powerpc/book3s64/pkeys: Inherit correctly on fork.

Child thread.kuap value is inherited from the parent in copy_thread_tls. We still
need to make sure when the child returns from a fork in the kernel we start with the kernel
default AMR value.
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Reviewed-by: Sandipan Das <sandipan@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201127044424.40686-12-aneesh.kumar@linux.ibm.com

arch/powerpc/kernel/process.c

@@ -1760,6 +1760,16 @@ int copy_thread(unsigned long clone_flags, unsigned long usp,
 		childregs->ppr = DEFAULT_PPR;
 
 	p->thread.tidr = 0;
+#endif
+	/*
+	 * Run with the current AMR value of the kernel
+	 */
+#ifdef CONFIG_PPC_PKEY
+	if (mmu_has_feature(MMU_FTR_BOOK3S_KUAP))
+		kregs->amr = AMR_KUAP_BLOCKED;
+
+	if (mmu_has_feature(MMU_FTR_BOOK3S_KUEP))
+		kregs->iamr = AMR_KUEP_BLOCKED;
 #endif
 	kregs->nip = ppc_function_entry(f);
 	return 0;