[PATCH net] ipv4: Check attribute length for RTA_GATEWAY

[David Ahern <dsahern@kernel.org>]

syzbot reported uninit-value:
============================================================
  BUG: KMSAN: uninit-value in fib_get_nhs+0xac4/0x1f80
  net/ipv4/fib_semantics.c:708
   fib_get_nhs+0xac4/0x1f80 net/ipv4/fib_semantics.c:708
   fib_create_info+0x2411/0x4870 net/ipv4/fib_semantics.c:1453
   fib_table_insert+0x45c/0x3a10 net/ipv4/fib_trie.c:1224
   inet_rtm_newroute+0x289/0x420 net/ipv4/fib_frontend.c:886

Add length checking before using the attribute.

Fixes: 4e902c57417c ("[IPv4]: FIB configuration using struct fib_config")
Reported-by: syzbot+d4b9a2851cc3ce998741@syzkaller.appspotmail.com
Signed-off-by: David Ahern <dsahern@kernel.org>
Cc: Thomas Graf <tgraf@suug.ch>
---
I do not have KMSAN setup, so this is based on a code analysis. Before
4e902c57417c fib_get_attr32 was checking the attribute length; the
switch to nla_get_u32 does not.

 net/ipv4/fib_semantics.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/ipv4/fib_semantics.c b/net/ipv4/fib_semantics.c
index 3cad543dc747..930843ba3b17 100644
--- a/net/ipv4/fib_semantics.c
+++ b/net/ipv4/fib_semantics.c
@@ -704,6 +704,10 @@ static int fib_get_nhs(struct fib_info *fi, struct rtnexthop *rtnh,
 				return -EINVAL;
 			}
 			if (nla) {
+				if (nla_len(nla) < sizeof(__be32)) {
+					NL_SET_ERR_MSG(extack, "Invalid IPv4 address in RTA_GATEWAY");
+					return -EINVAL;
+				}
 				fib_cfg.fc_gw4 = nla_get_in_addr(nla);
 				if (fib_cfg.fc_gw4)
 					fib_cfg.fc_gw_family = AF_INET;
-- 
2.24.3 (Apple Git-128)