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

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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 079096f103 ("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: e994b2f0fb ("tcp: do not lock listener to process SYN packets")
Fixes: 079096f103 ("tcp/dccp: install syn_recv requests into ehash table")
Fixes: c905dee622 ("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>
This commit is contained in:
Eric Dumazet 2026-11-02 08:54:41 +01:00 committed by Paolo Abeni
parent 58fa96691c
commit 8e069fd3fd
2 changed files with 6 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

3
include/linux/tcp.h
View File

@ -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)

5
net/ipv6/tcp_ipv6.c
View File

@ -578,8 +578,8 @@ static int tcp_v6_send_synack(const struct sock *sk, struct dst_entry *dst,
&ireq->ir_v6_rmt_addr);
fl6->daddr = ireq->ir_v6_rmt_addr;
if (inet6_test_bit(REPFLOW, sk) && ireq->pktopts)
fl6->flowlabel = ip6_flowlabel(ipv6_hdr(ireq->pktopts));
if (inet6_test_bit(REPFLOW, sk))
fl6->flowlabel = tcp_rsk(req)->syn_flowlabel;
tclass = READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_reflect_tos) ?
(tcp_rsk(req)->syn_tos & ~INET_ECN_MASK) |
@ -789,6 +789,7 @@ static void tcp_v6_init_req(struct request_sock *req,
ireq->ir_v6_loc_addr = ipv6_hdr(skb)->daddr;
ireq->ir_rmt_addr = LOOPBACK4_IPV6;
ireq->ir_loc_addr = LOOPBACK4_IPV6;
tcp_rsk(req)->syn_flowlabel = ip6_flowlabel(ipv6_hdr(skb));
/* So that link locals have meaning */
if ((!sk_listener->sk_bound_dev_if || l3_slave) &&