powerpc/mm/radix: Prevent kernel execution of user space

powerpc/mm/radix: Prevent kernel execution of user space

ISA 3 defines new encoded access authority that allows instruction
access prevention in privileged mode and allows normal access
to problem state. This patch just enables IAMR (Instruction Authority
Mask Register), enabling AMR would require more work.

I've tested this with a buggy driver and a simple payload. The payload
is specific to the build I've tested.

mpe: Also tested with LKDTM:

  # echo EXEC_USERSPACE > /sys/kernel/debug/provoke-crash/DIRECT
  lkdtm: Performing direct entry EXEC_USERSPACE
  lkdtm: attempting ok execution at c0000000005bf560
  lkdtm: attempting bad execution at 00003fff8d940000
  Unable to handle kernel paging request for instruction fetch
  Faulting instruction address: 0x3fff8d940000
  Oops: Kernel access of bad area, sig: 11 [#1]
  NIP: 00003fff8d940000 LR: c0000000005bfa58 CTR: 00003fff8d940000
  REGS: c0000000f1fcf900 TRAP: 0400   Not tainted  (4.9.0-rc5-compiler_gcc-6.2.0-00109-g956dbc06232a)
  MSR: 9000000010009033 <SF,HV,EE,ME,IR,DR,RI,LE>  CR: 48002222  XER: 00000000
  ...
  Call Trace:
    lkdtm_EXEC_USERSPACE+0x104/0x120 (unreliable)
    lkdtm_do_action+0x3c/0x80
    direct_entry+0x100/0x1b0
    full_proxy_write+0x94/0x100
    __vfs_write+0x3c/0x1b0
    vfs_write+0xcc/0x230
    SyS_write+0x60/0x110
    system_call+0x38/0xfc

Signed-off-by: Balbir Singh <bsingharora@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>