Documentation: Clarify f_cred vs current_cred() use

When making access control choices from a file-based context, f_cred
must be used instead of current_cred() to avoid confused deputy attacks
where an open file may get passed to a more privileged process. Add a
short paragraph to explicitly state the rationale.

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Signed-off-by: NKees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/202007031038.8833A35DE4@keescookSigned-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/security/credentials.rst

@@ -548,6 +548,10 @@ pointer will not change over the lifetime of the file struct, and nor will the
 contents of the cred struct pointed to, barring the exceptions listed above
 (see the Task Credentials section).
 
+To avoid "confused deputy" privilege escalation attacks, access control checks
+during subsequent operations on an opened file should use these credentials
+instead of "current"'s credentials, as the file may have been passed to a more
+privileged process.
 
 Overriding the VFS's Use of Credentials
 =======================================