linux-hardening - [PATCH 62/64] netlink: Avoid false-positive memcpy() warning

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Date:   Tue, 27 Jul 2021 13:58:53 -0700
From:   Kees Cook <keescook@...omium.org>
To:     linux-hardening@...r.kernel.org
Cc:     Kees Cook <keescook@...omium.org>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Keith Packard <keithpac@...zon.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-wireless@...r.kernel.org,
        netdev@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-staging@...ts.linux.dev, linux-block@...r.kernel.org,
        linux-kbuild@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: [PATCH 62/64] netlink: Avoid false-positive memcpy() warning

In preparation for FORTIFY_SOURCE performing compile-time and run-time
field bounds checking for memcpy(), memmove(), and memset(), avoid
intentionally writing across neighboring fields.

Add a flexible array member to mark the end of struct nlmsghdr, and
split the memcpy() to avoid false positive memcpy() warning:

memcpy: detected field-spanning write (size 32) of single field (size 16)

Signed-off-by: Kees Cook <keescook@...omium.org>
---
 include/uapi/linux/netlink.h | 1 +
 net/netlink/af_netlink.c     | 4 +++-
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/netlink.h b/include/uapi/linux/netlink.h
index 4c0cde075c27..ddeaa748df5e 100644
--- a/include/uapi/linux/netlink.h
+++ b/include/uapi/linux/netlink.h
@@ -47,6 +47,7 @@ struct nlmsghdr {
 	__u16		nlmsg_flags;	/* Additional flags */
 	__u32		nlmsg_seq;	/* Sequence number */
 	__u32		nlmsg_pid;	/* Sending process port ID */
+	__u8		contents[];
 };
 
 /* Flags values */
diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index 24b7cf447bc5..f2dd99e96822 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -2447,7 +2447,9 @@ void netlink_ack(struct sk_buff *in_skb, struct nlmsghdr *nlh, int err,
 			  NLMSG_ERROR, payload, flags);
 	errmsg = nlmsg_data(rep);
 	errmsg->error = err;
-	memcpy(&errmsg->msg, nlh, payload > sizeof(*errmsg) ? nlh->nlmsg_len : sizeof(*nlh));
+	memcpy(&errmsg->msg, nlh, sizeof(*nlh));
+	if (payload > sizeof(*errmsg))
+		memcpy(errmsg->msg.contents, nlh->contents, nlh->nlmsg_len - sizeof(*nlh));
 
 	if (nlk_has_extack && extack) {
 		if (extack->_msg) {
-- 
2.30.2