[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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