Subject: Re: [PATCH v4 bpf/net 3/6] sockmap: Fix use-after-free in udp_bpf_recvmsg().

From: Jakub Sitnicki <[email-masked]>
Date: Thu, 05 Mar 2026 12:39:20 +0100

On Sat, Feb 21, 2026 at 11:30 PM GMT, Kuniyuki Iwashima wrote:
> syzbot reported use-after-free of struct sk_msg in sk_msg_recvmsg(). [0]
>
> sk_msg_recvmsg() peeks sk_msg from psock->ingress_msg under a lock,
> but its processing is lockless.
>
> Thus, sk_msg_recvmsg() must be serialised by callers, otherwise
> multiple threads could touch the same sk_msg.
>
> For example, TCP uses lock_sock(), and AF_UNIX uses unix_sk(sk)->iolock.
>
> Initially, udp_bpf_recvmsg() had used lock_sock(), but the cited
> commit accidentally removed it.

FWIW, it doesn't sound like commit 9f2470fbc4cb ("skmsg: Improve
udp_bpf_recvmsg() accuracy") removed it by accident. The commit message
calls it out explicitly:

    Also, UDP does not lock the sock during BH Rx path, it makes
    no sense for its ->recvmsg() to lock the sock. It is always
    possible for ->recvmsg() to be called before packets actually
    arrive in the receive queue, we just use best effort to make
    it accurate here.

Looks like we just didn't understand the consequences at that time.

