CVE-2025-21707: mptcp: consolidate suboption status

In the Linux kernel, the following vulnerability has been resolved:

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 mptcpexpandseq net/mptcp/options.c:1030 [inline] BUG: KMSAN: uninit-value in mptcpexpandseq net/mptcp/protocol.h:864 [inline] BUG: KMSAN: uninit-value in ackupdatemsk net/mptcp/options.c:1060 [inline] BUG: KMSAN: uninit-value in mptcpincomingoptions+0x2036/0x3d30 net/mptcp/options.c:1209 mptcpexpandseq net/mptcp/options.c:1030 [inline] mptcpexpandseq net/mptcp/protocol.h:864 [inline] ackupdatemsk net/mptcp/options.c:1060 [inline] mptcpincomingoptions+0x2036/0x3d30 net/mptcp/options.c:1209 tcpdataqueue+0xb4/0x7be0 net/ipv4/tcpinput.c:5233 tcprcvestablished+0x1061/0x2510 net/ipv4/tcpinput.c:6264 tcpv4dorcv+0x7f3/0x11a0 net/ipv4/tcpipv4.c:1916 tcpv4rcv+0x51df/0x5750 net/ipv4/tcpipv4.c:2351 ipprotocoldeliverrcu+0x2a3/0x13d0 net/ipv4/ipinput.c:205 iplocaldeliverfinish+0x336/0x500 net/ipv4/ipinput.c:233 NFHOOK include/linux/netfilter.h:314 [inline] iplocaldeliver+0x21f/0x490 net/ipv4/ipinput.c:254 dstinput include/net/dst.h:460 [inline] iprcvfinish+0x4a2/0x520 net/ipv4/ipinput.c:447 NFHOOK include/linux/netfilter.h:314 [inline] iprcv+0xcd/0x380 net/ipv4/ipinput.c:567 netifreceiveskbonecore net/core/dev.c:5704 [inline] netifreceiveskb+0x319/0xa00 net/core/dev.c:5817 processbacklog+0x4ad/0xa50 net/core/dev.c:6149 napipoll+0xe7/0x980 net/core/dev.c:6902 napipoll net/core/dev.c:6971 [inline] netrxaction+0xa5a/0x19b0 net/core/dev.c:7093 handlesoftirqs+0x1a0/0x7c0 kernel/softirq.c:561 dosoftirq+0x14/0x1a kernel/softirq.c:595 dosoftirq+0x9a/0x100 kernel/softirq.c:462 localbhenableip+0x9f/0xb0 kernel/softirq.c:389 localbhenable include/linux/bottomhalf.h:33 [inline] rcureadunlockbh include/linux/rcupdate.h:919 [inline] devqueuexmit+0x2758/0x57d0 net/core/dev.c:4493 devqueuexmit include/linux/netdevice.h:3168 [inline] neighhhoutput include/net/neighbour.h:523 [inline] neighoutput include/net/neighbour.h:537 [inline] ipfinishoutput2+0x187c/0x1b70 net/ipv4/ipoutput.c:236 ipfinishoutput+0x287/0x810 ipfinishoutput+0x4b/0x600 net/ipv4/ipoutput.c:324 NFHOOKCOND include/linux/netfilter.h:303 [inline] ipoutput+0x15f/0x3f0 net/ipv4/ipoutput.c:434 dstoutput include/net/dst.h:450 [inline] iplocalout net/ipv4/ipoutput.c:130 [inline] ipqueuexmit+0x1f2a/0x20d0 net/ipv4/ipoutput.c:536 ipqueuexmit+0x60/0x80 net/ipv4/ipoutput.c:550 tcptransmitskb+0x3cea/0x4900 net/ipv4/tcpoutput.c:1468 tcptransmitskb net/ipv4/tcpoutput.c:1486 [inline] tcpwritexmit+0x3b90/0x9070 net/ipv4/tcpoutput.c:2829 tcppushpendingframes+0xc4/0x380 net/ipv4/tcpoutput.c:3012 tcpsendfin+0x9f6/0xf50 net/ipv4/tcpoutput.c:3618 tcpclose+0x140c/0x1550 net/ipv4/tcp.c:3130 mptcpclosessk+0x74e/0x16f0 net/mptcp/protocol.c:2496 mptcpclosessk+0x26b/0x2c0 net/mptcp/protocol.c:2550 mptcppmnlrmaddrorsubflow+0x635/0xd10 net/mptcp/pmnetlink.c:889 mptcppmnlrmsubflowreceived net/mptcp/pmnetlink.c:924 [inline] mptcppmflushaddrsandsubflows net/mptcp/pmnetlink.c:1688 [inline] mptcpnlflushaddrslist net/mptcp/pmnetlink.c:1709 [inline] mptcppmnlflushaddrsdoit+0xe10/0x1630 net/mptcp/pmnetlink.c:1750 genlfamilyrcvmsgdoit net/netlink/genetlink.c:1115 [inline] ---truncated---