[PATCH 1/2] netlink: Bounds-check nlmsg_len()

[Kees Cook]

The nlmsg_len() helper returned "int" from a u32 calculation that could
possible go negative. WARN() if this calculation ever goes negative
(instead returning 0), or if the result would be larger than INT_MAX
(instead returning INT_MAX).

Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
Cc: Jakub Kicinski <kuba@xxxxxxxxxx>
Cc: Paolo Abeni <pabeni@xxxxxxxxxx>
Cc: Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx>
Cc: Jozsef Kadlecsik <kadlec@xxxxxxxxxxxxx>
Cc: Florian Westphal <fw@xxxxxxxxx>
Cc: syzbot <syzkaller@xxxxxxxxxxxxxxxx>
Cc: Yajun Deng <yajun.deng@xxxxxxxxx>
Cc: netdev@xxxxxxxxxxxxxxx
Cc: netfilter-devel@xxxxxxxxxxxxxxx
Cc: coreteam@xxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 include/net/netlink.h | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/include/net/netlink.h b/include/net/netlink.h
index 7a2a9d3144ba..f8cb0543635e 100644
--- a/include/net/netlink.h
+++ b/include/net/netlink.h
@@ -576,7 +576,15 @@ static inline void *nlmsg_data(const struct nlmsghdr *nlh)
  */
 static inline int nlmsg_len(const struct nlmsghdr *nlh)
 {
-	return nlh->nlmsg_len - NLMSG_HDRLEN;
+	u32 nlmsg_contents_len;
+
+	if (WARN_ON_ONCE(check_sub_overflow(nlh->nlmsg_len,
+					    (u32)NLMSG_HDRLEN,
+					    &nlmsg_contents_len)))
+		return 0;
+	if (WARN_ON_ONCE(nlmsg_contents_len > INT_MAX))
+		return INT_MAX;
+	return nlmsg_contents_len;
 }
 
 /**
-- 
2.34.1