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[PATCH v2] netfilter: nfnetlink: Improve input length sanitization in nfnetlink_rcv

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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>
---
Changes in v2:
  - Compare skb->len against NLMSG_HDRLEN to avoid assuming the layout of
    the nlmsghdr structure.

 net/netfilter/nfnetlink.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/netfilter/nfnetlink.c b/net/netfilter/nfnetlink.c
index 80f5ecf2c3d7..1f9667f52be5 100644
--- a/net/netfilter/nfnetlink.c
+++ b/net/netfilter/nfnetlink.c
@@ -491,7 +491,8 @@ 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;
 
-- 
2.13.1.508.gb3defc5cc-goog

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[PATCH] netfilter: nfnetlink: Improve input length sanitization in nfnetlink_rcv Mateusz Jurczyk <mjurczyk@google.com> - 2017-06-07 14:40 +0200
  Re: [PATCH] netfilter: nfnetlink: Improve input length sanitization  in nfnetlink_rcv Eric Dumazet <eric.dumazet@gmail.com> - 2017-06-07 15:30 +0200
    [PATCH v2] netfilter: nfnetlink: Improve input length sanitization in nfnetlink_rcv Mateusz Jurczyk <mjurczyk@google.com> - 2017-06-07 16:00 +0200

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