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

On 19/04/2017 00:17, Kees Cook wrote:

On Tue, Mar 28, 2017 at 4:46 PM, Mickaël Salaün [off-list ref] wrote:

quoted

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: Mickaël Salaün <mic@digikod.net>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: David S. Miller <davem@davemloft.net>
Cc: James Morris <redacted>
Cc: Kees Cook <redacted>
Cc: Serge E. Hallyn <serge@hallyn.com>
---
 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.

This is useful to catch defined but unused hooks: error out if a
HOOK_NEW_FS(foo) is not used with a HOOK_INIT_FS(foo) in the struct
security_hook_list landlock_hooks.

quoted

[...]

@@ -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")?

Yes, this is ugly with the new security_add_hooks() with three arguments
but I wanted to split the hooks definition in multiple files.

The current security_add_hooks() use lsm_append(lsm, &lsm_names) which
is not exported. Unfortunately, calling multiple security_add_hooks()
with the same LSM name would register multiple names for the same LSM…
Is it OK if I modify this function to not add duplicated entries?

quoted

+}

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?

Right, this is the intend. I'm not sure it is the only way to register
hooks, though.

For an unprivileged access-control, we don't want to give the ability to
any process to do some checks, through an eBPF program, on kernel
objects (e.g. files) if they should not be accessible (because of a
following LSM hook check).

 Mickaël