netdev - Re: [PATCH] wireless: wext: allocate space for NULL-termination for 32byte SSIDs

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Date:	Wed, 16 Dec 2009 11:58:44 +0800
From:	Daniel Mack <daniel@...aq.de>
To:	Johannes Berg <johannes@...solutions.net>
Cc:	David Miller <davem@...emloft.net>, linux-kernel@...r.kernel.org,
	dcbw@...hat.com, m.hirsch@...mfeld.com, netdev@...r.kernel.org,
	libertas-dev@...ts.infradead.org, stable@...nel.org,
	linux-wireless@...r.kernel.org
Subject: Re: [PATCH] wireless: wext: allocate space for NULL-termination
 for 32byte SSIDs

On Tue, Dec 15, 2009 at 11:03:31AM +0100, Johannes Berg wrote:
> 
> > > -	/* kzalloc() ensures NULL-termination for essid_compat. */
> > > -	extra = kzalloc(extra_size, GFP_KERNEL);
> > > +	/* kzalloc() +1 ensures NULL-termination for essid_compat. */
> > > +	extra = kzalloc(extra_size + 1, GFP_KERNEL);
> 
> That doesn't seem correct.
> 
> If this is used in a SET, then it is purely an in-kernel thing and
> everything in the kernel is passed the length + data, and the kernel
> MUST NEVER treat the SSID as a NUL-terminated string.
> 
> If this is used in a GET, then it will be filled up to 32 bytes by the
> get handler, and the trailing \0 your patch reserves will never be
> copied into userspace.

The problem is the GET case. The libertas driver copies ssid_len
characters here and appends a trailing \0, which my patch caught now and
which caused memory corruption in before.

>From what I've seen, libertas _does_ treat the extra data correctly
at all places, I checked it several times now. (Btw, the %s format
string you pointed out all use print_ssid() to properly escape all
non-printable characters, so they're rules out, too).

I'll send a patch to fix the flaw in libertas.

Thanks,
Daniel

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