[PATCH v2] bpf, sockmap: Fix cork ownership in tcp_bpf_sendmsg()

[Chengfeng Ye <nicoyip.dev@gmail.com>]

tcp_bpf_sendmsg() keeps msg_tx across sk_stream_wait_memory(), which drops
and reacquires the socket lock.  Its error path used the current value of
psock->cork to decide whether msg_tx named the stack-local temporary
message.

Two senders can therefore interleave as follows:

  Thread A                         Thread B
  msg_tx = psock->cork
  sk_msg_alloc() fails
  sk_stream_wait_memory()
    releases the socket lock      acquires the socket lock
                                  completes the cork
                                  psock->cork = NULL
                                  frees the cork
    reacquires the socket lock
  msg_tx != psock->cork
  sk_msg_free(msg_tx)

The stale cork is mistaken for the local temporary message and freed again.
KASAN reported:

  BUG: KASAN: slab-use-after-free in sk_msg_free+0x49/0x50
  Read of size 4 at addr ffff88810c908800 by task poc/90
  Call Trace:
   sk_msg_free+0x49/0x50
   tcp_bpf_sendmsg+0x14f5/0x1cc0
   __sys_sendto+0x32c/0x3a0
   __x64_sys_sendto+0xdb/0x1b0
  Allocated by task 89:
   __kasan_kmalloc+0x8f/0xa0
   tcp_bpf_sendmsg+0x16b3/0x1cc0
  Freed by task 91:
   __kasan_slab_free+0x43/0x70
   kfree+0x131/0x3c0
   tcp_bpf_sendmsg+0xec3/0x1cc0

The same unclear ownership also leaves several error paths inconsistent.
A failed cork allocation leaves cork_bytes armed, a subsequent temporary
message can escape without releasing its pages, iterator errors are
returned as success, and a failed redirect of a corked message discards
data while reporting it as sent.  Re-evaluating an existing cork can also
copy the object onto itself.

Make temporary ownership explicit by freeing only &tmp at the common exit.
Reset cork_bytes when allocating the persistent cork fails, propagate
iterator errors, preserve redirect failures and clear the copied count
when corked data is discarded.  Skip the copy when the message is already
the persistent cork.

Fixes: 604326b41a6f ("bpf, sockmap: convert to generic sk_msg interface")
Cc: stable@vger.kernel.org
Suggested-by: Emil Tsalapatis <emil@etsalapatis.com>
Signed-off-by: Chengfeng Ye <nicoyip.dev@gmail.com>
---

Changes in v2:
- Address Sashiko review findings for stale cork state, iterator-error
  propagation, cork self-copy, and masked redirect failures.
- Keep the original stale-cork use-after-free fix.

Sashiko: https://sashiko.dev/#/patchset/20260719161630.2901208-1-nicoyip.dev%40gmail.com
Link: https://lore.kernel.org/netdev/20260719161630.2901208-1-nicoyip.dev@gmail.com/ [v1]

 net/ipv4/tcp_bpf.c | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/net/ipv4/tcp_bpf.c b/net/ipv4/tcp_bpf.c
index 8e905b50dead..0594ee4013b0 100644
--- a/net/ipv4/tcp_bpf.c
+++ b/net/ipv4/tcp_bpf.c
@@ -443,12 +443,14 @@ static int tcp_bpf_send_verdict(struct sock *sk, struct sk_psock *psock,
 			psock->cork = kzalloc_obj(*psock->cork,
 						  GFP_ATOMIC | __GFP_NOWARN);
 			if (!psock->cork) {
+				psock->cork_bytes = 0;
 				sk_msg_free(sk, msg);
 				*copied = 0;
 				return -ENOMEM;
 			}
 		}
-		memcpy(psock->cork, msg, sizeof(*msg));
+		if (psock->cork != msg)
+			memcpy(psock->cork, msg, sizeof(*msg));
 		return 0;
 	}
 
@@ -495,14 +497,15 @@ static int tcp_bpf_send_verdict(struct sock *sk, struct sk_psock *psock,
 		if (unlikely(ret < 0)) {
 			int free = sk_msg_free(sk, msg);
 
-			if (!cork)
+			if (cork)
+				*copied = 0;
+			else
 				*copied -= free;
 		}
 		if (cork) {
 			sk_msg_free(sk, msg);
 			kfree(msg);
 			msg = NULL;
-			ret = 0;
 		}
 		break;
 	case __SK_DROP:
@@ -579,6 +582,7 @@ static int tcp_bpf_sendmsg(struct sock *sk, struct msghdr *msg, size_t size)
 					       copy);
 		if (ret < 0) {
 			sk_msg_trim(sk, msg_tx, osize);
+			err = ret;
 			goto out_err;
 		}
 
@@ -603,13 +607,12 @@ static int tcp_bpf_sendmsg(struct sock *sk, struct msghdr *msg, size_t size)
 		set_bit(SOCK_NOSPACE, &sk->sk_socket->flags);
 wait_for_memory:
 		err = sk_stream_wait_memory(sk, &timeo);
-		if (err) {
-			if (msg_tx && msg_tx != psock->cork)
-				sk_msg_free(sk, msg_tx);
+		if (err)
 			goto out_err;
-		}
 	}
 out_err:
+	if (msg_tx == &tmp)
+		sk_msg_free(sk, msg_tx);
 	if (err < 0)
 		err = sk_stream_error(sk, msg->msg_flags, err);
 	release_sock(sk);
-- 
2.43.0