CVE-2026-74257: sockmap: Fix use-after-free in udp_bpf_recvmsg()

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

sockmap: Fix use-after-free in udpbpfrecvmsg()

syzbot reported use-after-free of struct skmsg in skmsgrecvmsg(). [0]

skmsgrecvmsg() peeks skmsg from psock->ingressmsg under a lock, but its processing is lockless.

Thus, skmsgrecvmsg() must be serialised by callers, otherwise multiple threads could touch the same skmsg.

For example, TCP uses locksock(), and AFUNIX uses unixsk(sk)->iolock.

Initially, udpbpfrecvmsg() had used locksock(), but the cited commit removed it.

Let's serialise skmsgrecvmsg() with locksock() in udpbpfrecvmsg().

Note that holding spinlockbh(&sk->skreceivequeue.lock) is not an option due to copypagetoiter() in skmsgrecvmsg().

[0]: BUG: KASAN: slab-use-after-free in skmsgrecvmsg+0xb54/0xc30 net/core/skmsg.c:428 Read of size 4 at addr ffff88814cdcf000 by task syz.0.24/6020

CPU: 1 UID: 0 PID: 6020 Comm: syz.0.24 Not tainted syzkaller #0 PREEMPT(full) Hardware name: Google Compute Engine/Google Compute Engine, BIOS Google 01/13/2026 Call Trace: <TASK> dumpstacklvl+0xe8/0x150 lib/dumpstack.c:120 printaddressdescription mm/kasan/report.c:378 [inline] printreport+0xba/0x230 mm/kasan/report.c:482 kasanreport+0x117/0x150 mm/kasan/report.c:595 skmsgrecvmsg+0xb54/0xc30 net/core/skmsg.c:428 udpbpfrecvmsg+0x4bd/0xe00 net/ipv4/udpbpf.c:84 inetrecvmsg+0x260/0x270 net/ipv4/afinet.c:891 sockrecvmsgnosec net/socket.c:1078 [inline] sockrecvmsg+0x1a8/0x270 net/socket.c:1100 sysrecvmsg+0x1e6/0x4a0 net/socket.c:2812 sysrecvmsg+0x215/0x590 net/socket.c:2854 dorecvmmsg+0x334/0x800 net/socket.c:2949 sysrecvmmsg net/socket.c:3023 [inline] dosysrecvmmsg net/socket.c:3046 [inline] sesysrecvmmsg net/socket.c:3039 [inline] x64sysrecvmmsg+0x198/0x250 net/socket.c:3039 dosyscallx64 arch/x86/entry/syscall64.c:63 [inline] dosyscall64+0xe2/0xf80 arch/x86/entry/syscall64.c:94 entrySYSCALL64afterhwframe+0x77/0x7f RIP: 0033:0x7fb319f9aeb9 Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48 RSP: 002b:00007fb31ad97028 EFLAGS: 00000246 ORIGRAX: 000000000000012b RAX: ffffffffffffffda RBX: 00007fb31a216090 RCX: 00007fb319f9aeb9 RDX: 0000000000000001 RSI: 0000200000000400 RDI: 0000000000000004 RBP: 00007fb31a008c1f R08: 0000000000000000 R09: 0000000000000000 R10: 0000000040000021 R11: 0000000000000246 R12: 0000000000000000 R13: 00007fb31a216128 R14: 00007fb31a216090 R15: 00007ffe21dd0a98 </TASK>

Allocated by task 6019: kasansavestack mm/kasan/common.c:57 [inline] kasansavetrack+0x3e/0x80 mm/kasan/common.c:78 poisonkmallocredzone mm/kasan/common.c:398 [inline] kasankmalloc+0x93/0xb0 mm/kasan/common.c:415 kasankmalloc include/linux/kasan.h:263 [inline] kmalloccachenoprof+0x3d1/0x6e0 mm/slub.c:5780 kmallocnoprof include/linux/slab.h:957 [inline] kzallocnoprof include/linux/slab.h:1094 [inline] allocskmsg net/core/skmsg.c:510 [inline] skpsockskbingressself+0x60/0x350 net/core/skmsg.c:612 skpsockverdictapply net/core/skmsg.c:1038 [inline] skpsockverdictrecv+0x7d9/0x8d0 net/core/skmsg.c:1236 udpreadskb+0x73e/0x7e0 net/ipv4/udp.c:2045 skpsockverdictdataready+0x12d/0x550 net/core/skmsg.c:1257 udpenqueuescheduleskb+0xc54/0x10b0 net/ipv4/udp.c:1789 udpqueuercvskb net/ipv4/udp.c:2346 [inline] udpqueuercvoneskb+0xac5/0x19c0 net/ipv4/udp.c:2475 udp4libmcastdeliver+0xc06/0xcf0 net/ipv4/udp.c:2585 udp4librcv+0x10f6/0x2620 net/ipv4/udp.c:2724 ipprotocoldeliverrcu+0x282/0x440 net/ipv4/ipinput.c:207 iplocaldeliverfinish+0x3bb/0x6f0 net/ipv4/ipinput.c:241 NFHOOK+0x336/0x3c0 include/linux/netfilter.h:318 dstinput include/net/dst.h:474 [inline] ipsublistrcvfinish+0x221/0x2a0 net/ipv4/ipinput.c:584 iplistrcvfinish net/ipv4/ipinp ---truncated---