[PATCH bpf v4 0/5] bpf, sockmap: Fix af_unix null-ptr-deref in proto update

[PATCH bpf v4 0/5] bpf, sockmap: Fix af_unix null-ptr-deref in proto update

[Michal Luczaj]

Updating sockmap/sockhash using a unix sock races unix_stream_connect():
when sock_map_sk_state_allowed() passes (sk_state == TCP_ESTABLISHED),
unix_peer(sk) in unix_stream_bpf_update_proto() may still return NULL.

Signed-off-by: Michal Luczaj <mhal@rbox.co>
---
Changes in v4:
- Circle back to v1 approach
- More details in commit messages [Martin]
- Make unix iter take the state lock [Kaniyuki]
- Link to v3: https://lore.kernel.org/r/20260306-unix-proto-update-null-ptr-deref-v3-0-2f0c7410c523@rbox.co

Changes in v3:
- Drop sparse annotations [Martin]
- Keep lock_sock() along the unix_state_lock() [Kaniyuki]
- Unify BPF iter af_unix locking [Kaniyuki, Martin]
- Link to v2: https://lore.kernel.org/r/20260207-unix-proto-update-null-ptr-deref-v2-0-9f091330e7cd@rbox.co

Changes in v2:
- Instead of probing for unix peer, make sockmap take the right lock [Martin]
- Annotate data races [Kaniyuki, Martin]
- Extend bpf unix iter selftest to attempt a deadlock
- Link to v1: https://lore.kernel.org/r/20260129-unix-proto-update-null-ptr-deref-v1-1-e1daeb7012fd@rbox.co

To: John Fastabend <john.fastabend@gmail.com>
To: Jakub Sitnicki <jakub@cloudflare.com>
To: Eric Dumazet <edumazet@google.com>
To: Kuniyuki Iwashima <kuniyu@google.com>
To: Paolo Abeni <pabeni@redhat.com>
To: Willem de Bruijn <willemb@google.com>
To: "David S. Miller" <davem@davemloft.net>
To: Jakub Kicinski <kuba@kernel.org>
To: Simon Horman <horms@kernel.org>
To: Yonghong Song <yhs@fb.com>
To: Andrii Nakryiko <andrii@kernel.org>
To: Eduard Zingerman <eddyz87@gmail.com>
To: Alexei Starovoitov <ast@kernel.org>
To: Daniel Borkmann <daniel@iogearbox.net>
To: Martin KaFai Lau <martin.lau@linux.dev>
To: Song Liu <song@kernel.org>
To: Yonghong Song <yonghong.song@linux.dev>
To: KP Singh <kpsingh@kernel.org>
To: Stanislav Fomichev <sdf@fomichev.me>
To: Hao Luo <haoluo@google.com>
To: Jiri Olsa <jolsa@kernel.org>
To: Shuah Khan <shuah@kernel.org>
To: Cong Wang <cong.wang@bytedance.com>
Cc: netdev@vger.kernel.org
Cc: bpf@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-kselftest@vger.kernel.org

---
Michal Luczaj (5):
      bpf, sockmap: Annotate af_unix sock::sk_state data-races
      bpf, sockmap: Fix af_unix iter deadlock
      selftests/bpf: Extend bpf_iter_unix to attempt deadlocking
      bpf, sockmap: Fix af_unix null-ptr-deref in proto update
      bpf, sockmap: Take state lock for af_unix iter

 net/core/sock_map.c                               |  4 ++--
 net/unix/af_unix.c                                |  9 +++++----
 net/unix/unix_bpf.c                               |  3 +++
 tools/testing/selftests/bpf/progs/bpf_iter_unix.c | 10 ++++++++++
 4 files changed, 20 insertions(+), 6 deletions(-)
---
base-commit: 0f00132132937ca01a99feaf8985109a9087c9ff
change-id: 20260129-unix-proto-update-null-ptr-deref-6a2733bcbbf8

Best regards,
--  
Michal Luczaj <mhal@rbox.co>

[PATCH bpf v4 1/5] bpf, sockmap: Annotate af_unix sock::sk_state data-races

[Michal Luczaj]

sock_map_sk_state_allowed() and sock_map_redirect_allowed() read af_unix
socket sk_state locklessly.

