[PATCH V4] netrom: Preventing the use of abnormal neighbor

Lizhi Xu
2025-10-23T13:50:50Z

The root cause of the problem is that multiple different tasks initiate
SIOCADDRT & NETROM_NODE commands to add new routes, there is no lock
between them to protect the same nr_neigh.

Task0 can add the nr_neigh.refcount value of 1 on Task1 to routes[2].
When Task2 executes nr_neigh_put(nr_node->routes[2].neighbour), it will
release the neighbour because its refcount value is 1.

In this case, the following situation causes a UAF on Task2:

Task0 Task1 Task2
===== ===== =====
nr_add_node()
nr_neigh_get_dev() nr_add_node()
nr_node_lock()
nr_node->routes[2].neighbour->count--
nr_neigh_put(nr_node->routes[2].neighbour);
nr_remove_neigh(nr_node->routes[2].neighbour)
nr_node_unlock()
nr_node_lock()
nr_node->routes[2].neighbour = nr_neigh
nr_neigh_hold(nr_neigh); nr_add_node()
nr_neigh_put()
if (nr_node->routes[2].neighbour->count
Description of the UAF triggering process:
First, Task 0 executes nr_neigh_get_dev() to set neighbor refcount to 3.
Then, Task 1 puts the same neighbor from its routes[2] and executes
nr_remove_neigh() because the count is 0. After these two operations,
the neighbor's refcount becomes 1. Then, Task 0 acquires the nr node
lock and writes it to its routes[2].neighbour.
Finally, Task 2 executes nr_neigh_put(nr_node->routes[2].neighbour) to
release the neighbor. The subsequent execution of the neighbor->count
check triggers a UAF.

Filter out neighbors with a refcount of 1 to avoid unsafe conditions.

syzbot reported:
BUG: KASAN: slab-use-after-free in nr_add_node+0x25db/0x2c00 net/netrom/nr_route.c:248
Read of size 4 at addr ffff888051e6e9b0 by task syz.1.2539/8741

Call Trace:
<TASK>
nr_add_node+0x25db/0x2c00 net/netrom/nr_route.c:248

Reported-by: syzbot+2860e75836a08b172755@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=2860e75836a08b172755
Signed-off-by: Lizhi Xu <lizhi.xu@windriver.com>
---
V1 -> V2: update comments for cause uaf
V2 -> V3: sync neighbor operations in ioctl and route frame, update comments
V3 -> V4: Preventing the use of neighbors with a reference count of 1

net/netrom/nr_route.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/net/netrom/nr_route.c b/net/netrom/nr_route.c
index b94cb2ffbaf8..1ef2743a5ec0 100644
--- a/net/netrom/nr_route.c
+++ b/net/netrom/nr_route.c
@@ -100,7 +100,7 @@ static int __must_check nr_add_node(ax25_address *nr, const char *mnemonic,
{
struct nr_node *nr_node;
struct nr_neigh *nr_neigh;
- int i, found;
+ int i, found, ret = 0;
struct net_device *odev;

if ((odev=nr_dev_get(nr)) != NULL) { /* Can't add routes to ourself */
@@ -212,6 +212,10 @@ static int __must_check nr_add_node(ax25_address *nr, const char *mnemonic,
return 0;
}
nr_node_lock(nr_node);
+ if (refcount_read(&nr_neigh->refcount) == 1) {
+ ret = -EINVAL;
+ goto out;
+ }

if (quality != 0)
strscpy(nr_node->mnemonic, mnemonic);
@@ -279,10 +283,11 @@ static int __must_check nr_add_node(ax25_address *nr, const char *mnemonic,
}
}

+out:
nr_neigh_put(nr_neigh);
nr_node_unlock(nr_node);
nr_node_put(nr_node);
- return 0;
+ return ret;
}

static void nr_remove_node_locked(struct nr_node *nr_node)
--
2.43.0