>
> Let's serialise sk_msg_recvmsg() with lock_sock() in udp_bpf_recvmsg().
>
> Note that holding spin_lock_bh(&sk->sk_receive_queue.lock) is not
> an option due to copy_page_to_iter() in sk_msg_recvmsg().
>
> [0]:
> BUG: KASAN: slab-use-after-free in sk_msg_recvmsg+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 Google Compute Engine/Google Compute Engine, BIOS Google 01/13/2026
> Call Trace:
>  <TASK>
>  dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
>  print_address_description mm/kasan/report.c:378 [inline]
>  print_report+0xba/0x230 mm/kasan/report.c:482
>  kasan_report+0x117/0x150 mm/kasan/report.c:595
>  sk_msg_recvmsg+0xb54/0xc30 net/core/skmsg.c:428
>  udp_bpf_recvmsg+0x4bd/0xe00 net/ipv4/udp_bpf.c:84
>  inet_recvmsg+0x260/0x270 net/ipv4/af_inet.c:891
>  sock_recvmsg_nosec net/socket.c:1078 [inline]
>  sock_recvmsg+0x1a8/0x270 net/socket.c:1100
>  ____sys_recvmsg+0x1e6/0x4a0 net/socket.c:2812
>  ___sys_recvmsg+0x215/0x590 net/socket.c:2854
>  do_recvmmsg+0x334/0x800 net/socket.c:2949
>  __sys_recvmmsg net/socket.c:3023 [inline]
>  __do_sys_recvmmsg net/socket.c:3046 [inline]
>  __se_sys_recvmmsg net/socket.c:3039 [inline]
>  __x64_sys_recvmmsg+0x198/0x250 net/socket.c:3039
>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>  do_syscall_64+0xe2/0xf80 arch/x86/entry/syscall_64.c:94
>  entry_SYSCALL_64_after_hwframe+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 ORIG_RAX: 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:
>  kasan_save_stack mm/kasan/common.c:57 [inline]
>  kasan_save_track+0x3e/0x80 mm/kasan/common.c:78
>  poison_kmalloc_redzone mm/kasan/common.c:398 [inline]
>  __kasan_kmalloc+0x93/0xb0 mm/kasan/common.c:415
>  kasan_kmalloc include/linux/kasan.h:263 [inline]
>  __kmalloc_cache_noprof+0x3d1/0x6e0 mm/slub.c:5780
>  kmalloc_noprof include/linux/slab.h:957 [inline]
>  kzalloc_noprof include/linux/slab.h:1094 [inline]
>  alloc_sk_msg net/core/skmsg.c:510 [inline]
>  sk_psock_skb_ingress_self+0x60/0x350 net/core/skmsg.c:612
>  sk_psock_verdict_apply net/core/skmsg.c:1038 [inline]
>  sk_psock_verdict_recv+0x7d9/0x8d0 net/core/skmsg.c:1236
>  udp_read_skb+0x73e/0x7e0 net/ipv4/udp.c:2045
>  sk_psock_verdict_data_ready+0x12d/0x550 net/core/skmsg.c:1257
>  __udp_enqueue_schedule_skb+0xc54/0x10b0 net/ipv4/udp.c:1789
>  __udp_queue_rcv_skb net/ipv4/udp.c:2346 [inline]
>  udp_queue_rcv_one_skb+0xac5/0x19c0 net/ipv4/udp.c:2475
>  __udp4_lib_mcast_deliver+0xc06/0xcf0 net/ipv4/udp.c:2585
>  __udp4_lib_rcv+0x10f6/0x2620 net/ipv4/udp.c:2724
>  ip_protocol_deliver_rcu+0x282/0x440 net/ipv4/ip_input.c:207
>  ip_local_deliver_finish+0x3bb/0x6f0 net/ipv4/ip_input.c:241
>  NF_HOOK+0x336/0x3c0 include/linux/netfilter.h:318
>  dst_input include/net/dst.h:474 [inline]
>  ip_sublist_rcv_finish+0x221/0x2a0 net/ipv4/ip_input.c:584
>  ip_list_rcv_finish net/ipv4/ip_input.c:628 [inline]
>  ip_sublist_rcv+0x5c6/0xa70 net/ipv4/ip_input.c:644
>  ip_list_rcv+0x3f1/0x450 net/ipv4/ip_input.c:678
>  __netif_receive_skb_list_ptype net/core/dev.c:6195 [inline]
>  __netif_receive_skb_list_core+0x7e5/0x810 net/core/dev.c:6242
>  __netif_receive_skb_list net/core/dev.c:6294 [inline]
>  netif_receive_skb_list_internal+0x995/0xcf0 net/core/dev.c:6385
>  netif_receive_skb_list+0x54/0x410 net/core/dev.c:6437
>  xdp_recv_frames net/bpf/test_run.c:269 [inline]
>  xdp_test_run_batch net/bpf/test_run.c:350 [inline]
>  bpf_test_run_xdp_live+0x1946/0x1cf0 net/bpf/test_run.c:379
>  bpf_prog_test_run_xdp+0x81c/0x1160 net/bpf/test_run.c:1396
>  bpf_prog_test_run+0x2c7/0x340 kernel/bpf/syscall.c:4703
>  __sys_bpf+0x5cb/0x920 kernel/bpf/syscall.c:6182
>  __do_sys_bpf kernel/bpf/syscall.c:6274 [inline]
>  __se_sys_bpf kernel/bpf/syscall.c:6272 [inline]
>  __x64_sys_bpf+0x7c/0x90 kernel/bpf/syscall.c:6272
>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>  do_syscall_64+0xe2/0xf80 arch/x86/entry/syscall_64.c:94
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
> Freed by task 6021:
>  kasan_save_stack mm/kasan/common.c:57 [inline]
>  kasan_save_track+0x3e/0x80 mm/kasan/common.c:78
>  kasan_save_free_info+0x46/0x50 mm/kasan/generic.c:584
>  poison_slab_object mm/kasan/common.c:253 [inline]
>  __kasan_slab_free+0x5c/0x80 mm/kasan/common.c:285
>  kasan_slab_free include/linux/kasan.h:235 [inline]
>  slab_free_hook mm/slub.c:2540 [inline]
>  slab_free mm/slub.c:6674 [inline]
>  kfree+0x1be/0x650 mm/slub.c:6882
>  kfree_sk_msg include/linux/skmsg.h:385 [inline]
>  sk_msg_recvmsg+0xaa8/0xc30 net/core/skmsg.c:483
>  udp_bpf_recvmsg+0x4bd/0xe00 net/ipv4/udp_bpf.c:84
>  inet_recvmsg+0x260/0x270 net/ipv4/af_inet.c:891
>  sock_recvmsg_nosec net/socket.c:1078 [inline]
>  sock_recvmsg+0x1a8/0x270 net/socket.c:1100
>  ____sys_recvmsg+0x1e6/0x4a0 net/socket.c:2812
>  ___sys_recvmsg+0x215/0x590 net/socket.c:2854
>  do_recvmmsg+0x334/0x800 net/socket.c:2949
>  __sys_recvmmsg net/socket.c:3023 [inline]
>  __do_sys_recvmmsg net/socket.c:3046 [inline]
>  __se_sys_recvmmsg net/socket.c:3039 [inline]
>  __x64_sys_recvmmsg+0x198/0x250 net/socket.c:3039
>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>  do_syscall_64+0xe2/0xf80 arch/x86/entry/syscall_64.c:94
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
> Fixes: 9f2470fbc4cb ("skmsg: Improve udp_bpf_recvmsg() accuracy")
> Reported-by: [email-masked]
> Closes: https://lore.kernel.org/netdev/[email-masked]/
> Signed-off-by: Kuniyuki Iwashima <[email-masked]>
> ---

Reviewed-by: Jakub Sitnicki <[email-masked]>