[PATCH RFC 4/4] fs: allow mknod in non-initial userns using cgroup device guard

[Michael Weiß]

If a container manager restricts its unprivileged (user namespaced)
children by a device cgroup, it is not necessary to deny mknod
anymore. Thus, user space applications may map devices on different
locations in the file system by using mknod() inside the container.

A use case for this, we also use in GyroidOS, is to run virsh for
VMs inside an unprivileged container. virsh creates device nodes,
e.g., "/var/run/libvirt/qemu/11-fgfg.dev/null" which currently fails
in a non-initial userns, even if a cgroup device white list with the
corresponding major, minor of /dev/null exists. Thus, in this case
the usual bind mounts or pre populated device nodes under /dev are
not sufficient.

To circumvent this limitation, we allow mknod() in fs/namei.c if a
bpf cgroup device guard is enabeld for the current task using
devcgroup_task_is_guarded() and check CAP_MKNOD for the current user
namespace by ns_capable() instead of the global CAP_MKNOD.

To avoid unusable device nodes on file systems mounted in
non-initial user namespace, may_open_dev() ignores the SB_I_NODEV
for cgroup device guarded tasks.

Signed-off-by: Michael Weiß <michael.weiss@xxxxxxxxxxxxxxxxxxx>
---
 fs/namei.c | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index e56ff39a79bc..ef4f22b9575c 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -3221,6 +3221,9 @@ EXPORT_SYMBOL(vfs_mkobj);
 
 bool may_open_dev(const struct path *path)
 {
+	if (devcgroup_task_is_guarded(current))
+		return !(path->mnt->mnt_flags & MNT_NODEV);
+
 	return !(path->mnt->mnt_flags & MNT_NODEV) &&
 		!(path->mnt->mnt_sb->s_iflags & SB_I_NODEV);
 }
@@ -3976,9 +3979,19 @@ int vfs_mknod(struct mnt_idmap *idmap, struct inode *dir,
 	if (error)
 		return error;
 
-	if ((S_ISCHR(mode) || S_ISBLK(mode)) && !is_whiteout &&
-	    !capable(CAP_MKNOD))
-		return -EPERM;
+	/*
+	 * In case of a device cgroup restirction allow mknod in user
+	 * namespace. Otherwise just check global capability; thus,
+	 * mknod is also disabled for user namespace other than the
+	 * initial one.
+	 */
+	if ((S_ISCHR(mode) || S_ISBLK(mode)) && !is_whiteout) {
+		if (devcgroup_task_is_guarded(current)) {
+			if (!ns_capable(current_user_ns(), CAP_MKNOD))
+				return -EPERM;
+		} else if (!capable(CAP_MKNOD))
+			return -EPERM;
+	}
 
 	if (!dir->i_op->mknod)
 		return -EPERM;

-- 
2.30.2