netlink: Don't reorder loads/stores before marking mmap netlink frame as available

commit a18e6a186f53af06937a2c268c72443336f4ab56 upstream.

Each mmap Netlink frame contains a status field which indicates
whether the frame is unused, reserved, contains data or needs to
be skipped. Both loads and stores may not be reordeded and must
complete before the status field is changed and another CPU might
pick up the frame for use. Use an smp_mb() to cover needs of both
types of callers to netlink_set_status(), callers which have been
reading data frame from the frame, and callers which have been
filling or releasing and thus writing to the frame.

- Example code path requiring a smp_rmb():
  memcpy(skb->data, (void *)hdr + NL_MMAP_HDRLEN, hdr->nm_len);
  netlink_set_status(hdr, NL_MMAP_STATUS_UNUSED);

- Example code path requiring a smp_wmb():
  hdr->nm_uid	= from_kuid(sk_user_ns(sk), NETLINK_CB(skb).creds.uid);
  hdr->nm_gid	= from_kgid(sk_user_ns(sk), NETLINK_CB(skb).creds.gid);
  netlink_frame_flush_dcache(hdr);
  netlink_set_status(hdr, NL_MMAP_STATUS_VALID);

Fixes: f9c228 ("netlink: implement memory mapped recvmsg()")
Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index 625593d866ac2a31ad9b28c5e282364e8fce88f6..256ce8b844563f7bf28a6d56ca7706e59171c7f4 100644 (file)
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -535,9 +535,9 @@ static enum nl_mmap_status netlink_get_status(const struct nl_mmap_hdr *hdr)
 static void netlink_set_status(struct nl_mmap_hdr *hdr,
                               enum nl_mmap_status status)
 {
+       smp_mb();
        hdr->nm_status = status;
        flush_dcache_page(pgvec_to_page(hdr));
-       smp_wmb();
 }
 
 static struct nl_mmap_hdr *