decnet: dn_rtmsg: Improve input length sanitization in dnrmg_receive_user_skb

Verify that the length of the socket buffer is sufficient to cover the
nlmsghdr structure before accessing the nlh->nlmsg_len field for further
input sanitization. If the client only supplies 1-3 bytes of data in
sk_buff, then nlh->nlmsg_len remains partially uninitialized and
contains leftover memory from the corresponding kernel allocation.
Operating on such data may result in indeterminate evaluation of the
nlmsg_len < sizeof(*nlh) expression.

The bug was discovered by a runtime instrumentation designed to detect
use of uninitialized memory in the kernel. The patch prevents this and
other similar tools (e.g. KMSAN) from flagging this behavior in the future.

Signed-off-by: Mateusz Jurczyk <mjurczyk@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/decnet/netfilter/dn_rtmsg.c b/net/decnet/netfilter/dn_rtmsg.c
index 1ed81ac..aa8ffec 100644 (file)
--- a/net/decnet/netfilter/dn_rtmsg.c
+++ b/net/decnet/netfilter/dn_rtmsg.c
@@ -102,7 +102,9 @@ static inline void dnrmg_receive_user_skb(struct sk_buff *skb)
 {
        struct nlmsghdr *nlh = nlmsg_hdr(skb);
 
-       if (nlh->nlmsg_len < sizeof(*nlh) || skb->len < nlh->nlmsg_len)
+       if (skb->len < sizeof(*nlh) ||
+           nlh->nlmsg_len < sizeof(*nlh) ||
+           skb->len < nlh->nlmsg_len)
                return;
 
        if (!netlink_capable(skb, CAP_NET_ADMIN))