[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <85ebb3a1-bd5e-9f12-6d02-c08d2c0acff5@schaufler-ca.com>
Date: Tue, 30 Mar 2021 10:19:12 -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>
Subject: Re: [PATCH v5 1/1] fs: Allow no_new_privs tasks to call chroot(2)
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.
It's possible to use minimal privilege with CAP_SYS_CHROOT.
It looks like namespaces provide alternatives for all your
use cases. The constraints required to make this work are quite
limiting. Where is the real value add?
>
> 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.
