CVE-2025-38491: mptcp: make fallback action and fallback decision atomic

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

mptcp: make fallback action and fallback decision atomic

Syzkaller reported the following splat:

WARNING: CPU: 1 PID: 7704 at net/mptcp/protocol.h:1223 mptcpdofallback net/mptcp/protocol.h:1223 [inline] WARNING: CPU: 1 PID: 7704 at net/mptcp/protocol.h:1223 mptcpdofallback net/mptcp/protocol.h:1244 [inline] WARNING: CPU: 1 PID: 7704 at net/mptcp/protocol.h:1223 checkfullyestablished net/mptcp/options.c:982 [inline] WARNING: CPU: 1 PID: 7704 at net/mptcp/protocol.h:1223 mptcpincomingoptions+0x21a8/0x2510 net/mptcp/options.c:1153 Modules linked in: CPU: 1 UID: 0 PID: 7704 Comm: syz.3.1419 Not tainted 6.16.0-rc3-gbd5ce2324dba #20 PREEMPT(voluntary) Hardware name: QEMU Ubuntu 24.04 PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014 RIP: 0010:mptcpdofallback net/mptcp/protocol.h:1223 [inline] RIP: 0010:mptcpdofallback net/mptcp/protocol.h:1244 [inline] RIP: 0010:checkfullyestablished net/mptcp/options.c:982 [inline] RIP: 0010:mptcpincomingoptions+0x21a8/0x2510 net/mptcp/options.c:1153 Code: 24 18 e8 bb 2a 00 fd e9 1b df ff ff e8 b1 21 0f 00 e8 ec 5f c4 fc 44 0f b7 ac 24 b0 00 00 00 e9 54 f1 ff ff e8 d9 5f c4 fc 90 <0f> 0b 90 e9 b8 f4 ff ff e8 8b 2a 00 fd e9 8d e6 ff ff e8 81 2a 00 RSP: 0018:ffff8880a3f08448 EFLAGS: 00010246 RAX: 0000000000000000 RBX: ffff8880180a8000 RCX: ffffffff84afcf45 RDX: ffff888090223700 RSI: ffffffff84afdaa7 RDI: 0000000000000001 RBP: ffff888017955780 R08: 0000000000000001 R09: 0000000000000000 R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000 R13: ffff8880180a8910 R14: ffff8880a3e9d058 R15: 0000000000000000 FS: 00005555791b8500(0000) GS:ffff88811c495000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 000000110c2800b7 CR3: 0000000058e44000 CR4: 0000000000350ef0 Call Trace: <IRQ> tcpreset+0x26f/0x2b0 net/ipv4/tcpinput.c:4432 tcpvalidateincoming+0x1057/0x1b60 net/ipv4/tcpinput.c:5975 tcprcvestablished+0x5b5/0x21f0 net/ipv4/tcpinput.c:6166 tcpv4dorcv+0x5dc/0xa70 net/ipv4/tcpipv4.c:1925 tcpv4rcv+0x3473/0x44a0 net/ipv4/tcpipv4.c:2363 ipprotocoldeliverrcu+0xba/0x480 net/ipv4/ipinput.c:205 iplocaldeliverfinish+0x2f1/0x500 net/ipv4/ipinput.c:233 NFHOOK include/linux/netfilter.h:317 [inline] NFHOOK include/linux/netfilter.h:311 [inline] iplocaldeliver+0x1be/0x560 net/ipv4/ipinput.c:254 dstinput include/net/dst.h:469 [inline] iprcvfinish net/ipv4/ipinput.c:447 [inline] NFHOOK include/linux/netfilter.h:317 [inline] NFHOOK include/linux/netfilter.h:311 [inline] iprcv+0x514/0x810 net/ipv4/ipinput.c:567 netifreceiveskbonecore+0x197/0x1e0 net/core/dev.c:5975 netifreceiveskb+0x1f/0x120 net/core/dev.c:6088 processbacklog+0x301/0x1360 net/core/dev.c:6440 napipoll.constprop.0+0xba/0x550 net/core/dev.c:7453 napipoll net/core/dev.c:7517 [inline] netrxaction+0xb44/0x1010 net/core/dev.c:7644 handlesoftirqs+0x1d0/0x770 kernel/softirq.c:579 dosoftirq+0x3f/0x90 kernel/softirq.c:480 </IRQ> <TASK> localbhenableip+0xed/0x110 kernel/softirq.c:407 localbhenable include/linux/bottomhalf.h:33 [inline] inetcsklistenstop+0x2c5/0x1070 net/ipv4/inetconnectionsock.c:1524 mptcpchecklistenstop.part.0+0x1cc/0x220 net/mptcp/protocol.c:2985 mptcpchecklistenstop net/mptcp/mib.h:118 [inline] mptcpclose+0x9b9/0xbd0 net/mptcp/protocol.c:3000 mptcpclose+0x2f/0x140 net/mptcp/protocol.c:3066 inetrelease+0xed/0x200 net/ipv4/afinet.c:435 inet6release+0x4f/0x70 net/ipv6/afinet6.c:487 sockrelease+0xb3/0x270 net/socket.c:649 sockclose+0x1c/0x30 net/socket.c:1439 fput+0x402/0xb70 fs/filetable.c:465 taskworkrun+0x150/0x240 kernel/taskwork.c:227 resumeusermodework include/linux/resumeusermode.h:50 [inline] exittousermodeloop+0xd4 ---truncated---