Use READ_ONCE(). Note that for sock_map_redirect_allowed() change affects
not only af_unix, but all non-TCP sockets (UDP, af_vsock).

Suggested-by: Kuniyuki Iwashima <kuniyu@google.com>
Suggested-by: Martin KaFai Lau <martin.lau@linux.dev>
Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Signed-off-by: Michal Luczaj <mhal@rbox.co>
---
 net/core/sock_map.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/core/sock_map.c b/net/core/sock_map.c
index b0e96337a269..02a68be3002a 100644
--- a/net/core/sock_map.c
+++ b/net/core/sock_map.c
@@ -530,7 +530,7 @@ static bool sock_map_redirect_allowed(const struct sock *sk)
 	if (sk_is_tcp(sk))
 		return sk->sk_state != TCP_LISTEN;
 	else
-		return sk->sk_state == TCP_ESTABLISHED;
+		return READ_ONCE(sk->sk_state) == TCP_ESTABLISHED;
 }
 
 static bool sock_map_sk_is_suitable(const struct sock *sk)
@@ -543,7 +543,7 @@ static bool sock_map_sk_state_allowed(const struct sock *sk)
 	if (sk_is_tcp(sk))
 		return (1 << sk->sk_state) & (TCPF_ESTABLISHED | TCPF_LISTEN);
 	if (sk_is_stream_unix(sk))
-		return (1 << sk->sk_state) & TCPF_ESTABLISHED;
+		return (1 << READ_ONCE(sk->sk_state)) & TCPF_ESTABLISHED;
 	if (sk_is_vsock(sk) &&
 	    (sk->sk_type == SOCK_STREAM || sk->sk_type == SOCK_SEQPACKET))
 		return (1 << sk->sk_state) & TCPF_ESTABLISHED;

-- 
2.53.0

[PATCH bpf v4 2/5] bpf, sockmap: Fix af_unix iter deadlock

[Michal Luczaj]

bpf_iter_unix_seq_show() may deadlock when lock_sock_fast() takes the fast
path and the iter prog attempts to update a sockmap. Which ends up spinning
at sock_map_update_elem()'s bh_lock_sock():

WARNING: possible recursive locking detected
test_progs/1393 is trying to acquire lock:
ffff88811ec25f58 (slock-AF_UNIX){+...}-{3:3}, at: sock_map_update_elem+0xdb/0x1f0

but task is already holding lock:
ffff88811ec25f58 (slock-AF_UNIX){+...}-{3:3}, at: __lock_sock_fast+0x37/0xe0

other info that might help us debug this:
 Possible unsafe locking scenario:

       CPU0
       ----
  lock(slock-AF_UNIX);
  lock(slock-AF_UNIX);

 *** DEADLOCK ***

 May be due to missing lock nesting notation

4 locks held by test_progs/1393:
 #0: ffff88814b59c790 (&p->lock){+.+.}-{4:4}, at: bpf_seq_read+0x59/0x10d0
 #1: ffff88811ec25fd8 (sk_lock-AF_UNIX){+.+.}-{0:0}, at: bpf_seq_read+0x42c/0x10d0
 #2: ffff88811ec25f58 (slock-AF_UNIX){+...}-{3:3}, at: __lock_sock_fast+0x37/0xe0
 #3: ffffffff85a6a7c0 (rcu_read_lock){....}-{1:3}, at: bpf_iter_run_prog+0x51d/0xb00

Call Trace:
 dump_stack_lvl+0x5d/0x80
 print_deadlock_bug.cold+0xc0/0xce
 __lock_acquire+0x130f/0x2590
 lock_acquire+0x14e/0x2b0
 _raw_spin_lock+0x30/0x40
 sock_map_update_elem+0xdb/0x1f0
 bpf_prog_2d0075e5d9b721cd_dump_unix+0x55/0x4f4
 bpf_iter_run_prog+0x5b9/0xb00
 bpf_iter_unix_seq_show+0x1f7/0x2e0
 bpf_seq_read+0x42c/0x10d0
 vfs_read+0x171/0xb20
 ksys_read+0xff/0x200
 do_syscall_64+0x6b/0x3a0
 entry_SYSCALL_64_after_hwframe+0x76/0x7e

