landlock: Add ptrace restrictions

Using ptrace(2) and related debug features on a target process can lead
to a privilege escalation.  Indeed, ptrace(2) can be used by an attacker
to impersonate another task and to remain undetected while performing
malicious activities.  Thanks to  ptrace_may_access(), various part of
the kernel can check if a tracer is more privileged than a tracee.

A landlocked process has fewer privileges than a non-landlocked process
and must then be subject to additional restrictions when manipulating
processes. To be allowed to use ptrace(2) and related syscalls on a
target process, a landlocked process must have a subset of the target
process's rules (i.e. the tracee must be in a sub-domain of the tracer).

Cc: James Morris <[email protected]>
Signed-off-by: Mickaël Salaün <[email protected]>
Reviewed-by: Jann Horn <[email protected]>
Acked-by: Serge Hallyn <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: James Morris <[email protected]>

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