bpf, sockmap: Fix memleak in tcp_bpf_sendmsg while sk msg is full

If tcp_bpf_sendmsg() is running while sk msg is full. When sk_msg_alloc()
returns -ENOMEM error, tcp_bpf_sendmsg() goes to wait_for_memory. If partial
memory has been alloced by sk_msg_alloc(), that is, msg_tx->sg.size is
greater than osize after sk_msg_alloc(), memleak occurs. To fix we use
sk_msg_trim() to release the allocated memory, then goto wait for memory.

Other call paths of sk_msg_alloc() have the similar issue, such as
tls_sw_sendmsg(), so handle sk_msg_trim logic inside sk_msg_alloc(),
as Cong Wang suggested.

This issue can cause the following info:
WARNING: CPU: 3 PID: 7950 at net/core/stream.c:208 sk_stream_kill_queues+0xd4/0x1a0
Call Trace:
 <TASK>
 inet_csk_destroy_sock+0x55/0x110
 __tcp_close+0x279/0x470
 tcp_close+0x1f/0x60
 inet_release+0x3f/0x80
 __sock_release+0x3d/0xb0
 sock_close+0x11/0x20
 __fput+0x92/0x250
 task_work_run+0x6a/0xa0
 do_exit+0x33b/0xb60
 do_group_exit+0x2f/0xa0
 get_signal+0xb6/0x950
 arch_do_signal_or_restart+0xac/0x2a0
 exit_to_user_mode_prepare+0xa9/0x200
 syscall_exit_to_user_mode+0x12/0x30
 do_syscall_64+0x46/0x80
 entry_SYSCALL_64_after_hwframe+0x44/0xae
 </TASK>

WARNING: CPU: 3 PID: 2094 at net/ipv4/af_inet.c:155 inet_sock_destruct+0x13c/0x260
Call Trace:
 <TASK>
 __sk_destruct+0x24/0x1f0
 sk_psock_destroy+0x19b/0x1c0
 process_one_work+0x1b3/0x3c0
 kthread+0xe6/0x110
 ret_from_fork+0x22/0x30
 </TASK>

Fixes: 604326b4 ("bpf, sockmap: convert to generic sk_msg interface")
Signed-off-by: Wang Yufen <wangyufen@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20220304081145.2037182-3-wangyufen@huawei.com

net/core/skmsg.c

@@ -27,6 +27,7 @@ int sk_msg_alloc(struct sock *sk, struct sk_msg *msg, int len,
 		 int elem_first_coalesce)
 {
 	struct page_frag *pfrag = sk_page_frag(sk);
+	u32 osize = msg->sg.size;
 	int ret = 0;
 
 	len -= msg->sg.size;
@@ -35,13 +36,17 @@ int sk_msg_alloc(struct sock *sk, struct sk_msg *msg, int len,
 		u32 orig_offset;
 		int use, i;
 
-		if (!sk_page_frag_refill(sk, pfrag))
-			return -ENOMEM;
+		if (!sk_page_frag_refill(sk, pfrag)) {
+			ret = -ENOMEM;
+			goto msg_trim;
+		}
 
 		orig_offset = pfrag->offset;
 		use = min_t(int, len, pfrag->size - orig_offset);
-		if (!sk_wmem_schedule(sk, use))
-			return -ENOMEM;
+		if (!sk_wmem_schedule(sk, use)) {
+			ret = -ENOMEM;
+			goto msg_trim;
+		}
 
 		i = msg->sg.end;
 		sk_msg_iter_var_prev(i);
@@ -71,6 +76,10 @@ int sk_msg_alloc(struct sock *sk, struct sk_msg *msg, int len,
 	}
 
 	return ret;
+
+msg_trim:
+	sk_msg_trim(sk, msg, osize);
+	return ret;
 }
 EXPORT_SYMBOL_GPL(sk_msg_alloc);