powerpc/powernv/idle: Restore IAMR after idle

commit a3f3072db6cad40895c585dce65e36aab997f042 upstream.

Without restoring the IAMR after idle, execution prevention on POWER9
with Radix MMU is overwritten and the kernel can freely execute
userspace without faulting.

This is necessary when returning from any stop state that modifies
user state, as well as hypervisor state.

To test how this fails without this patch, load the lkdtm driver and
do the following:

  $ echo EXEC_USERSPACE > /sys/kernel/debug/provoke-crash/DIRECT

which won't fault, then boot the kernel with powersave=off, where it
will fault. Applying this patch will fix this.

Fixes: 3b10d009 ("powerpc/mm/radix: Prevent kernel execution of user space")
Cc: stable@vger.kernel.org # v4.10+
Signed-off-by: Russell Currey <ruscur@russell.cc>
Reviewed-by: Akshay Adiga <akshay.adiga@linux.vnet.ibm.com>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>