[PATCH] net: skmsg: fix NULL pointer dereference in sk_msg_recvmsg()

[Pranav Tyagi <pranav.tyagi03@gmail.com>]

A NULL page from sg_page() in sk_msg_recvmsg() can reach
__kmap_local_page_prot() and crash the kernel. Add a check for the page
before calling copy_page_to_iter() and fail early with -EFAULT to
prevent the crash.

Reported-by: syzbot+b18872ea9631b5dcef3b@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=b18872ea9631b5dcef3b
Fixes: 2bc793e3272a ("skmsg: Extract __tcp_bpf_recvmsg() and tcp_bpf_wait_data()")
Signed-off-by: Pranav Tyagi <pranav.tyagi03@gmail.com>
---
 net/core/skmsg.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/core/skmsg.c b/net/core/skmsg.c
index 4d75ef9d24bf..f5367356a483 100644
--- a/net/core/skmsg.c
+++ b/net/core/skmsg.c
@@ -432,6 +432,10 @@ int sk_msg_recvmsg(struct sock *sk, struct sk_psock *psock, struct msghdr *msg,
 			sge = sk_msg_elem(msg_rx, i);
 			copy = sge->length;
 			page = sg_page(sge);
+			if (!page) {
+				copied = copied ? copied : -EFAULT;
+				goto out;
+			}
 			if (copied + copy > len)
 				copy = len - copied;
 			copy = copy_page_to_iter(page, sge->offset, copy, iter);
-- 
2.49.0