[PATCH net] tipc: fix infinite loop in __tipc_nl_compat_dumpit

[Helen Koike <koike@igalia.com>]

cmd->dumpit callback can return a negative errno, causing an infinite
loop due to the while(len) condition. As the loop never terminates,
genl_mutex is never released, and other tasks waiting on it starve in D
state.

Check dumpit's return value, propagate it and jump to err_out on error.

Reported-by: syzbot+85d0bec020d805014a3a@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=85d0bec020d805014a3a
Fixes: d0796d1ef63d ("tipc: convert legacy nl bearer dump to nl compat")
Signed-off-by: Helen Koike <koike@igalia.com>
---

Tested locally using syzbot reproducer.
---
 net/tipc/netlink_compat.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/tipc/netlink_compat.c b/net/tipc/netlink_compat.c
index 2a786c56c8c5..d9a4f94ea2d4 100644
--- a/net/tipc/netlink_compat.c
+++ b/net/tipc/netlink_compat.c
@@ -221,6 +221,10 @@ static int __tipc_nl_compat_dumpit(struct tipc_nl_compat_cmd_dump *cmd,
 		int rem;
 
 		len = (*cmd->dumpit)(buf, &cb);
+		if (len < 0) {
+			err = len;
+			goto err_out;
+		}
 
 		nlmsg_for_each_msg(nlmsg, nlmsg_hdr(buf), len, rem) {
 			err = nlmsg_parse_deprecated(nlmsg, GENL_HDRLEN,
-- 
2.54.0