LKML Archive on lore.kernel.org
help / color / mirror / Atom feed
From: "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
To: "Serge E. Hallyn" <serge@hallyn.com>
Cc: mtk.manpages@gmail.com,
	linux-security-module <linux-security-module@vger.kernel.org>,
	lkml <linux-kernel@vger.kernel.org>,
	Alejandro Colomar <alx.manpages@gmail.com>,
	Kir Kolyshkin <kolyshkin@gmail.com>,
	linux-man <linux-man@vger.kernel.org>
Subject: Documenting the requirement of CAP_SETFCAP to map UID 0
Date: Sun, 8 Aug 2021 11:09:30 +0200	[thread overview]
Message-ID: <14cbab6f-19f6-a28c-05d8-453ecca62180@gmail.com> (raw)

Hello Serge,

Your commit:

[[
commit db2e718a47984b9d71ed890eb2ea36ecf150de18
Author: Serge E. Hallyn <serge@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/

             reply	other threads:[~2021-08-08  9:09 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-08  9:09 Michael Kerrisk (man-pages) [this message]
2021-08-10 23:58 ` Serge E. Hallyn
2021-08-11 10:10   ` Michael Kerrisk (man-pages)

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=14cbab6f-19f6-a28c-05d8-453ecca62180@gmail.com \
    --to=mtk.manpages@gmail.com \
    --cc=alx.manpages@gmail.com \
    --cc=kolyshkin@gmail.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-man@vger.kernel.org \
    --cc=linux-security-module@vger.kernel.org \
    --cc=serge@hallyn.com \
    --subject='Re: Documenting the requirement of CAP_SETFCAP to map UID 0' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).