[PATCH net] net: mpls: initialize rtm_tos in mpls_getroute()

mpls_getroute() builds the RTM_NEWROUTE reply to an RTM_GETROUTE
request by filling a struct rtmsg allocated from an skb whose data
area is not zeroed (alloc_skb(NLMSG_GOODSIZE, ...)). It sets every
field of the header except rtm_tos:

	r = nlmsg_data(nlh);
	r->rtm_family	 = AF_MPLS;
	r->rtm_dst_len	= 20;
	r->rtm_src_len	= 0;
	r->rtm_table	= RT_TABLE_MAIN;
	r->rtm_type	= RTN_UNICAST;
	r->rtm_scope	= RT_SCOPE_UNIVERSE;
	r->rtm_protocol = rt->rt_protocol;
	r->rtm_flags	= 0;

struct rtmsg has no padding, so the one uninitialised byte rtm_tos
(offset 3) is copied straight to user space on recvmsg(), leaking a
byte of uninitialised heap memory. This is in contrast to
mpls_dump_route(), which fills the very same header and does set
rtm_tos = 0.

Initialize rtm_tos to 0, matching mpls_dump_route().

Reproduced with KMSAN by adding an MPLS route and issuing a
non-RTM_F_FIB_MATCH RTM_GETROUTE for its label:

  BUG: KMSAN: kernel-infoleak in _copy_to_iter+0x36c/0x33f0
   _copy_to_iter+0x36c/0x33f0
   __skb_datagram_iter+0x196/0x12c0
   skb_copy_datagram_iter+0x5b/0x210
   netlink_recvmsg+0x37b/0xef0
   ...
  Uninit was created at:
   __alloc_skb+0x8ca/0x10e0
   mpls_getroute+0x1280/0x3a40
   rtnetlink_rcv_msg+0x1138/0x15a0
   ...
  Byte 19 of 64 is uninitialized

(byte 19 = nlmsghdr(16) + rtmsg offset 3 = rtm_tos)

Fixes: 397fc9e5cefe ("mpls: route get support")
Signed-off-by: Yehyeong Lee <redacted>
---
 net/mpls/af_mpls.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/mpls/af_mpls.c b/net/mpls/af_mpls.c
index 318cb7e2ac5f..f1c9fc8504e1 100644
--- a/net/mpls/af_mpls.c
+++ b/net/mpls/af_mpls.c

@@ -2537,6 +2537,7 @@ static int mpls_getroute(struct sk_buff *in_skb, struct nlmsghdr *in_nlh,
 	r->rtm_family	 = AF_MPLS;
 	r->rtm_dst_len	= 20;
 	r->rtm_src_len	= 0;
+	r->rtm_tos	= 0;
 	r->rtm_table	= RT_TABLE_MAIN;
 	r->rtm_type	= RTN_UNICAST;
 	r->rtm_scope	= RT_SCOPE_UNIVERSE;

-- 
2.43.0