[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <77ec5d18-f88e-5c7c-7450-744f69654f69@schaufler-ca.com>
Date: Tue, 30 Mar 2021 11:40:27 -0700
From: Casey Schaufler <casey@...aufler-ca.com>
To: Mickaël Salaün <mic@...ikod.net>,
Al Viro <viro@...iv.linux.org.uk>, James Morris <jmorris@...ei.org>,
Serge Hallyn <serge@...lyn.com>, Andrew Morton <akpm@...ux-foundation.org>
Cc: Andy Lutomirski <luto@...capital.net>,
Christian Brauner <christian.brauner@...ntu.com>,
Christoph Hellwig <hch@....de>, David Howells <dhowells@...hat.com>,
Dominik Brodowski <linux@...inikbrodowski.net>,
"Eric W . Biederman" <ebiederm@...ssion.com>, Jann Horn <jannh@...gle.com>,
John Johansen <john.johansen@...onical.com>,
Kees Cook <keescook@...omium.org>, Kentaro Takeda <takedakn@...data.co.jp>,
Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
kernel-hardening@...ts.openwall.com, linux-fsdevel@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-security-module@...r.kernel.org,
Mickaël Salaün <mic@...ux.microsoft.com>,
Casey Schaufler <casey@...aufler-ca.com>
Subject: Re: [PATCH v5 1/1] fs: Allow no_new_privs tasks to call chroot(2)
On 3/30/2021 11:11 AM, Mickaël Salaün wrote:
> On 30/03/2021 19:19, Casey Schaufler wrote:
>> On 3/30/2021 10:01 AM, Mickaël Salaün wrote:
>>> Hi,
>>>
>>> Is there new comments on this patch? Could we move forward?
>> I don't see that new comments are necessary when I don't see
>> that you've provided compelling counters to some of the old ones.
> Which ones? I don't buy your argument about the beauty of CAP_SYS_CHROOT.
CAP_SYS_CHROOT, namespaces. Bind mounts. The restrictions on
"unprivileged" chroot being sufficiently onerous to make it
unlikely to be usable.
>> It's possible to use minimal privilege with CAP_SYS_CHROOT.
> CAP_SYS_CHROOT can lead to privilege escalation.
Not when used in conjunction with the same set of
restrictions you're requiring for "unprivileged" chroot.
>> It looks like namespaces provide alternatives for all your
>> use cases.
> I explained in the commit message why it is not the case. In a nutshell,
> namespaces bring complexity which may not be required.
So? I can use a Swiss Army Knife to cut a string even though it
has a corkscrew.
> When designing a
> secure system, we want to avoid giving access to such complexity to
> untrusted processes (i.e. more complexity leads to more bugs).
If you're *really* designing a secure system you can design it to
use existing mechanisms, like CAP_SYS_CHROOT!
> An
> unprivileged chroot would enable to give just the minimum feature to
> drop some accesses. Of course it is not enough on its own, but it can be
> combined with existing (and future) security features.
Like NO_NEW_PRIVS, namespaces and capabilities!
You don't need anything new!
>> The constraints required to make this work are quite
>> limiting. Where is the real value add?
> As explain in the commit message, it is useful when hardening
> applications (e.g. network services, browsers, parsers, etc.). We don't
> want an untrusted (or compromised) application to have CAP_SYS_CHROOT
> nor (complex) namespace access.
If you can ensure that an unprivileged application is
always run with NO_NEW_PRIVS you could also ensure that
it runs with only CAP_SYS_CHROOT or in an appropriate
namespace. I believe that it would be easier for your
particular use case. I don't believe that is sufficient.
>>> Regards,
>>> Mickaël
>>>
>>>
>>> On 16/03/2021 21:36, Mickaël Salaün wrote:
>>>> From: Mickaël Salaün <mic@...ux.microsoft.com>
>>>>
>>>> Being able to easily change root directories enables to ease some
>>>> development workflow and can be used as a tool to strengthen
>>>> unprivileged security sandboxes. chroot(2) is not an access-control
>>>> mechanism per se, but it can be used to limit the absolute view of the
>>>> filesystem, and then limit ways to access data and kernel interfaces
>>>> (e.g. /proc, /sys, /dev, etc.).
