[PATCH 6.6 211/254] netrom: fix double-free in nr_route_frame()

Greg Kroah-Hartman
2026-01-28T15:39:36Z

From: Jeongjun Park <aha310510@gmail.com>

commit ba1096c315283ee3292765f6aea4cca15816c4f7 upstream.

In nr_route_frame(), old_skb is immediately freed without checking if
nr_neigh->ax25 pointer is NULL. Therefore, if nr_neigh->ax25 is NULL,
the caller function will free old_skb again, causing a double-free bug.

Therefore, to prevent this, we need to modify it to check whether
nr_neigh->ax25 is NULL before freeing old_skb.

Cc: <stable@vger.kernel.org>
Reported-by: syzbot+999115c3bf275797dc27@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/69694d6f.050a0220.58bed.0029.GAE@google.com/
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Jeongjun Park <aha310510@gmail.com>
Link: https://patch.msgid.link/20260119063359.10604-1-aha310510@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
net/netrom/nr_route.c | 13 +++++++++----
1 file changed, 9 insertions(+), 4 deletions(-)

--- a/net/netrom/nr_route.c
+++ b/net/netrom/nr_route.c
@@ -752,7 +752,7 @@ int nr_route_frame(struct sk_buff *skb,
unsigned char *dptr;
ax25_cb *ax25s;
int ret;
- struct sk_buff *skbn;
+ struct sk_buff *nskb, *oskb;

/*
* Reject malformed packets early. Check that it contains at least 2
@@ -811,14 +811,16 @@ int nr_route_frame(struct sk_buff *skb,
/* We are going to change the netrom headers so we should get our
own skb, we also did not know until now how much header space
we had to reserve... - RXQ */
- if ((skbn=skb_copy_expand(skb, dev->hard_header_len, 0, GFP_ATOMIC)) == NULL) {
+ nskb = skb_copy_expand(skb, dev->hard_header_len, 0, GFP_ATOMIC);
+
+ if (!nskb) {
nr_node_unlock(nr_node);
nr_node_put(nr_node);
dev_put(dev);
return 0;
}
- kfree_skb(skb);
- skb=skbn;
+ oskb = skb;
+ skb = nskb;
skb->data[14]--;

dptr = skb_push(skb, 1);
@@ -837,6 +839,9 @@ int nr_route_frame(struct sk_buff *skb,
nr_node_unlock(nr_node);
nr_node_put(nr_node);

+ if (ret)
+ kfree_skb(oskb);
+
return ret;
}