Subject: Re: [PATCH net v1] net/rose: fix NULL pointer dereference in rose_transmit_link on reconnect

From: "Jiayuan Chen" <[email-masked]>
Date: Tue, 10 Mar 2026 11:28:25 +0000

March 10, 2026 at 18:24, "Eric Dumazet" <[email-masked] mailto:[email-masked]?to=%22Eric%20Dumazet%22%20%3Cedumazet%40google.com%3E > wrote:

> 
> On Tue, Mar 10, 2026 at 11:14 AM Jiayuan Chen <[email-masked]> wrote:
> 
> > 
> > From: Jiayuan Chen <[email-masked]>
> > 
> >  syzkaller reported a bug [1], and the reproducer is available at [2].
> > 
> >  When rose_connect() is called a second time on an already-connecting
> >  socket, it overwrites rose->neighbour with the result of rose_get_neigh()
> >  without releasing the previous reference. If rose_get_neigh() returns
> >  NULL, the socket is left in an inconsistent state: rose->state remains
> >  ROSE_STATE_1 from the first connect while rose->neighbour is NULL.
> > 
> >  When the socket is subsequently closed, rose_release() sees ROSE_STATE_1
> >  and calls rose_write_internal() -> rose_transmit_link(skb, NULL), causing
> >  a NULL pointer dereference when accessing neigh->loopback.
> > 
> >  Fix this by:
> >  1. Releasing the old neighbour reference before attempting a reconnect
> >  2. Resetting rose->state to ROSE_STATE_0 before the new connect attempt,
> >  so a failure leaves the socket in a clean state
> >  3. Setting rose->neighbour to NULL in all error paths after
> >  rose_neigh_put() to prevent use-after-free on subsequent reconnects
> > 
> >  [1] https://syzkaller.appspot.com/bug?extid=d00f90e0af54102fb271
> >  [2] https://gist.github.com/mrpre/9e6779e0d13e2c66779b1653fef80516
> > 
> >  Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> >  Reported-by: [email-masked]
> >  Closes: https://lore.kernel.org/all/[email-masked]/T/
> >  Cc: Jiayuan Chen <[email-masked]>
> >  Signed-off-by: Jiayuan Chen <[email-masked]>
> >  ---
> >  net/rose/af_rose.c | 11 +++++++++++
> >  1 file changed, 11 insertions(+)
> > 
> Normally, a connect() on a socket already connected should return an error.
> 
> disconnect is a special operation involving AF_UNSPEC
> 
> man connect
> ...
>  Some protocol sockets (e.g., TCP sockets as well as datagram
> sockets in the UNIX and Internet domains) may dissolve the association
> by
>  connecting to an address with the sa_family member of sockaddr
> set to AF_UNSPEC; thereafter, the socket can be connected to another
> ad‐
>  dress. (AF_UNSPEC is supported since Linux 2.2.)
>

Thanks for pointing this out. I should have checked the man page earlier.

diff --git a/net/rose/af_rose.c b/net/rose/af_rose.c
index 841d62481048..ba56213e0a2a 100644
--- a/net/rose/af_rose.c
+++ b/net/rose/af_rose.c
@@ -811,6 +811,11 @@ static int rose_connect(struct socket *sock, struct sockaddr_unsized *uaddr, int
                goto out_release;
        }

+       if (sk->sk_state == TCP_SYN_SENT) {
+               err = -EALREADY;
+               goto out_release;
+       }
+
        sk->sk_state   = TCP_CLOSE;
        sock->state = SS_UNCONNECTED;

Tested and the panic is gone. Will send v2 after the cool-down period.