CVE-2025-39894

In the Linux kernel, the following vulnerability has been resolved:

netfilter: br_netfilter: do not check confirmed bit in br_nf_local_in() after confirm

When send a broadcast packet to a tap device, which was added to a bridge, br_nf_local_in() is called to confirm the conntrack. If another conntrack with the same hash value is added to the hash table, which can be triggered by a normal packet to a non-bridge device, the below warning may happen.

————[ cut here ]———— WARNING: CPU: 1 PID: 96 at net/bridge/br_netfilter_hooks.c:632 br_nf_local_in+0x168/0x200 CPU: 1 UID: 0 PID: 96 Comm: tap_send Not tainted 6.17.0-rc2-dirty #44 PREEMPT(voluntary) RIP: 0010:br_nf_local_in+0x168/0x200 Call Trace: nf_hook_slow+0x3e/0xf0 br_pass_frame_up+0x103/0x180 br_handle_frame_finish+0x2de/0x5b0 br_nf_hook_thresh+0xc0/0x120 br_nf_pre_routing_finish+0x168/0x3a0 br_nf_pre_routing+0x237/0x5e0 br_handle_frame+0x1ec/0x3c0 __netif_receive_skb_core+0x225/0x1210 __netif_receive_skb_one_core+0x37/0xa0 netif_receive_skb+0x36/0x160 tun_get_user+0xa54/0x10c0 tun_chr_write_iter+0x65/0xb0 vfs_write+0x305/0x410 ksys_write+0x60/0xd0 do_syscall_64+0xa4/0x260 entry_SYSCALL_64_after_hwframe+0x77/0x7f —[ end trace 0000000000000000 ]—

To solve the hash conflict, nf_ct_resolve_clash() try to merge the conntracks, and update skb->_nfct. However, br_nf_local_in() still use the old ct from local variable nfct after confirm(), which leads to this warning.

If confirm() does not insert the conntrack entry and return NF_DROP, the warning may also occur. There is no need to reserve the WARN_ON_ONCE, just remove it.

Affected Software

References

• https://git.kernel.org/stable/c/479a54ab92087318514c82428a87af2d7af1a576
• https://git.kernel.org/stable/c/50db11e2bbb635e38e3dd096215580d6adb41fb0
• https://git.kernel.org/stable/c/a74abcf0f09f59daeecf7a3ba9c1d690808b0afe
• https://git.kernel.org/stable/c/c47ca77fee9071aa543bae592dd2a384f895c8b6
• https://git.kernel.org/stable/c/ccbad4803225eafe0175d3cb19f0d8d73b504a94
• https://git.kernel.org/stable/c/d00c8b0daf56012f69075e3377da67878c775e4c