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

Mickaël Salaün
2021-03-23T19:30:37Z

On 19/03/2021 20:19, Mickaël Salaün wrote:
>
> 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:

I think the inode helper would have to be in a separate patch focused on
fs/ (like all matches of your greps, except Landlock). Are you OK if I
send a patch for that once Landlock is merged?

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