Suggested-by: Kuniyuki Iwashima <kuniyu@google.com>
Suggested-by: Martin KaFai Lau <martin.lau@linux.dev>
Fixes: 2c860a43dd77 ("bpf: af_unix: Implement BPF iterator for UNIX domain socket.")
Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Signed-off-by: Michal Luczaj <mhal@rbox.co>
---
 net/unix/af_unix.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index b23c33df8b46..590a30d3b2f7 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -3731,15 +3731,14 @@ static int bpf_iter_unix_seq_show(struct seq_file *seq, void *v)
 	struct bpf_prog *prog;
 	struct sock *sk = v;
 	uid_t uid;
-	bool slow;
 	int ret;
 
 	if (v == SEQ_START_TOKEN)
 		return 0;
 
-	slow = lock_sock_fast(sk);
+	lock_sock(sk);
 
-	if (unlikely(sk_unhashed(sk))) {
+	if (unlikely(sock_flag(sk, SOCK_DEAD))) {
 		ret = SEQ_SKIP;
 		goto unlock;
 	}
@@ -3749,7 +3748,7 @@ static int bpf_iter_unix_seq_show(struct seq_file *seq, void *v)
 	prog = bpf_iter_get_info(&meta, false);
 	ret = unix_prog_seq_show(prog, &meta, v, uid);
 unlock:
-	unlock_sock_fast(sk, slow);
+	release_sock(sk);
 	return ret;
 }
 

-- 
2.53.0

[PATCH bpf v4 3/5] selftests/bpf: Extend bpf_iter_unix to attempt deadlocking

[Michal Luczaj]

Updating a sockmap from a unix iterator prog may lead to a deadlock.
Piggyback on the original selftest.

Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Signed-off-by: Michal Luczaj <mhal@rbox.co>
---
 tools/testing/selftests/bpf/progs/bpf_iter_unix.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/tools/testing/selftests/bpf/progs/bpf_iter_unix.c b/tools/testing/selftests/bpf/progs/bpf_iter_unix.c
index fea275df9e22..a2652c8c3616 100644
--- a/tools/testing/selftests/bpf/progs/bpf_iter_unix.c
+++ b/tools/testing/selftests/bpf/progs/bpf_iter_unix.c
@@ -7,6 +7,13 @@
 
 char _license[] SEC("license") = "GPL";
 
+SEC(".maps") struct {
+	__uint(type, BPF_MAP_TYPE_SOCKMAP);
+	__uint(max_entries, 1);
+	__type(key, __u32);
+	__type(value, __u64);
+} sockmap;
+
 static long sock_i_ino(const struct sock *sk)
 {
 	const struct socket *sk_socket = sk->sk_socket;
@@ -76,5 +83,8 @@ int dump_unix(struct bpf_iter__unix *ctx)
 
 	BPF_SEQ_PRINTF(seq, "\n");
 
+	/* Test for deadlock. */
+	bpf_map_update_elem(&sockmap, &(int){0}, sk, 0);
+
 	return 0;
 }

-- 
2.53.0

[PATCH bpf v4 4/5] bpf, sockmap: Fix af_unix null-ptr-deref in proto update

[Michal Luczaj]

unix_stream_connect() sets sk_state (`WRITE_ONCE(sk->sk_state,
TCP_ESTABLISHED)`) _before_ it assigns a peer (`unix_peer(sk) = newsk`).
sk_state == TCP_ESTABLISHED makes sock_map_sk_state_allowed() believe that
socket is properly set up, which would include having a defined peer. IOW,
there's a window when unix_stream_bpf_update_proto() can be called on
socket which still has unix_peer(sk) == NULL.

