powerpc/mm: Implement set_memory() routines

The set_memory_{ro/rw/nx/x}() functions are required for
STRICT_MODULE_RWX, and are generally useful primitives to have.  This
implementation is designed to be generic across powerpc's many MMUs.
It's possible that this could be optimised to be faster for specific
MMUs.

This implementation does not handle cases where the caller is attempting
to change the mapping of the page it is executing from, or if another
CPU is concurrently using the page being altered.  These cases likely
shouldn't happen, but a more complex implementation with MMU-specific code
could safely handle them.

On hash, the linear mapping is not kept in the linux pagetable, so this
will not change the protection if used on that range. Currently these
functions are not used on the linear map so just WARN for now.

apply_to_existing_page_range() does not work on huge pages so for now
disallow changing the protection of huge pages.

[jpn: - Allow set memory functions to be used without Strict RWX
      - Hash: Disallow certain regions
      - Have change_page_attr() take function pointers to manipulate ptes
      - Radix: Add ptesync after set_pte_at()]

Signed-off-by: Russell Currey <ruscur@russell.cc>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
Reviewed-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210609013431.9805-2-jniethe5@gmail.com

arch/powerpc/Kconfig

@@ -140,6 +140,7 @@ config PPC
 	select ARCH_HAS_PTE_DEVMAP		if PPC_BOOK3S_64
 	select ARCH_HAS_PTE_SPECIAL
 	select ARCH_HAS_SCALED_CPUTIME		if VIRT_CPU_ACCOUNTING_NATIVE && PPC_BOOK3S_64
+	select ARCH_HAS_SET_MEMORY
 	select ARCH_HAS_STRICT_KERNEL_RWX	if ((PPC_BOOK3S_64 || PPC32) && !HIBERNATION)
 	select ARCH_HAS_TICK_BROADCAST		if GENERIC_CLOCKEVENTS_BROADCAST
 	select ARCH_HAS_UACCESS_FLUSHCACHE

arch/powerpc/include/asm/set_memory.h

@@ -0,0 +1,32 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_POWERPC_SET_MEMORY_H
+#define _ASM_POWERPC_SET_MEMORY_H
+
+#define SET_MEMORY_RO	0
+#define SET_MEMORY_RW	1
+#define SET_MEMORY_NX	2
+#define SET_MEMORY_X	3
+
+int change_memory_attr(unsigned long addr, int numpages, long action);
+
+static inline int set_memory_ro(unsigned long addr, int numpages)
+{
+	return change_memory_attr(addr, numpages, SET_MEMORY_RO);
+}
+
+static inline int set_memory_rw(unsigned long addr, int numpages)
+{
+	return change_memory_attr(addr, numpages, SET_MEMORY_RW);
+}
+
+static inline int set_memory_nx(unsigned long addr, int numpages)
+{
+	return change_memory_attr(addr, numpages, SET_MEMORY_NX);
+}
+
+static inline int set_memory_x(unsigned long addr, int numpages)
+{
+	return change_memory_attr(addr, numpages, SET_MEMORY_X);
+}
+
+#endif

arch/powerpc/mm/Makefile

@@ -5,7 +5,7 @@
 
 ccflags-$(CONFIG_PPC64)	:= $(NO_MINIMAL_TOC)
 
-obj-y				:= fault.o mem.o pgtable.o mmap.o maccess.o \
+obj-y				:= fault.o mem.o pgtable.o mmap.o maccess.o pageattr.o \
 				   init_$(BITS).o pgtable_$(BITS).o \
 				   pgtable-frag.o ioremap.o ioremap_$(BITS).o \
 				   init-common.o mmu_context.o drmem.o \

arch/powerpc/mm/pageattr.c

@@ -0,0 +1,101 @@
+// SPDX-License-Identifier: GPL-2.0
+
+/*
+ * MMU-generic set_memory implementation for powerpc
+ *
+ * Copyright 2019-2021, IBM Corporation.
+ */
+
+#include <linux/mm.h>
+#include <linux/vmalloc.h>
+#include <linux/set_memory.h>
+
+#include <asm/mmu.h>
+#include <asm/page.h>
+#include <asm/pgtable.h>
+
+
+/*
+ * Updates the attributes of a page in three steps:
+ *
+ * 1. invalidate the page table entry
+ * 2. flush the TLB
+ * 3. install the new entry with the updated attributes
+ *
+ * Invalidating the pte means there are situations where this will not work
+ * when in theory it should.
+ * For example:
+ * - removing write from page whilst it is being executed
+ * - setting a page read-only whilst it is being read by another CPU
+ *
+ */
+static int change_page_attr(pte_t *ptep, unsigned long addr, void *data)
+{
+	long action = (long)data;
+	pte_t pte;
+
+	spin_lock(&init_mm.page_table_lock);
+
+	/* invalidate the PTE so it's safe to modify */
+	pte = ptep_get_and_clear(&init_mm, addr, ptep);
+	flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
+
+	/* modify the PTE bits as desired, then apply */
+	switch (action) {
+	case SET_MEMORY_RO:
+		pte = pte_wrprotect(pte);
+		break;
+	case SET_MEMORY_RW:
+		pte = pte_mkwrite(pte_mkdirty(pte));
+		break;
+	case SET_MEMORY_NX:
+		pte = pte_exprotect(pte);
+		break;
+	case SET_MEMORY_X:
+		pte = pte_mkexec(pte);
+		break;
+	default:
+		WARN_ON_ONCE(1);
+		break;
+	}
+
+	set_pte_at(&init_mm, addr, ptep, pte);
+
+	/* See ptesync comment in radix__set_pte_at() */
+	if (radix_enabled())
+		asm volatile("ptesync": : :"memory");
+	spin_unlock(&init_mm.page_table_lock);
+
+	return 0;
+}
+
+int change_memory_attr(unsigned long addr, int numpages, long action)
+{
+	unsigned long start = ALIGN_DOWN(addr, PAGE_SIZE);
+	unsigned long size = numpages * PAGE_SIZE;
+
+	if (!numpages)
+		return 0;
+
+	if (WARN_ON_ONCE(is_vmalloc_or_module_addr((void *)addr) &&
+			 is_vm_area_hugepages((void *)addr)))
+		return -EINVAL;
+
+#ifdef CONFIG_PPC_BOOK3S_64
+	/*
+	 * On hash, the linear mapping is not in the Linux page table so
+	 * apply_to_existing_page_range() will have no effect. If in the future
+	 * the set_memory_* functions are used on the linear map this will need
+	 * to be updated.
+	 */
+	if (!radix_enabled()) {
+		int region = get_region_id(addr);
+
+		if (WARN_ON_ONCE(region != VMALLOC_REGION_ID && region != IO_REGION_ID))
+			return -EINVAL;
+	}
+#endif
+
+	return apply_to_existing_page_range(&init_mm, start, size,
+					    change_page_attr, (void *)action);
+}