[PATCH 7/7] doc: clarify memif secret is not access control

[Stephen Hemminger <stephen@networkplumber.org>]

The secret option was described as a security option, which invites
using it as one. It is sent in cleartext in the connection request,
and when passed as a device argument it is visible to other local
users in the process arguments.

Describe it as what it is, a check against connecting mismatched
interfaces, and document what actually restricts access to an
interface. By default the control socket is in the abstract
namespace and has no filesystem entry to own or permission. Only
with socket-abstract=no do file permissions and the owner-uid and
owner-gid options apply.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Tested-by: Sriram Yagnaraman <sriram.yagnaraman@ericsson.com>
---
 doc/guides/nics/memif.rst | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/doc/guides/nics/memif.rst b/doc/guides/nics/memif.rst
index f8b629ab1f..5552d319d0 100644
--- a/doc/guides/nics/memif.rst
+++ b/doc/guides/nics/memif.rst
@@ -47,9 +47,27 @@ client.
    "owner-uid=1000", "Set socket listener owner uid. Only relevant to server with socket-abstract=no", "unchanged", "uid_t"
    "owner-gid=1000", "Set socket listener owner gid. Only relevant to server with socket-abstract=no", "unchanged", "gid_t"
    "mac=01:23:45:ab:cd:ef", "Mac address", "01:ab:23:cd:45:ef", ""
-   "secret=abc123", "Secret is an optional security option, which if specified, must be matched by peer", "", "string len 24"
+   "secret=abc123", "Optional identifier which, if specified, must be matched by peer", "", "string len 24"
    "zero-copy=yes", "Enable/disable zero-copy client mode. Only relevant to client, requires '--single-file-segments' eal argument", "no", "yes|no"
 
+**Access control**
+
+Any process able to connect to the socket of a server interface is able to
+reach its shared memory rings, so what restricts access to that socket is
+the security boundary.
+
+By default the socket is in the abstract namespace (``socket-abstract=yes``).
+An abstract socket has no filesystem entry.
+Use a network namespace to restrict access to such an interface.
+
+With ``socket-abstract=no`` the socket is a filesystem object and normal
+file permissions apply, together with the ``owner-uid`` and ``owner-gid``
+options.
+
+The ``secret`` option is *not* an access control mechanism.
+It only guards against connecting mismatched interfaces by mistake,
+for example where several interfaces share one socket.
+
 **Connection establishment**
 
 In order to create memif connection, two memif interfaces, each in separate
-- 
2.53.0