Documenting the requirement of CAP_SETFCAP to map UID 0

[Michael Kerrisk (man-pages)]

Hello Serge,

Your commit:

[[
commit db2e718a47984b9d71ed890eb2ea36ecf150de18
Author: Serge E. Hallyn <serge at hallyn.com>
Date:   Tue Apr 20 08:43:34 2021 -0500

    capabilities: require CAP_SETFCAP to map uid 0
]]

added a new requirement when updating a UID map a user namespace
with a value of '0 0 *'.

Kir sent a patch to briefly document this change, but I think much more
should be written. I've attempted to do so. Could you tell me whether the
following text (to be added in user_namespaces(7)) is accurate please:

[[
      In  order  for  a  process  to  write  to  the /proc/[pid]/uid_map
       (/proc/[pid]/gid_map) file, all of the following requirements must
       be met:

       [...]

       4. If  updating  /proc/[pid]/uid_map to create a mapping that maps
          UID 0 in the parent namespace, then one of the  following  must
          be true:

          *  if  writing process is in the parent user namespace, then it
             must have the CAP_SETFCAP capability in that user namespace;
             or

          *  if  the writing process is in the child user namespace, then
             the process that created the user namespace  must  have  had
             the CAP_SETFCAP capability when the namespace was created.

          This rule has been in place since Linux 5.12.  It eliminates an
          earlier security bug whereby a UID 0  process  that  lacks  the
          CAP_SETFCAP capability, which is needed to create a binary with
          namespaced file capabilities (as described in capabilities(7)),
          could  nevertheless  create  such  a  binary,  by the following
          steps:

          *  Create a new user namespace with the identity mapping (i.e.,
             UID  0 in the new user namespace maps to UID 0 in the parent
             namespace), so that UID 0 in both namespaces  is  equivalent
             to the same root user ID.

          *  Since  the  child process has the CAP_SETFCAP capability, it
             could create a binary with namespaced file capabilities that
             would  then  be  effective in the parent user namespace (be‐
             cause the root user IDs are the same in the two namespaces).

       [...]
]]

Thanks,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/