Re: [PATCH v12 02/26] securityfs: Extend securityfs with namespacing support

[Christian Brauner <brauner@kernel.org>]

On Fri, May 20, 2022 at 09:23:02PM -0500, Serge Hallyn wrote:
> On Wed, Apr 20, 2022 at 10:06:09AM -0400, Stefan Berger wrote:
> > Enable multiple instances of securityfs by keying each instance with a
> > pointer to the user namespace it belongs to.
> > 
> > Since we do not need the pinning of the filesystem for the virtualization
> > case, limit the usage of simple_pin_fs() and simpe_release_fs() to the
> > case when the init_user_ns is active. This simplifies the cleanup for the
> > virtualization case where usage of securityfs_remove() to free dentries
> > is therefore not needed anymore.
> > 
> > For the initial securityfs, i.e. the one mounted in the host userns mount,
> > nothing changes. The rules for securityfs_remove() are as before and it is
> > still paired with securityfs_create(). Specifically, a file created via
> > securityfs_create_dentry() in the initial securityfs mount still needs to
> > be removed by a call to securityfs_remove(). Creating a new dentry in the
> > initial securityfs mount still pins the filesystem like it always did.
> > Consequently, the initial securityfs mount is not destroyed on
> > umount/shutdown as long as at least one user of it still has dentries that
> > it hasn't removed with a call to securityfs_remove().
> > 
> > Prevent mounting of an instance of securityfs in another user namespace
> > than it belongs to. Also, prevent accesses to files and directories by
> > a user namespace that is neither the user namespace it belongs to
> > nor an ancestor of the user namespace that the instance of securityfs
> > belongs to. Do not prevent access if securityfs was bind-mounted and
> > therefore the init_user_ns is the owning user namespace.
> > 
> > Suggested-by: Christian Brauner <brauner@kernel.org>
> > Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
> > Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
> > 
> > ---
> > v11:
> >  - Formatted comment's first line to be '/*'
> > ---
> >  security/inode.c | 73 ++++++++++++++++++++++++++++++++++++++++--------
> >  1 file changed, 62 insertions(+), 11 deletions(-)
> > 
> > diff --git a/security/inode.c b/security/inode.c
> > index 13e6780c4444..84c9396792a9 100644
> > --- a/security/inode.c
> > +++ b/security/inode.c
> > @@ -21,9 +21,38 @@
> >  #include <linux/security.h>
> >  #include <linux/lsm_hooks.h>
> >  #include <linux/magic.h>
> > +#include <linux/user_namespace.h>
> >  
> > -static struct vfsmount *mount;
> > -static int mount_count;
> > +static struct vfsmount *init_securityfs_mount;
> > +static int init_securityfs_mount_count;
> > +
> > +static int securityfs_permission(struct user_namespace *mnt_userns,
> > +				 struct inode *inode, int mask)
> > +{
> > +	int err;
> > +
> > +	err = generic_permission(&init_user_ns, inode, mask);
> > +	if (!err) {
> > +		/*
> > +		 * Unless bind-mounted, deny access if current_user_ns() is not
> > +		 * ancestor.
> 
> This comment has confused me the last few times I looked at this.  I see
> now you're using "bind-mounted" as a shortcut for saying "bind mounted from
> the init_user_ns into a child_user_ns container".  I do think that needs
> to be made clearer in this comment.
> 
> Should the init_user_ns really be special here?  What if I'm running a
> first level container with uptodate userspace that mounts its own
> securityfs, but in that i want to run a nested older userspace that
> bind mounts the parent securityfs?  Is there a good reason to deny that?
> 
> It would seem to me the better check would be
> 
> 	if (!is_original_mounter_of(current_user_ns, inode->i_sb->s_user_ns) &&
> 	     !in_userns(current_user_ns(), inode->i_sb->s_user_ns))
> 		err = -EACCESS;
> 
> the is_original_mounter_of() would require the user_ns to cache first
> its parent securityfs userns, and, when a task in the user_ns mounts
> securityfs, then cache its own userns.  (without a reference).
