CVE-2025-21707 – Linux MPTCP Uninitialized Bitfield Vulnerability

mptcp: consolidate suboption status

MPTCP maintains the received sub-options status is the bitmask carrying
the received suboptions and in several bitfields carrying per suboption
additional info.

Zeroing the bitmask before parsing is not enough to ensure a consistent
status, and the MPTCP code has to additionally clear some bitfiled
depending on the actually parsed suboption.

The above schema is fragile, and syzbot managed to trigger a path where
a relevant bitfield is not cleared/initialized:

BUG: KMSAN: uninit-value in __mptcp_expand_seq net/mptcp/options.c:1030 [inline]
BUG: KMSAN: uninit-value in mptcp_expand_seq net/mptcp/protocol.h:864 [inline]
BUG: KMSAN: uninit-value in ack_update_msk net/mptcp/options.c:1060 [inline]
BUG: KMSAN: uninit-value in mptcp_incoming_options+0x2036/0x3d30 net/mptcp/options.c:1209
__mptcp_expand_seq net/mptcp/options.c:1030 [inline]
mptcp_expand_seq net/mptcp/protocol.h:864 [inline]
ack_update_msk net/mptcp/options.c:1060 [inline]
mptcp_incoming_options+0x2036/0x3d30 net/mptcp/options.c:1209
tcp_data_queue+0xb4/0x7be0 net/ipv4/tcp_input.c:5233
tcp_rcv_established+0x1061/0x2510 net/ipv4/tcp_input.c:6264
tcp_v4_do_rcv+0x7f3/0x11a0 net/ipv4/tcp_ipv4.c:1916
tcp_v4_rcv+0x51df/0x5750 net/ipv4/tcp_ipv4.c:2351
ip_protocol_deliver_rcu+0x2a3/0x13d0 net/ipv4/ip_input.c:205
ip_local_deliver_finish+0x336/0x500 net/ipv4/ip_input.c:233
NF_HOOK include/linux/netfilter.h:314 [inline]
ip_local_deliver+0x21f/0x490 net/ipv4/ip_input.c:254
dst_input include/net/dst.h:460 [inline]
ip_rcv_finish+0x4a2/0x520 net/ipv4/ip_input.c:447
NF_HOOK include/linux/netfilter.h:314 [inline]
ip_rcv+0xcd/0x380 net/ipv4/ip_input.c:567
__netif_receive_skb_one_core net/core/dev.c:5704 [inline]
__netif_receive_skb+0x319/0xa00 net/core/dev.c:5817
process_backlog+0x4ad/0xa50 net/core/dev.c:6149
__napi_poll+0xe7/0x980 net/core/dev.c:6902
napi_poll net/core/dev.c:6971 [inline]
net_rx_action+0xa5a/0x19b0 net/core/dev.c:7093
handle_softirqs+0x1a0/0x7c0 kernel/softirq.c:561
__do_softirq+0x14/0x1a kernel/softirq.c:595
do_softirq+0x9a/0x100 kernel/softirq.c:462
__local_bh_enable_ip+0x9f/0xb0 kernel/softirq.c:389
local_bh_enable include/linux/bottom_half.h:33 [inline]
rcu_read_unlock_bh include/linux/rcupdate.h:919 [inline]
__dev_queue_xmit+0x2758/0x57d0 net/core/dev.c:4493
dev_queue_xmit include/linux/netdevice.h:3168 [inline]
neigh_hh_output include/net/neighbour.h:523 [inline]
neigh_output include/net/neighbour.h:537 [inline]
ip_finish_output2+0x187c/0x1b70 net/ipv4/ip_output.c:236
__ip_finish_output+0x287/0x810
ip_finish_output+0x4b/0x600 net/ipv4/ip_output.c:324
NF_HOOK_COND include/linux/netfilter.h:303 [inline]
ip_output+0x15f/0x3f0 net/ipv4/ip_output.c:434
dst_output include/net/dst.h:450 [inline]
ip_local_out net/ipv4/ip_output.c:130 [inline]
__ip_queue_xmit+0x1f2a/0x20d0 net/ipv4/ip_output.c:536
ip_queue_xmit+0x60/0x80 net/ipv4/ip_output.c:550
__tcp_transmit_skb+0x3cea/0x4900 net/ipv4/tcp_output.c:1468
tcp_transmit_skb net/ipv4/tcp_output.c:1486 [inline]
tcp_write_xmit+0x3b90/0x9070 net/ipv4/tcp_output.c:2829
__tcp_push_pending_frames+0xc4/0x380 net/ipv4/tcp_output.c:3012
tcp_send_fin+0x9f6/0xf50 net/ipv4/tcp_output.c:3618
__tcp_close+0x140c/0x1550 net/ipv4/tcp.c:3130
__mptcp_close_ssk+0x74e/0x16f0 net/mptcp/protocol.c:2496
mptcp_close_ssk+0x26b/0x2c0 net/mptcp/protocol.c:2550
mptcp_pm_nl_rm_addr_or_subflow+0x635/0xd10 net/mptcp/pm_netlink.c:889
mptcp_pm_nl_rm_subflow_received net/mptcp/pm_netlink.c:924 [inline]
mptcp_pm_flush_addrs_and_subflows net/mptcp/pm_netlink.c:1688 [inline]
mptcp_nl_flush_addrs_list net/mptcp/pm_netlink.c:1709 [inline]
mptcp_pm_nl_flush_addrs_doit+0xe10/0x1630 net/mptcp/pm_netlink.c:1750
genl_family_rcv_msg_doit net/netlink/genetlink.c:1115 [inline]

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