Re: [PATCH v30 07/12] landlock: Support filesystem access-control

Kees Cook
2021-03-19T18:58:24Z

On Tue, Mar 16, 2021 at 09:42:47PM +0100, Mickaël Salaün wrote:
> From: Mickaël Salaün <mic@linux.microsoft.com>
>
> Using Landlock objects and ruleset, it is possible to tag inodes
> according to a process's domain. To enable an unprivileged process to
> express a file hierarchy, it first needs to open a directory (or a file)
> and pass this file descriptor to the kernel through
> landlock_add_rule(2). When checking if a file access request is
> allowed, we walk from the requested dentry to the real root, following
> the different mount layers. The access to each "tagged" inodes are
> collected according to their rule layer level, and ANDed to create
> access to the requested file hierarchy. This makes possible to identify
> a lot of files without tagging every inodes nor modifying the
> filesystem, while still following the view and understanding the user
> has from the filesystem.
>
> Add a new ARCH_EPHEMERAL_INODES for UML because it currently does not
> keep the same struct inodes for the same inodes whereas these inodes are
> in use.
>
> This commit adds a minimal set of supported filesystem access-control
> which doesn't enable to restrict all file-related actions. This is the
> result of multiple discussions to minimize the code of Landlock to ease
> review. Thanks to the Landlock design, extending this access-control
> without breaking user space will not be a problem. Moreover, seccomp
> filters can be used to restrict the use of syscall families which may
> not be currently handled by Landlock.
>
> Cc: Al Viro <viro@zeniv.linux.org.uk>
> Cc: Anton Ivanov <anton.ivanov@cambridgegreys.com>
> Cc: James Morris <jmorris@namei.org>
> Cc: Jann Horn <jannh@google.com>
> Cc: Jeff Dike <jdike@addtoit.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Richard Weinberger <richard@nod.at>
> Cc: Serge E. Hallyn <serge@hallyn.com>
> Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com>
> Link: https://lore.kernel.org/r/20210316204252.427806-8-mic@digikod.net
> [...]
> + spin_lock(&sb->s_inode_list_lock);
> + list_for_each_entry(inode, &sb->s_inodes, i_sb_list) {
> + struct landlock_object *object;
> +
> + /* Only handles referenced inodes. */
> + if (!atomic_read(&inode->i_count))
> + continue;
> +
> + /*
> + * Checks I_FREEING and I_WILL_FREE to protect against a race
> + * condition when release_inode() just called iput(), which
> + * could lead to a NULL dereference of inode->security or a
> + * second call to iput() for the same Landlock object. Also
> + * checks I_NEW because such inode cannot be tied to an object.
> + */
> + spin_lock(&inode->i_lock);
> + if (inode->i_state & (I_FREEING | I_WILL_FREE | I_NEW)) {
> + spin_unlock(&inode->i_lock);
> + continue;
> + }

This (and elsewhere here) seems like a lot of inode internals getting
exposed. Can any of this be repurposed into helpers? I see this test
scattered around the kernel a fair bit:

$ git grep I_FREEING | grep I_WILL_FREE | grep I_NEW | wc -l
9

> +static inline u32 get_mode_access(const umode_t mode)
> +{
> + switch (mode & S_IFMT) {
> + case S_IFLNK:
> + return LANDLOCK_ACCESS_FS_MAKE_SYM;
> + case 0:
> + /* A zero mode translates to S_IFREG. */
> + case S_IFREG:
> + return LANDLOCK_ACCESS_FS_MAKE_REG;
> + case S_IFDIR:
> + return LANDLOCK_ACCESS_FS_MAKE_DIR;
> + case S_IFCHR:
> + return LANDLOCK_ACCESS_FS_MAKE_CHAR;
> + case S_IFBLK:
> + return LANDLOCK_ACCESS_FS_MAKE_BLOCK;
> + case S_IFIFO:
> + return LANDLOCK_ACCESS_FS_MAKE_FIFO;
> + case S_IFSOCK:
> + return LANDLOCK_ACCESS_FS_MAKE_SOCK;
> + default:
> + WARN_ON_ONCE(1);
> + return 0;
> + }

I'm assuming this won't be reachable from userspace.

> [...]
> index a5d6ef334991..f8e8e980454c 100644
> --- a/security/landlock/setup.c
> +++ b/security/landlock/setup.c
> @@ -11,17 +11,24 @@
>
> #include "common.h"
> #include "cred.h"
> +#include "fs.h"
> #include "ptrace.h"
> #include "setup.h"
>
> +bool landlock_initialized __lsm_ro_after_init = false;
> +
> struct lsm_blob_sizes landlock_blob_sizes __lsm_ro_after_init = {
> .lbs_cred = sizeof(struct landlock_cred_security),
> + .lbs_inode = sizeof(struct landlock_inode_security),
> + .lbs_superblock = sizeof(struct landlock_superblock_security),
> };
>
> static int __init landlock_init(void)
> {
> landlock_add_cred_hooks();
> landlock_add_ptrace_hooks();
> + landlock_add_fs_hooks();
> + landlock_initialized = true;

I think this landlock_initialized is logically separate from the optional
DEFINE_LSM "enabled" variable, but I thought I'd double check. :)

It seems like it's used here to avoid releasing superblocks before
landlock_init() is called? What is the scenario where that happens?

> pr_info("Up and running.\n");
> return 0;
> }
> diff --git a/security/landlock/setup.h b/security/landlock/setup.h
> index 9fdbf33fcc33..1daffab1ab4b 100644
> --- a/security/landlock/setup.h
> +++ b/security/landlock/setup.h
> @@ -11,6 +11,8 @@
>
> #include <linux/lsm_hooks.h>
>
> +extern bool landlock_initialized;
> +
> extern struct lsm_blob_sizes landlock_blob_sizes;
>
> #endif /* _SECURITY_LANDLOCK_SETUP_H */
> --
> 2.30.2
>

The locking and inode semantics are pretty complex, but since, again,
it's got significant test and syzkaller coverage, it looks good to me.

With the inode helper cleanup:

Reviewed-by: Kees Cook <keescook@chromium.org>

--
Kees Cook