powerpc/64s/kuap: Restore AMR in fast_interrupt_return

Interrupts that use fast_interrupt_return actually do lock AMR, but
they have been ones which tend to come from userspace (or kernel bugs)
in radix mode. With kuap on hash, segment interrupts are taken in
kernel often, which quickly breaks due to the missing restore.

Fixes: 890274c2dc ("powerpc/64s: Implement KUAP for Radix MMU")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200429065654.1677541-6-npiggin@gmail.com

arch/powerpc/kernel/entry_64.S

@@ -472,15 +472,17 @@ END_FTR_SECTION_IFCLR(CPU_FTR_ARCH_207S)
 #ifdef CONFIG_PPC_BOOK3S
 	/*
 	 * If MSR EE/RI was never enabled, IRQs not reconciled, NVGPRs not
-	 * touched, AMR not set, no exit work created, then this can be used.
+	 * touched, no exit work created, then this can be used.
 	 */
 	.balign IFETCH_ALIGN_BYTES
 	.globl fast_interrupt_return
 fast_interrupt_return:
 _ASM_NOKPROBE_SYMBOL(fast_interrupt_return)
+	kuap_check_amr r3, r4
 	ld	r4,_MSR(r1)
 	andi.	r0,r4,MSR_PR
 	bne	.Lfast_user_interrupt_return
+	kuap_restore_amr r3
 	andi.	r0,r4,MSR_RI
 	li	r3,0 /* 0 return value, no EMULATE_STACK_STORE */
 	bne+	.Lfast_kernel_interrupt_return