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[PATCH v2] decnet: dn_rtmsg: Improve input length sanitization in dnrmg_receive_user_skb

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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 < 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>
---
Changes in v2:
  - Compare skb->len against sizeof(*nlh) instead of sizeof(nlh->nlmsg_len)
    to avoid assuming the layout of the nlmsghdr structure. This was
    motivated by Eric Dumazet's comment on a related patch submission.

 net/decnet/netfilter/dn_rtmsg.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/decnet/netfilter/dn_rtmsg.c b/net/decnet/netfilter/dn_rtmsg.c
index 1ed81ac6dd1a..aa8ffecc46a4 100644
--- 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))
-- 
2.13.1.508.gb3defc5cc-goog

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[PATCH] decnet: dn_rtmsg: Improve input length sanitization in dnrmg_receive_user_skb Mateusz Jurczyk <mjurczyk@google.com> - 2017-06-07 15:20 +0200
  Re: [PATCH v2] decnet: dn_rtmsg: Improve input length sanitization  in dnrmg_receive_user_skb Florian Westphal <fw@strlen.de> - 2017-06-07 16:20 +0200
    Re: [PATCH v2] decnet: dn_rtmsg: Improve input length sanitization in dnrmg_receive_user_skb Mateusz Jurczyk <mjurczyk@google.com> - 2017-06-07 16:50 +0200
      Re: [PATCH v2] decnet: dn_rtmsg: Improve input length sanitization  in dnrmg_receive_user_skb David Miller <davem@davemloft.net> - 2017-06-08 16:40 +0200
  [PATCH v2] decnet: dn_rtmsg: Improve input length sanitization in dnrmg_receive_user_skb Mateusz Jurczyk <mjurczyk@google.com> - 2017-06-07 16:20 +0200

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