linux-kernel - Re: [PATCH] rfcomm/core.c avoid dangling pointer, check session exists

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Message-ID: <20110530212441.GP3935@spacedout.fries.net>
Date:	Mon, 30 May 2011 16:24:41 -0500
From:	David Fries <david@...es.net>
To:	"Gustavo F. Padovan" <padovan@...fusion.mobi>
Cc:	akpm@...ux-foundation.org, linux-bluetooth@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Marcel Holtmann <marcel@...tmann.org>, linville@...driver.com
Subject: Re: [PATCH] rfcomm/core.c avoid dangling pointer, check session
 exists

On Mon, May 30, 2011 at 05:48:52PM -0300, Gustavo F. Padovan wrote:
> Hi David,
> 
> * David Fries <david@...es.net> [2011-05-21 14:02:53 -0500]:
> 
> > rfcomm_process_sessions is calling rfcomm_process_rx, but
> > in this case the session is closed and freed leaving a
> > dangling pointer that blows up when rfcomm_process_rx returns
> > and rfcomm_process_dlcs is called with the now dangling session
> > pointer.  Check to see if the entry is still in the list.
> > 
> > Signed-off-by: David Fries <David@...es.net>
> > Cc: Marcel Holtmann <marcel@...tmann.org>
> > Cc: "Gustavo F. Padovan" <padovan@...fusion.mobi>
> > ---
> > I sent out an ealier patch,
> > Date: Mon, 21 Mar 2011 21:38:10 -0500
> > Subject: [PATCH] rfcomm/core.c avoid dangling pointer, check session
> > 
> > That version added a return value to rfcomm_session_close to determine
> > if the session was closed.  I thought this would be cleaner.
> > 
> > I can reproduce using blueman-manager on desktop, and Motorola S305 bluetooth
> > headset, 2.6.39, but it can take a few attempts.  Start out with the
> > desktop as the last device the S305 paired with.
> > desktop, connect to the S305,
> > S305, turn on
> > desktop (connection fails)
> > desktop (connection automatically comes up now that S305 is on)
> > desktop disconnect S305
> > desktop (kernel panic)
> > 
> > While rfcomm_process_sessions looks symmetrical,
> > rfcomm_session_hold(s);
> > rfcomm_process_rx
> > rfcomm_process_dlcs
> > rfcomm_session_put(s);
> > 
> > rfcomm_process_rx
> > if (sk->sk_state == BT_CLOSED) {
> >         if (!s->initiator)
> >                 rfcomm_session_put(s);
> >         rfcomm_session_close(s, sk->sk_err);
> > 
> > Which isn't symmetrical.  I don't know enough about the subsystem to
> > know if there is a better cleaner way to fix this, or if my patch is
> > the best solution.
> > 
> >  net/bluetooth/rfcomm/core.c |    6 ++++++
> >  1 files changed, 6 insertions(+), 0 deletions(-)
> > 
> > diff --git a/net/bluetooth/rfcomm/core.c b/net/bluetooth/rfcomm/core.c
> > index c997393..ac47ef3 100644
> > --- a/net/bluetooth/rfcomm/core.c
> > +++ b/net/bluetooth/rfcomm/core.c
> > @@ -1952,6 +1952,12 @@ static inline void rfcomm_process_sessions(void)
> >  
> >  		default:
> >  			rfcomm_process_rx(s);
> > +			/* The current session can be closed as part of rx
> > +			 * in which case s is now dangling.  Check if it
> > +			 * has been removed.
> > +			 */
> > +			if(n->prev != p)
> > +				continue;
> >  			break;
> >  		}
> 
> I don't like this, it's not the proper fix. So I'm trying to figure out this
> and fix it. Can you try this patch:

Not without some explaination as to what it's trying to accomplish
(other than crash my computer), it's just a single CPU system, so it
isn't a very good test case for crashes in use after free anyway.
s->sock has been released by sock_release, s has been freed by kfree
(in rfcomm_session_del), which leaves s->sock-> completely dangling by
now.  That's why I was using something other than s to determine if
it's dangling, and looking to see if it was still in the list looked
like the best way to me.

I posted an earlier patch, 'Date: Mon, 21 Mar 2011 21:38:10 -0500'
where I added a return value to to rfcomm_session_put, as another way
to determine if s has been freed and now dangling.  The other question
is why check BT_CLOSED outside of default?  It's only
rfcomm_process_rx that will delete the session with something still
holding on to it.

I would rather see a fix where rfcomm_process_rx doesn't do the put
which caused this situation.  Also, now that I look at it,
rfcomm_process_rx is doing a close after it did a put, which means the
entire close is dealing with a dangling pointer.  Looks to me like the
order could be switched, pending the outcome of the propper fix.

	if (sk->sk_state == BT_CLOSED) {
		if (!s->initiator)
			rfcomm_session_put(s);

		rfcomm_session_close(s, sk->sk_err);
	}

> padovan bluetooth-next-2.6 $ git diff
> diff --git a/net/bluetooth/rfcomm/core.c b/net/bluetooth/rfcomm/core.c
> index 5759bb7..75c58ed 100644
> --- a/net/bluetooth/rfcomm/core.c
> +++ b/net/bluetooth/rfcomm/core.c
> @@ -1958,6 +1958,9 @@ static inline void rfcomm_process_sessions(void)
>                         break;
>                 }
>  
> +               if (s->sock->sk->sk_state == BT_CLOSED)
> +                       continue;
> +
>                 rfcomm_process_dlcs(s);
>  
>                 rfcomm_session_put(s);
> 
> -- 
> Gustavo F. Padovan
> http://profusion.mobi

-- 
David Fries <david@...es.net>    PGP pub CB1EE8F0
http://fries.net/~david/
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