>>>>
>>>> Users may not wish to expose namespace complexity to potentially
>>>> malicious processes, or limit their use because of limited resources.
>>>> The chroot feature is much more simple (and limited) than the mount
>>>> namespace, but can still be useful. As for containers, users of
>>>> chroot(2) should take care of file descriptors or data accessible by
>>>> other means (e.g. current working directory, leaked FDs, passed FDs,
>>>> devices, mount points, etc.). There is a lot of literature that discuss
>>>> the limitations of chroot, and users of this feature should be aware
of
>>>> the multiple ways to bypass it. Using chroot(2) for security purposes
>>>> can make sense if it is combined with other features (e.g. dedicated
>>>> user, seccomp, LSM access-controls, etc.).
>>>>
>>>> One could argue that chroot(2) is useless without a properly populated
>>>> root hierarchy (i.e. without /dev and /proc). However, there are
>>>> multiple use cases that don't require the chrooting process to create
>>>> file hierarchies with special files nor mount points, e.g.:
>>>> * A process sandboxing itself, once all its libraries are loaded, may
>>>> not need files other than regular files, or even no file at all.
>>>> * Some pre-populated root hierarchies could be used to chroot into,
>>>> provided for instance by development environments or tailored
>>>> distributions.
>>>> * Processes executed in a chroot may not require access to these special
>>>> files (e.g. with minimal runtimes, or by emulating some special files
>>>> with a LD_PRELOADed library or seccomp).
>>>>
>>>> Allowing a task to change its own root directory is not a threat to the
>>>> system if we can prevent confused deputy attacks, which could be
>>>> performed through execution of SUID-like binaries. This can be
>>>> prevented if the calling task sets PR_SET_NO_NEW_PRIVS on itself with
>>>> prctl(2). To only affect this task, its filesystem information must
not
>>>> be shared with other tasks, which can be achieved by not passing
>>>> CLONE_FS to clone(2). A similar no_new_privs check is already used by
>>>> seccomp to avoid the same kind of security issues. Furthermore, because
>>>> of its security use and to avoid giving a new way for attackers to get
>>>> out of a chroot (e.g. using /proc/<pid>/root, or chroot/chdir), an
>>>> unprivileged chroot is only allowed if the calling process is not
>>>> already chrooted. This limitation is the same as for creating user
>>>> namespaces.
>>>>
>>>> This change may not impact systems relying on other permission models
>>>> than POSIX capabilities (e.g. Tomoyo). Being able to use chroot(2) on
>>>> such systems may require to update their security policies.
>>>>
>>>> Only the chroot system call is relaxed with this no_new_privs check;
the
>>>> init_chroot() helper doesn't require such change.
>>>>
>>>> Allowing unprivileged users to use chroot(2) is one of the initial
>>>> objectives of no_new_privs:
>>>> https://www.kernel.org/doc/html/latest/userspace-api/no_new_privs.html
>>>> This patch is a follow-up of a previous one sent by Andy Lutomirski:
>>>> https://lore.kernel.org/lkml/0e2f0f54e19bff53a3739ecfddb4ffa9a6dbde4d.1327858005.git.luto@amacapital.net/
>>>>
>>>> Cc: Al Viro <viro@...iv.linux.org.uk>
>>>> Cc: Andy Lutomirski <luto@...capital.net>
>>>> Cc: Christian Brauner <christian.brauner@...ntu.com>
>>>> Cc: Christoph Hellwig <hch@....de>
>>>> Cc: David Howells <dhowells@...hat.com>
>>>> Cc: Dominik Brodowski <linux@...inikbrodowski.net>
>>>> Cc: Eric W. Biederman <ebiederm@...ssion.com>
>>>> Cc: James Morris <jmorris@...ei.org>
>>>> Cc: Jann Horn <jannh@...gle.com>
>>>> Cc: John Johansen <john.johansen@...onical.com>
>>>> Cc: Kentaro Takeda <takedakn@...data.co.jp>
>>>> Cc: Serge Hallyn <serge@...lyn.com>
>>>> Cc: Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
>>>> Signed-off-by: Mickaël Salaün <mic@...ux.microsoft.com>
>>>> Reviewed-by: Kees Cook <keescook@...omium.org>
>>>> Link: https://lore.kernel.org/r/20210316203633.424794-2-mic@digikod.net
>>>> ---
>>>>
>>>> Changes since v4:
>>>> * Use READ_ONCE(current->fs->users) (found by Jann Horn).
