From: "Mickaël Salaün" <email@example.com> To: "Eric W. Biederman" <firstname.lastname@example.org> Cc: "Al Viro" <email@example.com>, "James Morris" <firstname.lastname@example.org>, "Serge Hallyn" <email@example.com>, "Andy Lutomirski" <firstname.lastname@example.org>, "Christian Brauner" <email@example.com>, "Christoph Hellwig" <firstname.lastname@example.org>, "David Howells" <email@example.com>, "Dominik Brodowski" <firstname.lastname@example.org>, "John Johansen" <email@example.com>, "Kees Cook" <firstname.lastname@example.org>, "Kentaro Takeda" <email@example.com>, "Tetsuo Handa" <firstname.lastname@example.org>, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, "Mickaël Salaün" <email@example.com> Subject: Re: [PATCH v1 1/1] fs: Allow no_new_privs tasks to call chroot(2) Date: Wed, 10 Mar 2021 19:13:33 +0100 [thread overview] Message-ID: <firstname.lastname@example.org> (raw) In-Reply-To: <email@example.com> On 10/03/2021 17:56, Eric W. Biederman wrote: > Mickaël Salaün <firstname.lastname@example.org> writes: > >> From: Mickaël Salaün <email@example.com> >> >> Being able to easily change root directories enable 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.). > > Actually chroot does not so limit the view of things. It only limits > the default view. > > A process that is chrooted can always escape by something like > chroot("../../../../../../../../.."). Not with this patch. > > So I don't see the point of allowing chroot once you are in your locked > down sandbox. > >> 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), an unprivileged chroot is >> only allowed if the new root directory is the same or beneath the >> current one. This still allows a process to use a subset of its >> legitimate filesystem to chroot into and then further reduce its view of >> the filesystem. >> >> 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, but >> with less limitations: >> https://firstname.lastname@example.org/ > > Last time I remember talking architecture we agreed that user namespaces > would be used for enabling features and that no_new_privs would just be > used to lock-down userspace. That way no_new_privs could be kept simple > and trivial to audit and understand. chroot(2) is simple. > > You can build your sandbox and use chroot if you use a user namespace at > the start. A mount namespace would also help lock things down. Still > allowing chroot after the sanbox has been built, a seccomp filter has > been installed and no_new_privs has been enabled seems like it is asking > for trouble and may weaken existing sandboxes. Could you please provide a new attack scenario? > > So I think we need a pretty compelling use case to consider allowing > chroot(2). You haven't even mentioned what your usecase is at this > point so I don't know why we would tackle that complexity. They are explained in this commit message. > > Eric >
next prev parent reply other threads:[~2021-03-10 18:14 UTC|newest] Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-03-10 16:09 [PATCH v1 0/1] Unprivileged chroot Mickaël Salaün 2021-03-10 16:10 ` [PATCH v1 1/1] fs: Allow no_new_privs tasks to call chroot(2) Mickaël Salaün 2021-03-10 16:56 ` Eric W. Biederman 2021-03-10 18:13 ` Mickaël Salaün [this message] 2021-03-10 17:22 ` [PATCH v1 0/1] Unprivileged chroot Casey Schaufler 2021-03-10 18:17 ` Mickaël Salaün 2021-03-10 20:59 ` Casey Schaufler 2021-03-11 10:42 ` Mickaël Salaün
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 \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --subject='Re: [PATCH v1 1/1] fs: Allow no_new_privs tasks to call chroot(2)' \ /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 an external index of several public inboxes, see mirroring instructions on how to clone and mirror all data and code used by this external index.