Re: [PATCH v4 1/1] fs: Allow no_new_privs tasks to call chroot(2)

On 16/03/2021 20:24, Kees Cook wrote:

On Tue, Mar 16, 2021 at 08:04:09PM +0100, Jann Horn wrote:

quoted

On Tue, Mar 16, 2021 at 6:02 PM Mickaël Salaün [off-list ref] wrote:

quoted

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).

Unprivileged chroot is especially interesting for userspace developers
wishing to harden their applications.  For instance, chroot(2) and Yama
enable to build a capability-based security (i.e. remove filesystem
ambient accesses) by calling chroot/chdir with an empty directory and
accessing data through dedicated file descriptors obtained with
openat2(2) and RESOLVE_BENEATH/RESOLVE_IN_ROOT/RESOLVE_NO_MAGICLINKS.

I don't entirely understand. Are you writing this with the assumption
that a future change will make it possible to set these RESOLVE flags
process-wide, or something like that?

I thought it meant "open all out-of-chroot dirs as fds using RESOLVE_...
flags then chroot". As in, there's no way to then escape "up" for the
old opens, and the new opens stay in the chroot.

Yes, that was the idea.

quoted

[...]

quoted

diff --git a/fs/open.c b/fs/open.c

[...]

quoted

+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) && current->fs->users == 1 &&

this read of current->fs->users should be using READ_ONCE()

Ah yeah, good call. I should remember this when I think "can this race?"
:P