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

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/ (local)
Signed-off-by: Zhao ShiRong <redacted>
---
  net/bridge/br_input.c | 6 ++++++
  1 file changed, 6 insertions(+)

diff --git a/net/bridge/br_input.c b/net/bridge/br_input.c
--- a/net/bridge/br_input.c
+++ b/net/bridge/br_input.c

@@ -26,6 +26,12 @@ static int
  br_netif_receive_skb(struct net *net, struct sock *sk, struct sk_buff *skb)
  {
  	br_drop_fake_rtable(skb);
+
+	/* Re-injected for local delivery: do not let generic XDP run on the
+	 * bridge device a second time, it could reallocate the head via
+	 * pskb_expand_head() and free a buffer still in use.
+	 */
+	skb_set_redirected_noclear(skb, false);
  	return netif_receive_skb(skb);
  }
  
--

2.43.0