         CPU0 bpf                            CPU1 connect
         --------                            ------------

                                WRITE_ONCE(sk->sk_state, TCP_ESTABLISHED)
sock_map_sk_state_allowed(sk)
...
sk_pair = unix_peer(sk)
sock_hold(sk_pair)
                                sock_hold(newsk)
                                smp_mb__after_atomic()
                                unix_peer(sk) = newsk

BUG: kernel NULL pointer dereference, address: 0000000000000080
RIP: 0010:unix_stream_bpf_update_proto+0xa0/0x1b0
Call Trace:
  sock_map_link+0x564/0x8b0
  sock_map_update_common+0x6e/0x340
  sock_map_update_elem_sys+0x17d/0x240
  __sys_bpf+0x26db/0x3250
  __x64_sys_bpf+0x21/0x30
  do_syscall_64+0x6b/0x3a0
  entry_SYSCALL_64_after_hwframe+0x76/0x7e

Initial idea was to move peer assignment _before_ the sk_state update[1],
but that involved an additional memory barrier, and changing the hot path
was rejected.
Then a NULL check during proto update in unix_stream_bpf_update_proto() was
considered[2], but the follow-up discussion[3] focused on the root cause,
i.e. sockmap update taking a wrong lock. Or, more specifically, missing
unix_state_lock()[4].
In the end it was concluded that teaching sockmap about the af_unix locking
would be unnecessarily complex[5].
Complexity aside, since BPF_PROG_TYPE_SCHED_CLS and BPF_PROG_TYPE_SCHED_ACT
are allowed to update sockmaps, sock_map_update_elem() taking the unix
lock, as it is currently implemented in unix_state_lock():
spin_lock(&unix_sk(s)->lock), would be problematic. unix_state_lock() taken
in a process context, followed by a softirq-context TC BPF program
attempting to take the same spinlock -- deadlock[6].
This way we circled back to the peer check idea[2].

[1]: https://lore.kernel.org/netdev/ba5c50aa-1df4-40c2-ab33-a72022c5a32e@rbox.co/
[2]: https://lore.kernel.org/netdev/20240610174906.32921-1-kuniyu@amazon.com/
[3]: https://lore.kernel.org/netdev/7603c0e6-cd5b-452b-b710-73b64bd9de26@linux.dev/
[4]: https://lore.kernel.org/netdev/CAAVpQUA+8GL_j63CaKb8hbxoL21izD58yr1NvhOhU=j+35+3og@mail.gmail.com/
[5]: https://lore.kernel.org/bpf/CAAVpQUAHijOMext28Gi10dSLuMzGYh+jK61Ujn+fZ-wvcODR2A@mail.gmail.com/
[6]: https://lore.kernel.org/bpf/dd043c69-4d03-46fe-8325-8f97101435cf@linux.dev/

Summary of scenarios where af_unix/stream connect() may race a sockmap
update:

1. connect() vs. bpf(BPF_MAP_UPDATE_ELEM), i.e. sock_map_update_elem_sys()

   Implemented NULL check is sufficient. Once assigned, socket peer won't
   be released until socket fd is released. And that's not an issue because
   sock_map_update_elem_sys() bumps fd refcnf.

2. connect() vs BPF program doing update

   Update restricted per verifier.c:may_update_sockmap() to

      BPF_PROG_TYPE_TRACING/BPF_TRACE_ITER
      BPF_PROG_TYPE_SOCK_OPS (bpf_sock_map_update() only)
      BPF_PROG_TYPE_SOCKET_FILTER
      BPF_PROG_TYPE_SCHED_CLS
      BPF_PROG_TYPE_SCHED_ACT
      BPF_PROG_TYPE_XDP
      BPF_PROG_TYPE_SK_REUSEPORT
      BPF_PROG_TYPE_FLOW_DISSECTOR
      BPF_PROG_TYPE_SK_LOOKUP

   Plus one more race to consider:

            CPU0 bpf                            CPU1 connect
            --------                            ------------

                                   WRITE_ONCE(sk->sk_state, TCP_ESTABLISHED)
   sock_map_sk_state_allowed(sk)
                                   sock_hold(newsk)
                                   smp_mb__after_atomic()
                                   unix_peer(sk) = newsk
   sk_pair = unix_peer(sk)
   if (unlikely(!sk_pair))
      return -EINVAL;

