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Subject[RFC PATCH] powerpc/mm: Implement STRICT_MODULE_RWX
Date
Strict module RWX is just like strict kernel RWX, but for modules - so
loadable modules aren't marked both writable and executable at the same
time. This is handled by the generic code in kernel/module.c, and
simply requires the architecture to implement the set_memory() set of
functions, declared with ARCH_HAS_SET_MEMORY.

There's nothing other than these functions required to turn
ARCH_HAS_STRICT_MODULE_RWX on, so turn that on too.

With STRICT_MODULE_RWX enabled, there are as many W+X pages at runtime
as there are with CONFIG_MODULES=n (none), so in Russel's testing it works
well on both Hash and Radix book3s64.

There's a TODO in the code for also applying the page permission changes
to the backing pages in the linear mapping: this is pretty simple for
Radix and (seemingly) a lot harder for Hash, so I've left it for now
since there's still a notable security benefit for the patch as-is.

Technically can be enabled without STRICT_KERNEL_RWX, but
that doesn't gets you a whole lot, so we should leave it off by default
until we can get STRICT_KERNEL_RWX to the point where it's enabled by
default.

Signed-off-by: Russell Currey <ruscur@russell.cc>
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
---
Generic implementation based on Russel's patch ("powerpc/mm/book3s64: Implement STRICT_MODULE_RWX")
Untested

arch/powerpc/Kconfig | 2 +
arch/powerpc/include/asm/set_memory.h | 32 +++++++++++++
arch/powerpc/mm/Makefile | 2 +-
arch/powerpc/mm/pageattr.c | 85 +++++++++++++++++++++++++++++++++++
4 files changed, 120 insertions(+), 1 deletion(-)
create mode 100644 arch/powerpc/include/asm/set_memory.h
create mode 100644 arch/powerpc/mm/pageattr.c

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index d7996cfaceca..1f1423e3d818 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -131,7 +131,9 @@ config PPC
select ARCH_HAS_PTE_SPECIAL
select ARCH_HAS_MEMBARRIER_CALLBACKS
select ARCH_HAS_SCALED_CPUTIME if VIRT_CPU_ACCOUNTING_NATIVE && PPC64
+ select ARCH_HAS_SET_MEMORY
select ARCH_HAS_STRICT_KERNEL_RWX if ((PPC_BOOK3S_64 || PPC32) && !RELOCATABLE && !HIBERNATION)
+ select ARCH_HAS_STRICT_MODULE_RWX if PPC_BOOK3S_64 || PPC32
select ARCH_HAS_TICK_BROADCAST if GENERIC_CLOCKEVENTS_BROADCAST
select ARCH_HAS_UACCESS_FLUSHCACHE if PPC64
select ARCH_HAS_UBSAN_SANITIZE_ALL
diff --git a/arch/powerpc/include/asm/set_memory.h b/arch/powerpc/include/asm/set_memory.h
new file mode 100644
index 000000000000..4b9683f3b3dd
--- /dev/null
+++ b/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 1
+#define SET_MEMORY_RW 2
+#define SET_MEMORY_NX 3
+#define SET_MEMORY_X 4
+
+int change_memory(unsigned long addr, int numpages, int action);
+
+static inline int set_memory_ro(unsigned long addr, int numpages)
+{
+ return change_memory(addr, numpages, SET_MEMORY_RO);
+}
+
+static inline int set_memory_rw(unsigned long addr, int numpages)
+{
+ return change_memory(addr, numpages, SET_MEMORY_RW);
+}
+
+static inline int set_memory_nx(unsigned long addr, int numpages)
+{
+ return change_memory(addr, numpages, SET_MEMORY_NX);
+}
+
+static inline int set_memory_x(unsigned long addr, int numpages)
+{
+ return change_memory(addr, numpages, SET_MEMORY_X);
+}
+
+#endif
diff --git a/arch/powerpc/mm/Makefile b/arch/powerpc/mm/Makefile
index 0f499db315d6..b683d1c311b3 100644
--- a/arch/powerpc/mm/Makefile
+++ b/arch/powerpc/mm/Makefile
@@ -7,7 +7,7 @@ ccflags-$(CONFIG_PPC64) := $(NO_MINIMAL_TOC)

