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Date: Thu, 11 Mar 2021 11:42:55 +0100
From: Mickaël Salaün <>
To: Casey Schaufler <>,
 Al Viro <>, James Morris <>,
 Serge Hallyn <>
Cc: Andy Lutomirski <>,
 Christian Brauner <>,
 Christoph Hellwig <>, David Howells <>,
 Dominik Brodowski <>,
 Eric Biederman <>,
 John Johansen <>,
 Kees Cook <>, Kentaro Takeda <>,
 Tetsuo Handa <>,,,,
Subject: Re: [PATCH v1 0/1] Unprivileged chroot

On 10/03/2021 21:59, Casey Schaufler wrote:
> On 3/10/2021 10:17 AM, Mickaël Salaün wrote:
>> On 10/03/2021 18:22, Casey Schaufler wrote:
>>> On 3/10/2021 8:09 AM, Mickaël Salaün wrote:
>>>> Hi,
>>>> The chroot system call is currently limited to be used by processes with
>>>> the CAP_SYS_CHROOT capability.  This protects against malicious
>>>> procesess willing to trick SUID-like binaries.  The following patch
>>>> allows unprivileged users to safely use chroot(2).
>>> Mount namespaces have pretty well obsoleted chroot(). CAP_SYS_CHROOT is
>>> one of the few fine grained capabilities. We're still finding edge cases
>>> (e.g. ptrace) where no_new_privs is imperfect. I doesn't seem that there
>>> is a compelling reason to remove the privilege requirement on chroot().
>> What is the link between chroot and ptrace?
> The possibility of sophisticated interactions with no_new_privs.

Would you mind giving some practical examples?

>> What is interesting with CAP_SYS_CHROOT?
> CAP_SYS_CHROOT is specific to chroot. It doesn't give you privilege
> beyond what you expect, unlike CAP_CHOWN or CAP_SYS_ADMIN. Making chroot
> unprivileged is silly when it's possibly the best example of how the
> capability mechanism is supposed to work.

Why would it be silly to make the use of this feature safe for any
processes instead of giving the right (with CAP_SYS_CHROOT) to some
processes to use it unsafely?

>>>> This patch is a follow-up of a previous one sent by Andy Lutomirski some
>>>> time ago:
>>>> This patch can be applied on top of v5.12-rc2 .  I would really
>>>> appreciate constructive reviews.
>>>> Regards,
>>>> Mickaël Salaün (1):
>>>>   fs: Allow no_new_privs tasks to call chroot(2)
>>>>  fs/open.c | 64 ++++++++++++++++++++++++++++++++++++++++++++++++++++---
>>>>  1 file changed, 61 insertions(+), 3 deletions(-)
>>>> base-commit: a38fd8748464831584a19438cbb3082b5a2dab15

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