Re: [Resend PATCH] RDS: fix race condition when sending a message on unbound socket

[Quentin Casasnovas]

On Wed, Nov 25, 2015 at 12:21:45PM +0000, David Laight wrote:
> From: Santosh Shilimkar
> > Sent: 24 November 2015 22:13
> ...
> > Sasha's found a NULL pointer dereference in the RDS connection code when
> > sending a message to an apparently unbound socket.  The problem is caused
> > by the code checking if the socket is bound in rds_sendmsg(), which checks
> > the rs_bound_addr field without taking a lock on the socket.  This opens a
> > race where rs_bound_addr is temporarily set but where the transport is not
> > in rds_bind(), leading to a NULL pointer dereference when trying to
> > dereference 'trans' in __rds_conn_create().
> > 
> > Vegard wrote a reproducer for this issue, so kindly ask him to share if
> > you're interested.
> ...
> > diff --git a/net/rds/send.c b/net/rds/send.c
> > index 827155c..c9cdb35 100644
> > --- a/net/rds/send.c
> > +++ b/net/rds/send.c
> > @@ -1013,11 +1013,13 @@ int rds_sendmsg(struct socket *sock, struct msghdr *msg, size_t payload_len)
> >  		release_sock(sk);
> 
> This is falling though into an unconditional lock_sock().
> No need to unlock and relock immediately.
> 
> >  	}
> > 
> > -	/* racing with another thread binding seems ok here */
> > +	lock_sock(sk);
> >  	if (daddr == 0 || rs->rs_bound_addr == 0) {
> > +		release_sock(sk);
> >  		ret = -ENOTCONN; /* XXX not a great errno */
> >  		goto out;
> >  	}
> > +	release_sock(sk);
> > 
> 
> On the face of it the above looks somewhat dubious.
> Locks usually tie together two action (eg a test and use of a value),
> In this case you only have a test inside the lock.
> That either means that the state can change after you release the lock
> (ie rs->rs_bound_addr = 0 is executed somewhere), or you don't
> really need the lock.
>

If you look at rds_bind(), you'll see that it does something like the
following:

    lock_sock(sk);
    ...
1:  rds_add_bound();  # This sets rs->rs_bound_addr
    ...
    if (!trans) {
            ...
2:          rds_remove_bound(rs);  # This unsets rs->rs_bound_addr
  ...
  release_sock(sk);

So any code checking rs_bound_addr without taking that lock could
potentially think the socket is bound, when in fact rds_bind() has failed.
This can happen if checking rs_bound_addr happens exactly between [1] and
[2] above.  So the usage of the lock in this particular case is to get a
consistent view of the sk.

The only other case where rs_bound_addr is cleared is on socket release, so
I didn't _think_ there was a problem here but maybe you can see another
race?

Thanks,
Quentin

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