[PATCH RFC 4/7] fs: support FD_FAILFS_ROOT in fchroot()

Allow a process to move its root directory into failfs via
fchroot(FD_FAILFS_ROOT). From that point on every absolute path lookup
and every absolute symlink fails with EOPNOTSUPP. Combined with
fchdir(FD_FAILFS_ROOT) this leaves the process with lookups anchored
at explicit directory file descriptors only. It is the fs_struct
equivalent of RESOLVE_BENEATH. This allows taks to drop their filesystem
state completely.

Callers with CAP_SYS_CHROOT in their user namespace may always do
this, mirroring chroot(2). Unprivileged callers are subject to two
requirements (which may be loosened later):

(1) no_new_privs must be set

    After entering failfs suid binaries on regular mounts remain
    reachable via inherited directory file descriptors or the working
    directory. A setuid program executing with an unusable root
    directory might be tricked by this. I'm not 100% convinced that this
    is needed but it feels more secure initially and it also forces more
    no_new_privs on userspace. So win-win imo.

(2) The caller must not already be chrooted.

    The root directory is what confines .. resolution. The failfs root
    can never be reached by walking up a real mount tree. A task whose
    root is failfs has no .. barrier left below the top of its mount
    tree. A .. walk from any real directory fd it still holds climbs
    to the mount-namespace root. Which is kinda the point if you want to
    do fd-based lookup only. If failfs prevented you from doing that
    then it doesn't make a lot of sense.

    A task that a privileged manager chrooted into a subtree could use
    chroot()ing into failfs as a way to allow for an inherited fd to
    resolve it again.

    So reject already-chrooted callers closing that issue without losing
    anything for the intended self-sandboxing use case.

There's also some thought needed around shared fs_struct state. It's
obviously possible to chroot into failfs with a shared fs_struct if the
caller has CAP_SYS_CHROOT and shares the fs_struct or if the caller is
no_new_privs and shares the fs_struct. The non-chrooted-currently
requirement still applies.

Once entered, failfs is a throw-away-the-key moment. The task is
considered chrooted so it cannot create user namespaces to regain
CAP_SYS_CHROOT. chroot()/fchroot() back out require CAP_SYS_CHROOT.

The only other exit is setns() to a mount namespace file descriptor
which requires CAP_SYS_ADMIN over the target namespace plus
CAP_SYS_CHROOT and CAP_SYS_ADMIN in the caller's user namespace and
resets both root and working directory. A process that closes or never
had such file descriptors and restricts *chdir()/*chroot()/setns() via
seccomp has thrown away the key.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 fs/open.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/fs/open.c b/fs/open.c
index c57f641f2e29..8adc9f00889a 100644
--- a/fs/open.c
+++ b/fs/open.c

@@ -618,6 +618,27 @@ SYSCALL_DEFINE1(chroot, const char __user *, filename)
 	return error;
 }
 
+static int fchroot_failfs(void)
+{
+	struct path path;
+	int error;
+
+	if (!ns_capable(current_user_ns(), CAP_SYS_CHROOT)) {
+		if (!task_no_new_privs(current))
+			return -EPERM;
+		/* Moving the root to failfs lifts the old root's ".." barrier. */
+		if (current_chrooted())
+			return -EPERM;
+	}
+
+	failfs_get_root(&path);
+	error = security_path_chroot(&path);
+	if (!error)
+		set_fs_root(current->fs, &path);
+	path_put(&path);
+	return error;
+}
+
 SYSCALL_DEFINE2(fchroot, int, fd, unsigned int, flags)
 {
 	int error;

@@ -625,6 +646,9 @@ SYSCALL_DEFINE2(fchroot, int, fd, unsigned int, flags)
 	if (flags)
 		return -EINVAL;
 
+	if (fd == FD_FAILFS_ROOT)
+		return fchroot_failfs();
+
 	CLASS(fd_raw, f)(fd);
 	if (fd_empty(f))
 		return -EBADF;

-- 
2.53.0