From: "Serge E. Hallyn" <serge@hallyn.com>
To: "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
Cc: "Serge E. Hallyn" <serge@hallyn.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: Re: Documenting the requirement of CAP_SETFCAP to map UID 0
Date: Tue, 10 Aug 2021 18:58:38 -0500 [thread overview]
Message-ID: <20210810235838.GA4561@mail.hallyn.com> (raw)
In-Reply-To: <14cbab6f-19f6-a28c-05d8-453ecca62180@gmail.com>
On Sun, Aug 08, 2021 at 11:09:30AM +0200, Michael Kerrisk (man-pages) wrote:
> 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:
Sorry for the delay - this did not go into my main mailbox.
The text looks good. Thanks!
> [[
> 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/
next prev parent reply other threads:[~2021-08-11 0:03 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-08-08 9:09 Documenting the requirement of CAP_SETFCAP to map UID 0 Michael Kerrisk (man-pages)
2021-08-10 23:58 ` Serge E. Hallyn [this message]
2021-08-11 10:10 ` Michael Kerrisk (man-pages)
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