obj-y := fault.o mem.o pgtable.o mmap.o \
init_$(BITS).o pgtable_$(BITS).o \
- pgtable-frag.o \
+ pgtable-frag.o pageattr.o \
init-common.o mmu_context.o drmem.o
obj-$(CONFIG_PPC_MMU_NOHASH) += nohash/
obj-$(CONFIG_PPC_BOOK3S_32) += book3s32/
diff --git a/arch/powerpc/mm/pageattr.c b/arch/powerpc/mm/pageattr.c
new file mode 100644
index 000000000000..3e8f2c203a00
--- /dev/null
+++ b/arch/powerpc/mm/pageattr.c
@@ -0,0 +1,85 @@
+// SPDX-License-Identifier: GPL-2.0+
+
+/*
+ * Page attribute and set_memory routines
+ *
+ * Derived from the arm64 implementation.
+ *
+ * Author: Russell Currey <ruscur@russell.cc>
+ *
+ * Copyright 2019, IBM Corporation.
+ *
+ */
+
+#include <linux/mm.h>
+#include <linux/set_memory.h>
+#include <linux/vmalloc.h>
+
+#include <asm/mmu.h>
+#include <asm/page.h>
+#include <asm/pgtable.h>
+
+static int change_page_ro(pte_t *ptep, pgtable_t token, unsigned long addr, void *data)
+{
+ set_pte_at(&init_mm, addr, ptep, pte_wrprotect(READ_ONCE(*ptep)));
+ return 0;
+}
+
+static int change_page_rw(pte_t *ptep, pgtable_t token, unsigned long addr, void *data)
+{
+ set_pte_at(&init_mm, addr, ptep, pte_mkwrite(READ_ONCE(*ptep)));
+ return 0;
+}
+
+static int change_page_nx(pte_t *ptep, pgtable_t token, unsigned long addr, void *data)
+{
+ set_pte_at(&init_mm, addr, ptep, pte_exprotect(READ_ONCE(*ptep)));
+ return 0;
+}
+
+static int change_page_x(pte_t *ptep, pgtable_t token, unsigned long addr, void *data)
+{
+ set_pte_at(&init_mm, addr, ptep, pte_mkexec(READ_ONCE(*ptep)));
+ return 0;
+}
+
+int change_memory(unsigned long addr, int numpages, int action)
+{
+ unsigned long size = numpages * PAGE_SIZE;
+ unsigned long start = ALIGN_DOWN(addr, PAGE_SIZE);
+ unsigned long end = start + size;
+ struct vm_struct *area;
+ int ret;
+
+ if (!numpages)
+ return 0;
+
+ // only operate on VM areas for now
+ area = find_vm_area((void *)addr);
+ if (!area || end > (unsigned long)area->addr + area->size ||
+ !(area->flags & VM_ALLOC))
+ return -EINVAL;
+
+ // TODO: also apply change to the backing pages in the linear mapping
+
+ switch (action) {
+ case SET_MEMORY_RO:
+ ret = apply_to_page_range(&init_mm, start, size, change_page_ro, NULL);
+ break;
+ case SET_MEMORY_RW:
+ ret = apply_to_page_range(&init_mm, start, size, change_page_rw, NULL);
+ break;
+ case SET_MEMORY_NX:
+ ret = apply_to_page_range(&init_mm, start, size, change_page_nx, NULL);
+ break;
+ case SET_MEMORY_X:
+ ret = apply_to_page_range(&init_mm, start, size, change_page_x, NULL);
+ break;
+ default:
+ WARN_ON(true);
+ return -EINVAL;
+ }
+
+ flush_tlb_kernel_range(start, end);
+ return ret;
+}
--
2.13.3
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