                                                 CPU1 close
                                                 ----------

                                   skpair = unix_peer(sk);
                                   unix_peer(sk) = NULL;
                                   sock_put(skpair)
   // use after free?
   sock_hold(sk_pair)

   2.1 BPF program invoking helper function bpf_sock_map_update() ->
       BPF_CALL_4(bpf_sock_map_update(), ...)

       Helper limited to BPF_PROG_TYPE_SOCK_OPS. Nevertheless, a unix sock
       might be accessible via bpf_map_lookup_elem(). Which implies sk
       already having psock, which in turn implies sk already having
       sk_pair. Since sk_psock_destroy() is queued as RCU work, sk_pair
       won't go away while BPF executes the update.

   2.2 BPF program invoking helper function bpf_map_update_elem() ->
       sock_map_update_elem()

       2.2.1 Unix sock accessible to BPF prog only via sockmap lookup in
             BPF_PROG_TYPE_SOCKET_FILTER, BPF_PROG_TYPE_SCHED_CLS,
             BPF_PROG_TYPE_SCHED_ACT, BPF_PROG_TYPE_XDP,
             BPF_PROG_TYPE_SK_REUSEPORT, BPF_PROG_TYPE_FLOW_DISSECTOR,
             BPF_PROG_TYPE_SK_LOOKUP.

             Pretty much the same as case 2.1.

       2.2.2 Unix sock accessible to BPF program directly:
             BPF_PROG_TYPE_TRACING, narrowed down to BPF_TRACE_ITER.

             Sockmap iterator (sock_map_seq_ops) is safe: unix sock
             residing in a sockmap means that the sock already went through
             the proto update step.

             Unix sock iterator (bpf_iter_unix_seq_ops), on the other hand,
             gives access to socks that may still be unconnected. Which
             means iterator prog can race sockmap/proto update against
             connect().

             BUG: KASAN: null-ptr-deref in unix_stream_bpf_update_proto+0x253/0x4d0
             Write of size 4 at addr 0000000000000080 by task test_progs/3140
             Call Trace:
              dump_stack_lvl+0x5d/0x80
              kasan_report+0xe4/0x1c0
              kasan_check_range+0x125/0x200
              unix_stream_bpf_update_proto+0x253/0x4d0
              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

             While the introduced NULL check prevents null-ptr-deref in the
             BPF program path as well, it is insufficient to guard against
             a poorly timed close() leading to a use-after-free. This will
             be addressed in a subsequent patch.

Reported-by: Michal Luczaj <mhal@rbox.co>
Closes: https://lore.kernel.org/netdev/ba5c50aa-1df4-40c2-ab33-a72022c5a32e@rbox.co/
Reported-by: 钱一铭 <yimingqian591@gmail.com>
Suggested-by: Kuniyuki Iwashima <kuniyu@google.com>
Suggested-by: Martin KaFai Lau <martin.lau@linux.dev>
Fixes: c63829182c37 ("af_unix: Implement ->psock_update_sk_prot()")
Signed-off-by: Michal Luczaj <mhal@rbox.co>
---
 net/unix/unix_bpf.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/unix/unix_bpf.c b/net/unix/unix_bpf.c
index e0d30d6d22ac..57f3124c9d8d 100644
--- a/net/unix/unix_bpf.c
+++ b/net/unix/unix_bpf.c
@@ -185,6 +185,9 @@ int unix_stream_bpf_update_proto(struct sock *sk, struct sk_psock *psock, bool r
 	 */
 	if (!psock->sk_pair) {
 		sk_pair = unix_peer(sk);
+		if (unlikely(!sk_pair))
+			return -EINVAL;
+
 		sock_hold(sk_pair);
 		psock->sk_pair = sk_pair;
 	}

-- 
2.53.0

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

[Michal Luczaj]

