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

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Message-ID: <20150413160227.GF12442@chrystal.uk.oracle.com>
Date:	Mon, 13 Apr 2015 18:02:27 +0200
From:	Quentin Casasnovas <quentin.casasnovas@...cle.com>
To:	Adam Lee <adam8157@...il.com>
Cc:	Quentin Casasnovas <quentin.casasnovas@...cle.com>,
	lkml <linux-kernel@...r.kernel.org>,
	linux-usb <linux-usb@...r.kernel.org>,
	Phil Turnbull <phil.turnbull@...cle.com>,
	Oliver Neukum <oneukum@...e.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] cdc-acm: prevent infinite loop when parsing CDC headers.

On Mon, Apr 13, 2015 at 11:48:27PM +0800, Adam Lee wrote:
> On Mon, Apr 13, 2015 at 05:24:04PM +0200, Quentin Casasnovas 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.
> 
> Yes, "elength = buffer[0]" should be moved to the front of
> USB_DT_CS_INTERFACE check, ACK this part.
> 
> But I don't know in what case the buffer[0] could be zero, if it
> happens, better to set elength 1 then goto next_desc? (I'm not a
> maintainer, pleas also consider others' opinion)
> 

Hi Adam,

I'm happy to change it as you suggest, though at that point we'll probably
be trying to parse garbage anyway.

If nobody had a different opinion in the meantime, I'll send a v2 tomorrow.

Thanks for the review :)

Quentin
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