> If current_user_ns() has mounted a securityfs for a user_ns other than
> inode->i_sb->s_user_ns (or init_user_ns), then reject the mount.
> Otherwise check current_user_ns()->parent, etc, until init_user_ns.
> If you reach init_user_ns, or an ns which mounted inode->i_sb->s_user_ns,
> then allow, else deny.
> 
> It's the kind of special casing we've worked hard to avoid in other
> namespaces.
> 
> > +		 */
> > +		if (inode->i_sb->s_user_ns != &init_user_ns &&
> > +		    !in_userns(current_user_ns(), inode->i_sb->s_user_ns))
> > +			err = -EACCES;
> > +	}
> > +
> > +	return err;
> > +}
> > +
> > +static const struct inode_operations securityfs_dir_inode_operations = {
> > +	.permission	= securityfs_permission,
> > +	.lookup		= simple_lookup,
> > +};
> > +
> > +static const struct inode_operations securityfs_file_inode_operations = {
> > +	.permission	= securityfs_permission,
> > +};
> >  
> >  static void securityfs_free_inode(struct inode *inode)
> >  {
> > @@ -40,20 +69,25 @@ static const struct super_operations securityfs_super_operations = {
> >  static int securityfs_fill_super(struct super_block *sb, struct fs_context *fc)
> >  {
> >  	static const struct tree_descr files[] = {{""}};
> > +	struct user_namespace *ns = fc->user_ns;
> >  	int error;
> >  
> > +	if (WARN_ON(ns != current_user_ns()))
> > +		return -EINVAL;
> > +
> >  	error = simple_fill_super(sb, SECURITYFS_MAGIC, files);
> >  	if (error)
> >  		return error;
> >  
> >  	sb->s_op = &securityfs_super_operations;
> > +	sb->s_root->d_inode->i_op = &securityfs_dir_inode_operations;
> >  
> >  	return 0;
> >  }
> >  
> >  static int securityfs_get_tree(struct fs_context *fc)
> >  {
> > -	return get_tree_single(fc, securityfs_fill_super);
> > +	return get_tree_keyed(fc, securityfs_fill_super, fc->user_ns);
> >  }
> >  
> >  static const struct fs_context_operations securityfs_context_ops = {
> > @@ -71,6 +105,7 @@ static struct file_system_type fs_type = {
> >  	.name =		"securityfs",
> >  	.init_fs_context = securityfs_init_fs_context,
> >  	.kill_sb =	kill_litter_super,
> > +	.fs_flags =	FS_USERNS_MOUNT,
> >  };
> >  
> >  /**
> > @@ -109,6 +144,7 @@ static struct dentry *securityfs_create_dentry(const char *name, umode_t mode,
> >  					const struct file_operations *fops,
> >  					const struct inode_operations *iops)
> >  {
> > +	struct user_namespace *ns = current_user_ns();
> >  	struct dentry *dentry;
> >  	struct inode *dir, *inode;
> >  	int error;
> > @@ -118,12 +154,19 @@ static struct dentry *securityfs_create_dentry(const char *name, umode_t mode,
> >  
> >  	pr_debug("securityfs: creating file '%s'\n",name);
> >  
> > -	error = simple_pin_fs(&fs_type, &mount, &mount_count);
> > -	if (error)
> > -		return ERR_PTR(error);
> > +	if (ns == &init_user_ns) {
> > +		error = simple_pin_fs(&fs_type, &init_securityfs_mount,
> > +				      &init_securityfs_mount_count);
> 
> So ...  it's less work for the kernel to skip the simple_pin_fs()
> here, but it's more code, and more confusing code, to skip it.
> 
> So I just want to ask, to make sure:  is it worth it?  Or should
> it just be done for all namespaces here (and below and for release),
> for shorter, simpler, easier to read and grok code?

I think you might've skipped a few version of the thread.
It would be more code and a lot more confusing to try and keep the
simple_pin_fs(). You will need a per-userns vfsmount pointer and you
still need to change securityfs_create_dentry and securityfs_remove. For
more context see [1] and [2].

[1]: https://lore.kernel.org/lkml/20211206172600.1495968-12-stefanb@linux.ibm.com
[2]: https://lore.kernel.org/lkml/20211206172600.1495968-13-stefanb@linux.ibm.com

The fs pinning logic is most suited for single-superblock, almost
system-lifetime bound pseudo filesystems such as debugfs or configfs.
It becomes a rather huge burden when a pseudo fs is supposed to
support multiple superblocks (in this case keyed by userns).