Re: [RFC v2 06/10] landlock: Add LSM hooks

On Aug 25, 2016 12:34 PM, "Mickaël Salaün" [off-list ref] wrote:

Add LSM hooks which can be used by userland through Landlock (eBPF)
programs. This programs are limited to a whitelist of functions (cf.
next commit). The eBPF program context is depicted by the struct
landlock_data (cf. include/uapi/linux/bpf.h):
* hook: LSM hook ID (useful when using the same program for multiple LSM
  hooks);
* cookie: the 16-bit value from the seccomp filter that triggered this
  Landlock program;
* args[6]: array of LSM hook arguments.

The LSM hook arguments can contain raw values as integers or
(unleakable) pointers. The only way to use the pointers are to pass them
to an eBPF function according to their types (e.g. the
bpf_landlock_cmp_fs_beneath_with_struct_file function can use a struct
file pointer).

For now, there is three hooks for file system access control:
* file_open;
* file_permission;
* mmap_file.

What's the purpose of exposing struct cred * to userspace?  It's
primarily just an optimization to save a bit of RAM, and it's a
dubious optimization at that.  What are you using it for?  Would it
make more sense to use struct task_struct * or struct pid * instead?

Also, exposing struct cred * has a really weird side-effect: it allows
(maybe even encourages) checking for pointer equality between two
struct cred * objects.  Doing so will have erratic results.