ipv4: provide stronger user input validation in nl_fib_input()

[ Upstream commit c64c0b3cac4c5b8cb093727d2c19743ea3965c0b ]

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 2 insertions, 1 deletions

diff --git a/net/ipv4/fib_frontend.c b/net/ipv4/fib_frontend.c
index 5b03d7f3b255..6789e48b7085 100644
--- a/net/ipv4/fib_frontend.c
+++ b/net/ipv4/fib_frontend.c
@@ -1081,7 +1081,8 @@ 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;