Re: [PATCH bpf v4 5/5] bpf, sockmap: Take state lock for af_unix iter

[Kuniyuki Iwashima]

On Tue, Apr 14, 2026 at 7:13 AM Michal Luczaj <mhal@xxxxxxx> wrote:
>
> When a BPF iterator program updates a sockmap, there is a race condition in
> unix_stream_bpf_update_proto() where the `peer` pointer can become stale[1]
> during a state transition TCP_ESTABLISHED -> TCP_CLOSE.
>
>         CPU0 bpf                          CPU1 close
>         --------                          ----------
> // unix_stream_bpf_update_proto()
> sk_pair = unix_peer(sk)
> if (unlikely(!sk_pair))
>    return -EINVAL;
>                                      // unix_release_sock()
>                                      skpair = unix_peer(sk);
>                                      unix_peer(sk) = NULL;
>                                      sock_put(skpair)
> sock_hold(sk_pair) // UaF
>
> More practically, this fix guarantees that the iterator program is
> consistently provided with a unix socket that remains stable during
> iterator execution.
>
> [1]:
> BUG: KASAN: slab-use-after-free in unix_stream_bpf_update_proto+0x155/0x490
> Write of size 4 at addr ffff8881178c9a00 by task test_progs/2231
> Call Trace:
>  dump_stack_lvl+0x5d/0x80
>  print_report+0x170/0x4f3
>  kasan_report+0xe4/0x1c0
>  kasan_check_range+0x125/0x200
>  unix_stream_bpf_update_proto+0x155/0x490
>  sock_map_link+0x71c/0xec0
>  sock_map_update_common+0xbc/0x600
>  sock_map_update_elem+0x19a/0x1f0
>  bpf_prog_bbbf56096cdd4f01_selective_dump_unix+0x20c/0x217
>  bpf_iter_run_prog+0x21e/0xae0
>  bpf_iter_unix_seq_show+0x1e0/0x2a0
>  bpf_seq_read+0x42c/0x10d0
>  vfs_read+0x171/0xb20
>  ksys_read+0xff/0x200
>  do_syscall_64+0xf7/0x5e0
>  entry_SYSCALL_64_after_hwframe+0x76/0x7e
>
> Allocated by task 2236:
>  kasan_save_stack+0x30/0x50
>  kasan_save_track+0x14/0x30
>  __kasan_slab_alloc+0x63/0x80
>  kmem_cache_alloc_noprof+0x1d5/0x680
>  sk_prot_alloc+0x59/0x210
>  sk_alloc+0x34/0x470
>  unix_create1+0x86/0x8a0
>  unix_stream_connect+0x318/0x15b0
>  __sys_connect+0xfd/0x130
>  __x64_sys_connect+0x72/0xd0
>  do_syscall_64+0xf7/0x5e0
>  entry_SYSCALL_64_after_hwframe+0x76/0x7e
>
> Freed by task 2236:
>  kasan_save_stack+0x30/0x50
>  kasan_save_track+0x14/0x30
>  kasan_save_free_info+0x3b/0x70
>  __kasan_slab_free+0x47/0x70
>  kmem_cache_free+0x11c/0x590
>  __sk_destruct+0x432/0x6e0
>  unix_release_sock+0x9b3/0xf60
>  unix_release+0x8a/0xf0
>  __sock_release+0xb0/0x270
>  sock_close+0x18/0x20
>  __fput+0x36e/0xac0
>  fput_close_sync+0xe5/0x1a0
>  __x64_sys_close+0x7d/0xd0
>  do_syscall_64+0xf7/0x5e0
>  entry_SYSCALL_64_after_hwframe+0x76/0x7e
>
> Suggested-by: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx>
> Fixes: 2c860a43dd77 ("bpf: af_unix: Implement BPF iterator for UNIX domain socket.")
> Signed-off-by: Michal Luczaj <mhal@xxxxxxx>

Reviewed-by: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx>

Thanks for the fixes, Michal !