[PATCH net] mpls: fix NULL deref in mpls_valid_fib_dump_req() on CONFIG_INET=n

[Xiang Mei <xmei5@asu.edu>]

On CONFIG_INET=n, mpls_valid_fib_dump_req()'s local strict-check loop
enters the RTA_OIF arm on the index alone and calls nla_get_u32(tb[i]).
Since nlmsg_parse_deprecated_strict() leaves tb[i] NULL for any omitted
attribute, a dump without RTA_OIF dereferences NULL and oopses.

Skip unset attributes before dispatching on the index, mirroring
ip_valid_fib_dump_req().

  Oops: general protection fault, probably for non-canonical address
  0xdffffc0000000000: 0000 [#1] SMP KASAN NOPTI
  KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
  RIP: 0010:mpls_valid_fib_dump_req (net/mpls/af_mpls.c:2189)
  Call Trace:
   mpls_dump_routes (net/mpls/af_mpls.c:2236)
   netlink_dump (net/netlink/af_netlink.c:2331)
   __netlink_dump_start (net/netlink/af_netlink.c:2446)
   rtnetlink_rcv_msg (net/core/rtnetlink.c:7041)
   netlink_rcv_skb (net/netlink/af_netlink.c:2556)
   netlink_unicast (net/netlink/af_netlink.c:1345)
   netlink_sendmsg (net/netlink/af_netlink.c:1900)
   __sock_sendmsg (net/socket.c:790)
   ____sys_sendmsg (net/socket.c:2684)
   ___sys_sendmsg (net/socket.c:2738)
   __sys_sendmsg (net/socket.c:2770)
   do_syscall_64 (arch/x86/entry/syscall_64.c:94)
   entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)
  Kernel panic - not syncing: Fatal exception

Fixes: 196cfebf8972 ("net/mpls: Handle kernel side filtering of route dumps")
Reported-by: Weiming Shi <bestswngs@gmail.com>
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Xiang Mei <xmei5@asu.edu>
---
 net/mpls/af_mpls.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/mpls/af_mpls.c b/net/mpls/af_mpls.c
index ca504d9626cf..1ad07df89d5b 100644
--- a/net/mpls/af_mpls.c
+++ b/net/mpls/af_mpls.c
@@ -2185,13 +2185,16 @@ static int mpls_valid_fib_dump_req(struct net *net, const struct nlmsghdr *nlh,
 	for (i = 0; i <= RTA_MAX; ++i) {
 		int ifindex;
 
+		if (!tb[i])
+			continue;
+
 		if (i == RTA_OIF) {
 			ifindex = nla_get_u32(tb[i]);
 			filter->dev = dev_get_by_index_rcu(net, ifindex);
 			if (!filter->dev)
 				return -ENODEV;
 			filter->filter_set = 1;
-		} else if (tb[i]) {
+		} else {
 			NL_SET_ERR_MSG_MOD(extack, "Unsupported attribute in dump request");
 			return -EINVAL;
 		}
-- 
2.43.0