>>>> * Remove ambiguous example in commit description.
>>>> * Add Reviewed-by Kees Cook.
>>>>
>>>> Changes since v3:
>>>> * Move the new permission checks to a dedicated helper
>>>> current_chroot_allowed() to make the code easier to read and align
>>>> with user_path_at(), path_permission() and security_path_chroot()
>>>> calls (suggested by Kees Cook).
>>>> * Remove now useless included file.
>>>> * Extend commit description.
>>>> * Rebase on v5.12-rc3 .
>>>>
>>>> Changes since v2:
>>>> * Replace path_is_under() check with current_chrooted() to gain the same
>>>> protection as create_user_ns() (suggested by Jann Horn). See commit
>>>> 3151527ee007 ("userns: Don't allow creation if the user is chrooted")
>>>>
>>>> Changes since v1:
>>>> * Replace custom is_path_beneath() with existing path_is_under().
>>>> ---
>>>> fs/open.c | 23 +++++++++++++++++++++--
>>>> 1 file changed, 21 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/fs/open.c b/fs/open.c
>>>> index e53af13b5835..480010a551b2 100644
>>>> --- a/fs/open.c
>>>> +++ b/fs/open.c
>>>> @@ -532,6 +532,24 @@ SYSCALL_DEFINE1(fchdir, unsigned int, fd)
>>>> return error;
>>>> }
>>>>
>>>> +static inline int current_chroot_allowed(void)
>>>> +{
>>>> + /*
>>>> + * Changing the root directory for the calling task (and its future
>>>> + * children) requires that this task has CAP_SYS_CHROOT in its
>>>> + * namespace, or be running with no_new_privs and not sharing its
>>>> + * fs_struct and not escaping its current root (cf. create_user_ns()).
>>>> + * As for seccomp, checking no_new_privs avoids scenarios where
>>>> + * unprivileged tasks can affect the behavior of privileged children.
>>>> + */
>>>> + if (task_no_new_privs(current) && READ_ONCE(current->fs->users) ==
>> 1 &&
>>>> + !current_chrooted())
>>>> + return 0;
>>>> + if (ns_capable(current_user_ns(), CAP_SYS_CHROOT))
>>>> + return 0;
>>>> + return -EPERM;
>>>> +}
>>>> +
>>>> SYSCALL_DEFINE1(chroot, const char __user *, filename)
>>>> {
>>>> struct path path;
>>>> @@ -546,9 +564,10 @@ SYSCALL_DEFINE1(chroot, const char __user *, filename)
>>>> if (error)
>>>> goto dput_and_out;
>>>>
>>>> - error = -EPERM;
>>>> - if (!ns_capable(current_user_ns(), CAP_SYS_CHROOT))
>>>> + error = current_chroot_allowed();
>>>> + if (error)
>>>> goto dput_and_out;
>>>> +
>>>> error = security_path_chroot(&path);
>>>> if (error)
>>>> goto dput_and_out;
>>>>
Powered by blists - more mailing lists
Confused about mailing lists and their use?
Read about mailing lists on Wikipedia
and check out these
guidelines on proper formatting of your messages.
