General flags to turn things off (getrandom, pid lookup, etc)

[Andy Lutomirski]

[new thread because this sort of combines two threads]

There is recent interest in having a way to turn generally-available
kernel features off.  Maybe we should add a good one so we can stop
bikeshedding and avoid proliferating dumb interfaces.

Things that might want to be turn-off-able include:
 - getrandom with GRND_RANDOM [from the getrandom threads]
 - Any lookup of a non-self pid [from the capsicum thread]
 - Any lookup of a pid outside the caller thread group [capsicum]
 - Various architectural things (personal wishlist), e.g.:
    - RDTSC and userspace HPET access
    - CPUID?
    - 32-bit GDT code segments [huge attack surface]
    - 64-bit GDT code segments [probably pointless]

I would propose a new syscall for this:

long restrict_userspace(int mode, int type, int value, int flags);

mode is RESTRICT_SET, RESTRICT_GET, or RESTRICT_LOCK.

type is RESTRICT_GRND_RANDOM, RESTRICT_PID_SCOPE, RESTRICT_X86_TIMING, etc.

Value is zero if RESTRICT_GET.  Otherwise value is the desired value,
generally 0 or 1.  For RESTRICT_PID_SCOPE, value would be
RESTRICT_PID_SCOPE_ANY, RESTRICT_PID_SCOPE_THREADGROUP, or
RESTRICT_PID_SCOPE_SELF.

flags must be zero.  Someday, someone will propose a thread-sync flag.

restrict_userspace requires either no_new_privs or CAP_SYS_ADMIN in
the current user namespace.

Thoughts?

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC
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