[PATCH bpf v2] bpf: Fix potential UAF in bpf_netns_link_update_prog

[Pu Lehui]

From: Pu Lehui <pulehui@xxxxxxxxxx>

In bpf_netns_link_update_prog, the checks for old_prog and prog type
are currently performed locklessly before acquiring netns_bpf_mutex.
This creates a race condition that can lead to a UAF issue.

If two threads concurrently execute BPF_LINK_UPDATE on the same netns
link, the following execution path can trigger a UAF:

CPU0                                          CPU1
bpf_netns_link_update_prog
  if (old_prog && old_prog != link->prog)
    return -EPERM;
                                              bpf_netns_link_update_prog
                                                if (old_prog && old_prog != link->prog)
                                                ...
                                                old_prog = xchg(&link->prog, new_prog);
                                                bpf_prog_put(old_prog);
  if (new_prog->type != link->prog->type) <-- trigger UAF

Fix this by moving the old_prog and prog->type checks inside the
netns_bpf_mutex critical section. Meanwhile, use guard() to simplify
lock management and avoid all the goto jumping.

Fixes: 7f045a49fee0 ("bpf: Add link-based BPF program attachment to network namespace")
Reported-by: Sashiko <sashiko-bot@xxxxxxxxxx>
Reviewed-by: Amery Hung <ameryhung@xxxxxxxxx>
Reviewed-by: Emil Tsalapatis <emil@xxxxxxxxxxxxxxx>
Signed-off-by: Pu Lehui <pulehui@xxxxxxxxxx>
---
v2:
- Use guard() to avoid all the goto jumping. (Andrii)
- Add Reviewed-by tags by Amery and Emil.
- Separate from patchset [0]. (Andrii)

Link: https://lore.kernel.org/bpf/f87b53c0-8f00-45a6-82db-8242fa9b143f@xxxxxxxxxxxxxxx [0]

v1: https://lore.kernel.org/bpf/20260720134547.1289964-1-pulehui@xxxxxxxxxxxxxxx

 kernel/bpf/net_namespace.c | 17 ++++++-----------
 1 file changed, 6 insertions(+), 11 deletions(-)

diff --git a/kernel/bpf/net_namespace.c b/kernel/bpf/net_namespace.c
index 25f30f9edaef..81006a242618 100644
--- a/kernel/bpf/net_namespace.c
+++ b/kernel/bpf/net_namespace.c
@@ -171,33 +171,28 @@ static int bpf_netns_link_update_prog(struct bpf_link *link,
 	struct net *net;
 	int idx, ret;
 
+	guard(mutex)(&netns_bpf_mutex);
+
 	if (old_prog && old_prog != link->prog)
 		return -EPERM;
 	if (new_prog->type != link->prog->type)
 		return -EINVAL;
 
-	mutex_lock(&netns_bpf_mutex);
-
 	net = net_link->net;
-	if (!net || !check_net(net)) {
+	if (!net || !check_net(net))
 		/* Link auto-detached or netns dying */
-		ret = -ENOLINK;
-		goto out_unlock;
-	}
+		return -ENOLINK;
 
 	run_array = rcu_dereference_protected(net->bpf.run_array[type],
 					      lockdep_is_held(&netns_bpf_mutex));
 	idx = link_index(net, type, net_link);
 	ret = bpf_prog_array_update_at(run_array, idx, new_prog);
 	if (ret)
-		goto out_unlock;
+		return ret;
 
 	old_prog = xchg(&link->prog, new_prog);
 	bpf_prog_put(old_prog);
-
-out_unlock:
-	mutex_unlock(&netns_bpf_mutex);
-	return ret;
+	return 0;
 }
 
 static int bpf_netns_link_fill_info(const struct bpf_link *link,
-- 
2.34.1