netfilter: nfnetlink: Improve input length sanitization in nfnetlink_rcv

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 < NLMSG_HDRLEN 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: Pablo Neira Ayuso <pablo@netfilter.org>

1 files changed, 3 insertions, 3 deletions

diff --git a/net/netfilter/nfnetlink.c b/net/netfilter/nfnetlink.c
index 92b05e188fd1..733d3e4a30d8 100644
--- a/net/netfilter/nfnetlink.c
+++ b/net/netfilter/nfnetlink.c
@@ -472,8 +472,7 @@ static void nfnetlink_rcv_skb_batch(struct sk_buff *skb, struct nlmsghdr *nlh)
         if (msglen > skb->len)
                 msglen = skb->len;
 
-        if (nlh->nlmsg_len < NLMSG_HDRLEN ||
-            skb->len < NLMSG_HDRLEN + sizeof(struct nfgenmsg))
+        if (skb->len < NLMSG_HDRLEN + sizeof(struct nfgenmsg))
                 return;
 
         err = nla_parse(cda, NFNL_BATCH_MAX, attr, attrlen, nfnl_batch_policy,
@@ -500,7 +499,8 @@ static void nfnetlink_rcv(struct sk_buff *skb)
 {
         struct nlmsghdr *nlh = nlmsg_hdr(skb);
 
-        if (nlh->nlmsg_len < NLMSG_HDRLEN ||
+        if (skb->len < NLMSG_HDRLEN ||
+            nlh->nlmsg_len < NLMSG_HDRLEN ||
             skb->len < nlh->nlmsg_len)
                 return;