ipv4: provide stronger user input validation in nl_fib_input()

Alexander reported a KMSAN splat caused by reads of uninitialized
field (tb_id_in) from user provided struct fib_result_nl

It turns out nl_fib_input() sanity tests on user input is a bit
wrong :

User can pretend nlh->nlmsg_len is big enough, but provide
at sendmsg() time a too small buffer.

Reported-by: Alexander Potapenko <glider@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/ipv4/fib_frontend.c b/net/ipv4/fib_frontend.c
index 42bfd08..8f2133f 100644 (file)
--- a/net/ipv4/fib_frontend.c
+++ b/net/ipv4/fib_frontend.c
@@ -1083,7 +1083,8 @@ static void nl_fib_input(struct sk_buff *skb)
 
        net = sock_net(skb->sk);
        nlh = nlmsg_hdr(skb);
-       if (skb->len < NLMSG_HDRLEN || skb->len < nlh->nlmsg_len ||
+       if (skb->len < nlmsg_total_size(sizeof(*frn)) ||
+           skb->len < nlh->nlmsg_len ||
            nlmsg_len(nlh) < sizeof(*frn))
                return;