On 2020-11-11 11:04, Xie He wrote:
The x25_disconnect function in x25_subr.c would decrease the refcount
of
"x25->neighbour" (struct x25_neigh) and reset this pointer to NULL.
However:
1) When we receive a connection, the x25_rx_call_request function in
af_x25.c does not increase the refcount when it assigns the pointer.
When we disconnect, x25_disconnect is called and the struct's refcount
is decreased without being increased in the first place.
Yes, this is a problem and should be fixed. As an alternative to your
approach, you could also go the way to prevent the call of
x25_neigh_put(nb) in x25_lapb_receive_frame() in case of a Call Request.
However, this would require more effort.
This causes frequent kernel crashes when using AF_X25 sockets.
2) When we initiate a connection but the connection is refused by the
remote side, x25_disconnect is called which decreases the refcount and
resets the pointer to NULL. But the x25_connect function in af_x25.c,
which is waiting for the connection to be established, notices the
failure and then tries to decrease the refcount again, resulting in a
NULL-pointer-dereference error.
This crashes the kernel every time a connection is refused by the
remote
side.
For this bug I already sent a fix some time ago (last time I sent a
RESEND yesterday), but unfortunately it was not merged yet:
https://lore.kernel.org/patchwork/patch/1334917/
Fixes: 4becb7ee5b3d ("net/x25: Fix x25_neigh refcnt leak when x25
disconnect")
Cc: Martin Schiller <m...@dev.tdt.de>
Signed-off-by: Xie He <xie.he.0...@gmail.com>
---
net/x25/af_x25.c | 11 +++++++----
1 file changed, 7 insertions(+), 4 deletions(-)
diff --git a/net/x25/af_x25.c b/net/x25/af_x25.c
index 0bbb283f23c9..8e59f9ecbeab 100644
--- a/net/x25/af_x25.c
+++ b/net/x25/af_x25.c
@@ -826,10 +826,12 @@ static int x25_connect(struct socket *sock,
struct sockaddr *uaddr,
rc = 0;
out_put_neigh:
if (rc) {
- read_lock_bh(&x25_list_lock);
- x25_neigh_put(x25->neighbour);
- x25->neighbour = NULL;
- read_unlock_bh(&x25_list_lock);
+ if (x25->neighbour) {
+ read_lock_bh(&x25_list_lock);
+ x25_neigh_put(x25->neighbour);
+ x25->neighbour = NULL;
+ read_unlock_bh(&x25_list_lock);
+ }
x25->state = X25_STATE_0;
}
out_put_route:
@@ -1050,6 +1052,7 @@ int x25_rx_call_request(struct sk_buff *skb,
struct x25_neigh *nb,
makex25->lci = lci;
makex25->dest_addr = dest_addr;
makex25->source_addr = source_addr;
+ x25_neigh_hold(nb);
makex25->neighbour = nb;
makex25->facilities = facilities;
makex25->dte_facilities= dte_facilities;
