tcp: ipv6: do not read ireq->pktopts from tcp_v6_send_synack()

When IPV6_FL_F_REFLECT is set on a listener, or the
net.ipv6.flowlabel_reflect sysctl has FLOWLABEL_REFLECT_ESTABLISHED,
tcp_v6_send_synack() reflects the flowlabel of the SYN packet,
reading it from the skb stored in ireq->pktopts.

Since commit 079096f103fa ("tcp/dccp: install syn_recv requests
into ehash table"), request sockets are processed without holding
the listener lock. A cpu can then process a retransmitted SYN for a
request socket, and send a SYNACK from tcp_check_req(), while another
cpu receives the ACK completing the 3WHS. tcp_v6_syn_recv_sock()
consumes ireq->pktopts when it wins the hashdance, and the first cpu
reads a freed skb:

  BUG: KASAN: slab-use-after-free in tcp_v6_send_synack (include/linux/skbuff.h:3152 include/linux/ipv6.h:110 net/ipv6/tcp_ipv6.c:582)
  Read of size 8 at addr ffff88802b08b608 by task poc-ipv6-pktopt/5076

  Call Trace:
   <IRQ>
   tcp_v6_send_synack (include/linux/skbuff.h:3152 include/linux/ipv6.h:110 net/ipv6/tcp_ipv6.c:582)
   tcp_rtx_synack (net/ipv4/tcp_output.c:4719)
   tcp_check_req (net/ipv4/tcp_minisocks.c:762)
   tcp_v6_rcv (net/ipv6/tcp_ipv6.c:1826)
   ip6_protocol_deliver_rcu (net/ipv6/ip6_input.c:479)
   ip6_input_finish (net/ipv6/ip6_input.c:534)
   ip6_input (include/linux/netfilter.h:325 include/linux/netfilter.h:319 net/ipv6/ip6_input.c:545)
   ipv6_rcv (include/net/dst.h:481 net/ipv6/ip6_input.c:119 include/linux/netfilter.h:325 include/linux/netfilter.h:319 net/ipv6/ip6_input.c:351)

  Freed by task 5074 on cpu 0 at 15.412249s:
   kmem_cache_free (include/linux/kasan.h:235 mm/slub.c:2780 mm/slub.c:6630 mm/slub.c:6757)
   tcp_v6_syn_recv_sock (net/ipv6/tcp_ipv6.c:1528)
   tcp_check_req (net/ipv4/tcp_minisocks.c:934)
   tcp_v6_rcv (net/ipv6/tcp_ipv6.c:1826)
   ip6_protocol_deliver_rcu (net/ipv6/ip6_input.c:479)
   ip6_input_finish (net/ipv6/ip6_input.c:534)

Request socket migration has the same issue: reqsk_migrate_reset()
transfers ireq->pktopts to the clone, which can be freed while
another cpu holding a reference on the old request socket is still
sending a SYNACK.

The SYNACK timer itself is not an issue, because
inet_csk_reqsk_queue_drop() calls timer_delete_sync().

Save the flowlabel of the SYN in the request socket instead, so that
tcp_v6_send_synack() no longer needs ireq->pktopts.

As a side effect, the flowlabel is also reflected for SYN packets
reusing a TIME_WAIT socket, as ireq->pktopts is not set in this case.

Fixes: e994b2f0fb92 ("tcp: do not lock listener to process SYN packets")
Fixes: 079096f103fa ("tcp/dccp: install syn_recv requests into ehash table")
Fixes: c905dee62232 ("tcp: Migrate TCP_NEW_SYN_RECV requests at retransmitting SYN+ACKs.")
Reported-by: Anvik Loitongbam <permissiontogallop@gmail.com>
Reported-by: Yingjie Wang <x-professor@outlook.com>
Closes: https://lore.kernel.org/netdev/82c31e4d5043f3dbd11b44177cfba779d335aae3.1793599929.git.x-professor@outlook.com/
Signed-off-by: Eric Dumazet <edumazet@kernel.org>
Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Reviewed-by: Neal Cardwell <ncardwell@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20261102075441.3771550-1-edumazet@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/include/linux/tcp.h b/include/linux/tcp.h
index d18f7bc1e5d0..5a4c61fc240e 100644
--- a/include/linux/tcp.h
+++ b/include/linux/tcp.h
@@ -179,6 +179,9 @@ struct tcp_request_sock {
 	u8				ao_rcv_next;
 	bool				used_tcp_ao;
 #endif
+#if IS_ENABLED(CONFIG_IPV6)
+	__be32				syn_flowlabel;
+#endif
 };
 
 #define tcp_rsk(ptr) container_of_const(ptr, struct tcp_request_sock, req.req)