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

[David Drysdale]

On Fri, Jul 25, 2014 at 7:30 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> [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.

Today, me: proposed :-)

> restrict_userspace requires either no_new_privs or CAP_SYS_ADMIN in
> the current user namespace.
>
> Thoughts?
>
> --Andy
>
> --
> Andy Lutomirski
> AMA Capital Management, LLC
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/