[PATCH bpf-next] skmsg: Fix wrong last sg check in sk_msg_recvmsg()

[Liu Jian <liujian56@huawei.com>]

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 7303524e04af ("skmsg: Lose offset info in sk_psock_skb_ingress"),
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: 7303524e04af ("skmsg: Lose offset info in sk_psock_skb_ingress")
Signed-off-by: Liu Jian <liujian56@huawei.com>
---
 net/core/skmsg.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/core/skmsg.c b/net/core/skmsg.c
index 81627892bdd4..385ae23580a5 100644
--- a/net/core/skmsg.c
+++ b/net/core/skmsg.c
@@ -462,7 +462,7 @@ 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);
@@ -472,7 +472,7 @@ 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);
 		}
-- 
2.17.1