Re: [PATCH net-next v6 04/11] landlock: Add LSM hooks related to filesystem

[Kees Cook]

On Tue, Mar 28, 2017 at 4:46 PM, MickaÃl SalaÃn <mic@xxxxxxxxxxx> wrote:
> Handle 33 filesystem-related LSM hooks for the Landlock filesystem
> event: LANDLOCK_SUBTYPE_EVENT_FS.
>
> A Landlock event wrap LSM hooks for similar kernel object types (e.g.
> struct file, struct path...). Multiple LSM hooks can trigger the same
> Landlock event.
>
> Landlock handle nine coarse-grained actions: read, write, execute, new,
> get, remove, ioctl, lock and fcntl. Each of them abstract LSM hook
> access control in a way that can be extended in the future.
>
> The Landlock LSM hook registration is done after other LSM to only run
> actions from user-space, via eBPF programs, if the access was granted by
> major (privileged) LSMs.
>
> Changes since v5:
> * split hooks.[ch] into hooks.[ch] and hooks_fs.[ch]
> * add more documentation
> * cosmetic fixes
>
> Changes since v4:
> * add LSM hook abstraction called Landlock event
>   * use the compiler type checking to verify hooks use by an event
>   * handle all filesystem related LSM hooks (e.g. file_permission,
>     mmap_file, sb_mount...)
> * register BPF programs for Landlock just after LSM hooks registration
> * move hooks registration after other LSMs
> * add failsafes to check if a hook is not used by the kernel
> * allow partial raw value access form the context (needed for programs
>   generated by LLVM)
>
> Changes since v3:
> * split commit
> * add hooks dealing with struct inode and struct path pointers:
>   inode_permission and inode_getattr
> * add abstraction over eBPF helper arguments thanks to wrapping structs
>
> Signed-off-by: MickaÃl SalaÃn <mic@xxxxxxxxxxx>
> Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> Cc: David S. Miller <davem@xxxxxxxxxxxxx>
> Cc: James Morris <james.l.morris@xxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Serge E. Hallyn <serge@xxxxxxxxxx>
> ---
>  include/linux/lsm_hooks.h    |   5 +
>  security/landlock/Makefile   |   4 +-
>  security/landlock/hooks.c    | 115 +++++++++
>  security/landlock/hooks.h    | 177 ++++++++++++++
>  security/landlock/hooks_fs.c | 563 +++++++++++++++++++++++++++++++++++++++++++
>  security/landlock/hooks_fs.h |  19 ++
>  security/landlock/init.c     |  13 +
>  security/security.c          |   7 +-
>  8 files changed, 901 insertions(+), 2 deletions(-)
>  create mode 100644 security/landlock/hooks.c
>  create mode 100644 security/landlock/hooks.h
>  create mode 100644 security/landlock/hooks_fs.c
>  create mode 100644 security/landlock/hooks_fs.h
>
> diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
> index e29d4c62a3c8..884289166a0e 100644
> --- a/include/linux/lsm_hooks.h
> +++ b/include/linux/lsm_hooks.h
> @@ -1920,5 +1920,10 @@ void __init loadpin_add_hooks(void);
>  #else
>  static inline void loadpin_add_hooks(void) { };
>  #endif
> +#ifdef CONFIG_SECURITY_LANDLOCK
> +extern void __init landlock_add_hooks(void);
> +#else
> +static inline void __init landlock_add_hooks(void) { }
> +#endif /* CONFIG_SECURITY_LANDLOCK */
>
>  #endif /* ! __LINUX_LSM_HOOKS_H */
> diff --git a/security/landlock/Makefile b/security/landlock/Makefile
> index 7205f9a7a2ee..c0db504a6335 100644
> --- a/security/landlock/Makefile
> +++ b/security/landlock/Makefile
> @@ -1,3 +1,5 @@
> +ccflags-$(CONFIG_SECURITY_LANDLOCK) += -Werror=unused-function

Why is this needed? If it can't be avoided, a comment should exist
here explaining why.

> [...]
> @@ -127,3 +132,11 @@ static struct bpf_prog_type_list bpf_landlock_type __ro_after_init = {
>         .ops = &bpf_landlock_ops,
>         .type = BPF_PROG_TYPE_LANDLOCK,
>  };
> +
> +void __init landlock_add_hooks(void)
> +{
> +       pr_info("landlock: Version %u", LANDLOCK_VERSION);
> +       landlock_add_hooks_fs();
> +       security_add_hooks(NULL, 0, "landlock");
> +       bpf_register_prog_type(&bpf_landlock_type);

I'm confused by the separation of hook registration here. The call to
security_add_hooks is with count=0 is especially weird. Why isn't this
just a single call with security_add_hooks(landlock_hooks,
ARRAY_SIZE(landlock_hooks), "landlock")?

> +}
> diff --git a/security/security.c b/security/security.c
> index d0e07f269b2d..a3e9f4625991 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -64,10 +64,15 @@ int __init security_init(void)
>         loadpin_add_hooks();
>
>         /*
> -        * Load all the remaining security modules.
> +        * Load all remaining privileged security modules.
>          */
>         do_security_initcalls();
>
> +       /*
> +        * Load potentially-unprivileged security modules at the end.
> +        */
> +       landlock_add_hooks();

Oh, is this to make it last in the list? Is there a reason it has to be last?

-Kees

-- 
Kees Cook
Pixel Security