skmsg: Fix wrong last sg check in sk_msg_recvmsg()

Fix one kernel NULL pointer dereference as below:

[  224.462334] Call Trace:
[  224.462394]  __tcp_bpf_recvmsg+0xd3/0x380
[  224.462441]  ? sock_has_perm+0x78/0xa0
[  224.462463]  tcp_bpf_recvmsg+0x12e/0x220
[  224.462494]  inet_recvmsg+0x5b/0xd0
[  224.462534]  __sys_recvfrom+0xc8/0x130
[  224.462574]  ? syscall_trace_enter+0x1df/0x2e0
[  224.462606]  ? __do_page_fault+0x2de/0x500
[  224.462635]  __x64_sys_recvfrom+0x24/0x30
[  224.462660]  do_syscall_64+0x5d/0x1d0
[  224.462709]  entry_SYSCALL_64_after_hwframe+0x65/0xca

In commit 9974d37 ("skmsg: Fix invalid last sg check in
sk_msg_recvmsg()"), we change last sg check to sg_is_last(),
but in sockmap redirection case (without stream_parser/stream_verdict/
skb_verdict), we did not mark the end of the scatterlist. Check the
sk_msg_alloc, sk_msg_page_add, and bpf_msg_push_data functions, they all
do not mark the end of sg. They are expected to use sg.end for end
judgment. So the judgment of '(i != msg_rx->sg.end)' is added back here.

Fixes: 9974d37 ("skmsg: Fix invalid last sg check in sk_msg_recvmsg()")
Signed-off-by: Liu Jian <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Acked-by: John Fastabend <[email protected]>
Acked-by: Jakub Sitnicki <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

Author: liujian56

net/core/skmsg.c

@@ -461,7 +461,7 @@ int sk_msg_recvmsg(struct sock *sk, struct sk_psock *psock, struct msghdr *msg,
 
 			if (copied == len)
 				break;
-		} while (!sg_is_last(sge));
+		} while ((i != msg_rx->sg.end) && !sg_is_last(sge));
 
 		if (unlikely(peek)) {
 			msg_rx = sk_psock_next_msg(psock, msg_rx);
@@ -471,7 +471,7 @@ int sk_msg_recvmsg(struct sock *sk, struct sk_psock *psock, struct msghdr *msg,
 		}
 
 		msg_rx->sg.start = i;
-		if (!sge->length && sg_is_last(sge)) {
+		if (!sge->length && (i == msg_rx->sg.end || sg_is_last(sge))) {
 			msg_rx = sk_psock_dequeue_msg(psock);
 			kfree_sk_msg(msg_rx);
 		}