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

Kees Cook
2021-07-27T21:19:23Z

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@chromium.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