.c
> +++ b/drivers/usb/class/cdc-acm.c
> @@ -985,11 +985,16 @@ static int acm_probe(struct usb_interface *intf,
>  	}
>  
>  	while (buflen > 0) {
> +		elength = buffer[0];
> +		if (!elength) {
> +			dev_err(&intf->dev, "skipping garbage byte\n");
> +			elength = 1;
> +			goto next_desc;
> +		}
>  		if (buffer[1] != USB_DT_CS_INTERFACE) {
>  			dev_err(&intf->dev, "skipping garbage\n");
>  			goto next_desc;
>  		}
> -		elength = buffer[0];
>  
>  		switch (buffer[2]) {
>  		case USB_CDC_UNION_TYPE: /* we've found it */
>