Re: [PATCH] cdc-acm: prevent infinite loop when parsing CDC headers.

[Sergei Shtylyov]

Hello.

On 4/13/2015 7:20 PM, Sergei Shtylyov wrote:

> > Phil and I found out a problem with commit:

> >    7e860a6e ("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: 7e860a6e ("cdc-acm: add sanity checks")

>     12-digit SHA1 hash is required here.

> > Signed-off-by: Phil Turnbull <phil.turnbull@xxxxxxxxxx>
> > Signed-off-by: Quentin Casasnovas <quentin.casasnovas@xxxxxxxxxx>
> > CC: Oliver Neukum <oneukum@xxxxxxx>
> > CC: Adam Lee <adam8157@xxxxxxxxx>
> > ---
> >   drivers/usb/class/cdc-acm.c | 3 ++-
> >   1 file changed, 2 insertions(+), 1 deletion(-)

> > diff --git a/drivers/usb/class/cdc-acm.c b/drivers/usb/class/cdc-acm.c
> > index 6836177..1ac4587 100644
> > --- a/drivers/usb/class/cdc-acm.c
> > +++ b/drivers/usb/class/cdc-acm.c
> > @@ -1133,11 +1133,12 @@ static int acm_probe(struct usb_interface *intf,
> >       }
> >
> >       while (buflen > 0) {
> > +        if ((elength = buffer[0]) == 0)

>     Please run your patches thru scripts/checkpatch.pl. Assignments in the
> *if* operator are not allowed.

   s/operator/statement/, of course. :-)

> [...]

WBR, Sergei

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