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@google.com>
Fixes: 2c860a43dd77 ("bpf: af_unix: Implement BPF iterator for UNIX domain socket.")
Signed-off-by: Michal Luczaj <mhal@rbox.co>
---
 net/unix/af_unix.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 590a30d3b2f7..15b48cc6e9b0 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -3737,6 +3737,7 @@ static int bpf_iter_unix_seq_show(struct seq_file *seq, void *v)
 		return 0;
 
 	lock_sock(sk);
+	unix_state_lock(sk);
 
 	if (unlikely(sock_flag(sk, SOCK_DEAD))) {
 		ret = SEQ_SKIP;
@@ -3748,6 +3749,7 @@ static int bpf_iter_unix_seq_show(struct seq_file *seq, void *v)
 	prog = bpf_iter_get_info(&meta, false);
 	ret = unix_prog_seq_show(prog, &meta, v, uid);
 unlock:
+	unix_state_unlock(sk);
 	release_sock(sk);
 	return ret;
 }

-- 
2.53.0

Re: [PATCH bpf v4 4/5] bpf, sockmap: Fix af_unix null-ptr-deref in proto update

[Kuniyuki Iwashima]

On Tue, Apr 14, 2026 at 7:13 AM Michal Luczaj <mhal@rbox.co> wrote:
>
> unix_stream_connect() sets sk_state (`WRITE_ONCE(sk->sk_state,
> TCP_ESTABLISHED)`) _before_ it assigns a peer (`unix_peer(sk) = newsk`).
> sk_state == TCP_ESTABLISHED makes sock_map_sk_state_allowed() believe that
> socket is properly set up, which would include having a defined peer. IOW,
> there's a window when unix_stream_bpf_update_proto() can be called on
> socket which still has unix_peer(sk) == NULL.
>
>          CPU0 bpf                            CPU1 connect
>          --------                            ------------
>
>                                 WRITE_ONCE(sk->sk_state, TCP_ESTABLISHED)
> sock_map_sk_state_allowed(sk)
> ...
> sk_pair = unix_peer(sk)
> sock_hold(sk_pair)
>                                 sock_hold(newsk)
>                                 smp_mb__after_atomic()
>                                 unix_peer(sk) = newsk
>
> BUG: kernel NULL pointer dereference, address: 0000000000000080
> RIP: 0010:unix_stream_bpf_update_proto+0xa0/0x1b0
> Call Trace:
>   sock_map_link+0x564/0x8b0
>   sock_map_update_common+0x6e/0x340
>   sock_map_update_elem_sys+0x17d/0x240
>   __sys_bpf+0x26db/0x3250
>   __x64_sys_bpf+0x21/0x30
>   do_syscall_64+0x6b/0x3a0
>   entry_SYSCALL_64_after_hwframe+0x76/0x7e
>
> Initial idea was to move peer assignment _before_ the sk_state update[1],
> but that involved an additional memory barrier, and changing the hot path
> was rejected.
> Then a NULL check during proto update in unix_stream_bpf_update_proto() was
> considered[2], but the follow-up discussion[3] focused on the root cause,
> i.e. sockmap update taking a wrong lock. Or, more specifically, missing
> unix_state_lock()[4].
> In the end it was concluded that teaching sockmap about the af_unix locking
> would be unnecessarily complex[5].
> Complexity aside, since BPF_PROG_TYPE_SCHED_CLS and BPF_PROG_TYPE_SCHED_ACT
> are allowed to update sockmaps, sock_map_update_elem() taking the unix
> lock, as it is currently implemented in unix_state_lock():
> spin_lock(&unix_sk(s)->lock), would be problematic. unix_state_lock() taken
> in a process context, followed by a softirq-context TC BPF program
> attempting to take the same spinlock -- deadlock[6].
> This way we circled back to the peer check idea[2].
>
> [1]: https://lore.kernel.org/netdev/ba5c50aa-1df4-40c2-ab33-a72022c5a32e@rbox.co/
> [2]: https://lore.kernel.org/netdev/20240610174906.32921-1-kuniyu@amazon.com/
> [3]: https://lore.kernel.org/netdev/7603c0e6-cd5b-452b-b710-73b64bd9de26@linux.dev/
> [4]: https://lore.kernel.org/netdev/CAAVpQUA+8GL_j63CaKb8hbxoL21izD58yr1NvhOhU=j+35+3og@mail.gmail.com/
> [5]: https://lore.kernel.org/bpf/CAAVpQUAHijOMext28Gi10dSLuMzGYh+jK61Ujn+fZ-wvcODR2A@mail.gmail.com/
> [6]: https://lore.kernel.org/bpf/dd043c69-4d03-46fe-8325-8f97101435cf@linux.dev/
>
> Summary of scenarios where af_unix/stream connect() may race a sockmap
> update:
>
> 1. connect() vs. bpf(BPF_MAP_UPDATE_ELEM), i.e. sock_map_update_elem_sys()
>
>    Implemented NULL check is sufficient. Once assigned, socket peer won't
>    be released until socket fd is released. And that's not an issue because
