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

Mickaël Salaün
2021-03-19T19:20:51Z

On 19/03/2021 19:57, Kees Cook wrote:
> 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

Dealing with the filesystem is complex. Some helpers could probably be
added, but with a series dedicated to the filesystem. I can work on that
once this series is merged.

>
>> +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.

It should not, only a bogus kernel code could.

>
>> [...]
>> 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. :)

An LSM can be marked as enabled (at boot) but not yet initialized.

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

It is a condition for LSM hooks, syscalls and superblock management.

>
>> 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>
>