Re: [PATCH net] bridge: skip generic XDP on locally re-injected packets

[Nikolay Aleksandrov <razor@blackwall.org>]

On 31/08/2026 15:25, Ido Schimmel wrote:
> On Mon, Aug 31, 2026 at 07:30:51PM +0800, Zhao ShiRong wrote:
>> Packets locally delivered by the bridge are re-injected into the
>> receive path via br_pass_frame_up() -> br_netif_receive_skb() ->
>> netif_receive_skb() with skb->dev set to the bridge device.  If the
>> bridge device has an XDP program attached, __netif_receive_skb_core()
>> runs do_xdp_generic() a second time on such packets.
>>
>> A locally-delivered packet that was allocated on the TX path (e.g. an
>> MLD packet built by mld_newpack()) does not carry the
>> XDP_PACKET_HEADROOM that generic XDP requires, so
>> netif_skb_check_for_xdp() calls pskb_expand_head() and reallocates the
>> skb head buffer.  This frees the head that the bridge rx path
>> (br_handle_frame() / br_handle_frame_finish()) is still using, leading
>> to a use-after-free read in br_handle_frame():
>>
>>    BUG: KASAN: slab-use-after-free in is_multicast_ether_addr [inline]
>>    BUG: KASAN: slab-use-after-free in is_valid_ether_addr [inline]
>>    BUG: KASAN: slab-use-after-free in br_handle_frame+0xcfb/0x1510 net/bridge/br_input.c:349
>>
>> netif_receive_generic_xdp() already refuses to run generic XDP on
>> reinjected packets by checking skb_is_redirected().  Reuse that marker:
>> set it right before the bridge re-injects the packet, so generic XDP is
>> skipped and the head buffer is left intact.
>>
>> Reported-by: syzbot+128e9f5a0f85a51215b1@syzkaller.appspotmail.com
>> Link: https://lore.kernel.org/all/6a6d4406.2d659fcc.1d46f5.01ad.GAE@google.com/T/
>> Signed-off-by: Zhao ShiRong <shxzhaosr@163.com>
> 
> Eric sent a fix for this issue earlier today:
> 
> https://lore.kernel.org/netdev/20260831095952.3385596-1-edumazet@google.com/

Oh I missed that patch, it fixes the root cause and we don't have to do anything in
the bridge. Good thing :)

Thanks,
  Nik