>    sock_map_update_elem_sys() bumps fd refcnf.
>
> 2. connect() vs BPF program doing update
>
>    Update restricted per verifier.c:may_update_sockmap() to
>
>       BPF_PROG_TYPE_TRACING/BPF_TRACE_ITER
>       BPF_PROG_TYPE_SOCK_OPS (bpf_sock_map_update() only)
>       BPF_PROG_TYPE_SOCKET_FILTER
>       BPF_PROG_TYPE_SCHED_CLS
>       BPF_PROG_TYPE_SCHED_ACT
>       BPF_PROG_TYPE_XDP
>       BPF_PROG_TYPE_SK_REUSEPORT
>       BPF_PROG_TYPE_FLOW_DISSECTOR
>       BPF_PROG_TYPE_SK_LOOKUP
>
>    Plus one more race to consider:
>
>             CPU0 bpf                            CPU1 connect
>             --------                            ------------
>
>                                    WRITE_ONCE(sk->sk_state, TCP_ESTABLISHED)
>    sock_map_sk_state_allowed(sk)
>                                    sock_hold(newsk)
>                                    smp_mb__after_atomic()
>                                    unix_peer(sk) = newsk
>    sk_pair = unix_peer(sk)
>    if (unlikely(!sk_pair))
>       return -EINVAL;
>
>                                                  CPU1 close
>                                                  ----------
>
>                                    skpair = unix_peer(sk);
>                                    unix_peer(sk) = NULL;
>                                    sock_put(skpair)
>    // use after free?
>    sock_hold(sk_pair)
>
>    2.1 BPF program invoking helper function bpf_sock_map_update() ->
>        BPF_CALL_4(bpf_sock_map_update(), ...)
>
>        Helper limited to BPF_PROG_TYPE_SOCK_OPS. Nevertheless, a unix sock
>        might be accessible via bpf_map_lookup_elem(). Which implies sk
>        already having psock, which in turn implies sk already having
>        sk_pair. Since sk_psock_destroy() is queued as RCU work, sk_pair
>        won't go away while BPF executes the update.
>
>    2.2 BPF program invoking helper function bpf_map_update_elem() ->
>        sock_map_update_elem()
>
>        2.2.1 Unix sock accessible to BPF prog only via sockmap lookup in
>              BPF_PROG_TYPE_SOCKET_FILTER, BPF_PROG_TYPE_SCHED_CLS,
>              BPF_PROG_TYPE_SCHED_ACT, BPF_PROG_TYPE_XDP,
>              BPF_PROG_TYPE_SK_REUSEPORT, BPF_PROG_TYPE_FLOW_DISSECTOR,
>              BPF_PROG_TYPE_SK_LOOKUP.
>
>              Pretty much the same as case 2.1.
>
>        2.2.2 Unix sock accessible to BPF program directly:
>              BPF_PROG_TYPE_TRACING, narrowed down to BPF_TRACE_ITER.
>
>              Sockmap iterator (sock_map_seq_ops) is safe: unix sock
>              residing in a sockmap means that the sock already went through
>              the proto update step.
>
>              Unix sock iterator (bpf_iter_unix_seq_ops), on the other hand,
>              gives access to socks that may still be unconnected. Which
>              means iterator prog can race sockmap/proto update against
>              connect().
>
>              BUG: KASAN: null-ptr-deref in unix_stream_bpf_update_proto+0x253/0x4d0
>              Write of size 4 at addr 0000000000000080 by task test_progs/3140
>              Call Trace:
>               dump_stack_lvl+0x5d/0x80
>               kasan_report+0xe4/0x1c0
>               kasan_check_range+0x125/0x200
>               unix_stream_bpf_update_proto+0x253/0x4d0
>               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
>
>              While the introduced NULL check prevents null-ptr-deref in the
>              BPF program path as well, it is insufficient to guard against
>              a poorly timed close() leading to a use-after-free. This will
>              be addressed in a subsequent patch.
>
> Reported-by: Michal Luczaj <mhal@rbox.co>
> Closes: https://lore.kernel.org/netdev/ba5c50aa-1df4-40c2-ab33-a72022c5a32e@rbox.co/
> Reported-by: 钱一铭 <yimingqian591@gmail.com>
> Suggested-by: Kuniyuki Iwashima <kuniyu@google.com>
> Suggested-by: Martin KaFai Lau <martin.lau@linux.dev>
> Fixes: c63829182c37 ("af_unix: Implement ->psock_update_sk_prot()")
> Signed-off-by: Michal Luczaj <mhal@rbox.co>

Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>

Re: [PATCH bpf v4 3/5] selftests/bpf: Extend bpf_iter_unix to attempt deadlocking

[Kuniyuki Iwashima]

On Tue, Apr 14, 2026 at 7:13 AM Michal Luczaj <mhal@rbox.co> wrote:
>
> Updating a sockmap from a unix iterator prog may lead to a deadlock.
> Piggyback on the original selftest.
>
> Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>
> Signed-off-by: Michal Luczaj <mhal@rbox.co>

Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>

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@rbox.co> 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@google.com>
> Fixes: 2c860a43dd77 ("bpf: af_unix: Implement BPF iterator for UNIX domain socket.")
> Signed-off-by: Michal Luczaj <mhal@rbox.co>

Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>

Thanks for the fixes, Michal !

Re: [PATCH bpf v4 0/5] bpf, sockmap: Fix af_unix null-ptr-deref in proto update

[patchwork-bot+netdevbpf]

Hello:

This series was applied to bpf/bpf.git (master)
by Martin KaFai Lau <martin.lau@kernel.org>:

On Tue, 14 Apr 2026 16:13:14 +0200 you wrote:
> Updating sockmap/sockhash using a unix sock races unix_stream_connect():
> when sock_map_sk_state_allowed() passes (sk_state == TCP_ESTABLISHED),
> unix_peer(sk) in unix_stream_bpf_update_proto() may still return NULL.
> 
> Signed-off-by: Michal Luczaj <mhal@rbox.co>
> ---
> Changes in v4:
> - Circle back to v1 approach
> - More details in commit messages [Martin]
> - Make unix iter take the state lock [Kaniyuki]
> - Link to v3: https://lore.kernel.org/r/20260306-unix-proto-update-null-ptr-deref-v3-0-2f0c7410c523@rbox.co
> 
> [...]

Here is the summary with links:
  - [bpf,v4,1/5] bpf, sockmap: Annotate af_unix sock::sk_state data-races
    https://git.kernel.org/bpf/bpf/c/a25566084e39
  - [bpf,v4,2/5] bpf, sockmap: Fix af_unix iter deadlock
    https://git.kernel.org/bpf/bpf/c/4d328dd69538
  - [bpf,v4,3/5] selftests/bpf: Extend bpf_iter_unix to attempt deadlocking
    https://git.kernel.org/bpf/bpf/c/997b8483d44c
  - [bpf,v4,4/5] bpf, sockmap: Fix af_unix null-ptr-deref in proto update
    https://git.kernel.org/bpf/bpf/c/dca38b7734d2
  - [bpf,v4,5/5] bpf, sockmap: Take state lock for af_unix iter
    https://git.kernel.org/bpf/bpf/c/64c2f93fc325

You are awesome, thank you!
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