[PATCH] powerpc/mm/book3s64: Implement STRICT_MODULE_RWX

[Christophe Leroy]

Le 15/05/2019 à 03:30, Russell Currey a écrit :
> 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.
> 
> The set_memory() family of functions are implemented for book3s64
> MMUs (so Hash and Radix), however they could likely be adapted to work
> for other platforms as well and made more generic.  I did it this way
> since they're the platforms I have the most understanding of and ability
> to test.

Based on this patch, I have drafted a generic implementation. Please 
comment and test. I'll test on my side on PPC32.

Christophe

> 
> 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 my testing it works
> well on both Hash and Radix.
> 
> 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 I don't think
> that gets you a whole lot, so I think 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 at russell.cc>
> ---
>   arch/powerpc/Kconfig                  |   2 +
>   arch/powerpc/include/asm/set_memory.h |  12 +++
>   arch/powerpc/mm/book3s64/Makefile     |   2 +-
>   arch/powerpc/mm/book3s64/pageattr.c   | 106 ++++++++++++++++++++++++++
>   4 files changed, 121 insertions(+), 1 deletion(-)
>   create mode 100644 arch/powerpc/include/asm/set_memory.h
>   create mode 100644 arch/powerpc/mm/book3s64/pageattr.c
> 
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index d7996cfaceca..9e1bfa81bc5a 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		if PPC_BOOK3S_64
>   	select ARCH_HAS_STRICT_KERNEL_RWX	if ((PPC_BOOK3S_64 || PPC32) && !RELOCATABLE && !HIBERNATION)
> +	select ARCH_HAS_STRICT_MODULE_RWX	if PPC_BOOK3S_64
>   	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..5323a8b06f98
> --- /dev/null
> +++ b/arch/powerpc/include/asm/set_memory.h
> @@ -0,0 +1,12 @@
> +/* SPDX-License-Identifier: GPL-2.0+ */
> +#ifndef _ASM_POWERPC_SET_MEMORY_H
> +#define _ASM_POWERPC_SET_MEMORY_H
> +
> +#ifdef CONFIG_PPC_BOOK3S_64
> +int set_memory_ro(unsigned long addr, int numpages);
> +int set_memory_rw(unsigned long addr, int numpages);
> +int set_memory_nx(unsigned long addr, int numpages);
> +int set_memory_x(unsigned long addr, int numpages);
> +#endif
> +
> +#endif
> diff --git a/arch/powerpc/mm/book3s64/Makefile b/arch/powerpc/mm/book3s64/Makefile
> index 974b4fc19f4f..09c5afadf235 100644
> --- a/arch/powerpc/mm/book3s64/Makefile
> +++ b/arch/powerpc/mm/book3s64/Makefile
> @@ -5,7 +5,7 @@ ccflags-y	:= $(NO_MINIMAL_TOC)
>   CFLAGS_REMOVE_slb.o = $(CC_FLAGS_FTRACE)
>   
>   obj-y				+= hash_pgtable.o hash_utils.o slb.o \
> -				   mmu_context.o pgtable.o hash_tlb.o
> +				   mmu_context.o pgtable.o hash_tlb.o pageattr.o
>   obj-$(CONFIG_PPC_NATIVE)	+= hash_native.o
>   obj-$(CONFIG_PPC_RADIX_MMU)	+= radix_pgtable.o radix_tlb.o
>   obj-$(CONFIG_PPC_4K_PAGES)	+= hash_4k.o
> diff --git a/arch/powerpc/mm/book3s64/pageattr.c b/arch/powerpc/mm/book3s64/pageattr.c
> new file mode 100644
> index 000000000000..d6afa89fb407
> --- /dev/null
> +++ b/arch/powerpc/mm/book3s64/pageattr.c
> @@ -0,0 +1,106 @@
> +// SPDX-License-Identifier: GPL-2.0+
> +
> +/*
> + * Page attribute and set_memory routines for Radix and Hash MMUs
> + *
> + * Derived from the arm64 implementation.
> + *
> + * Author: Russell Currey <ruscur at russell.cc>
> + *
> + * Copyright 2019, IBM Corporation.
> + *
> + * This program is free software; you can redistribute it and/or modify it
> + * under the terms of the GNU General Public License as published by the
> + * Free Software Foundation; either version 2 of the License, or (at your
> + * option) any later version.

Above text should be removed as it is redundant with the 
SPDX-Licence-Identifier

> + */
> +
> +#include <linux/mm.h>
> +#include <linux/vmalloc.h>
> +
> +#include <asm/mmu.h>
> +#include <asm/page.h>
> +#include <asm/pgtable.h>
> +
> +// we need this to have a single pointer to pass into apply_to_page_range()
> +struct page_change_data {
> +	pgprot_t set_mask;
> +	pgprot_t clear_mask;
> +};
> +
> +static pte_t clear_pte_bit(pte_t pte, pgprot_t prot)
> +{
> +	return __pte(pte_val(pte) & ~pgprot_val(prot));
> +}
> +
> +static pte_t set_pte_bit(pte_t pte, pgprot_t prot)
> +{
> +	return __pte(pte_val(pte) | pgprot_val(prot));
> +}
> +
> +static int change_page_range(pte_t *ptep, pgtable_t token, unsigned long addr,
> +			     void *data)
> +{
> +	struct page_change_data *cdata = data;
> +	pte_t pte = READ_ONCE(*ptep);
> +
> +	pte = clear_pte_bit(pte, cdata->clear_mask);
> +	pte = set_pte_bit(pte, cdata->set_mask);
> +
> +	set_pte_at(&init_mm, addr, ptep, pte);
> +	return 0;
> +}
> +
> +static int change_memory(unsigned long addr, int numpages, pgprot_t set_mask,
> +			 pgprot_t clear_mask)
> +{
> +	unsigned long size = numpages * PAGE_SIZE;
> +	unsigned long start = ALIGN_DOWN(addr, PAGE_SIZE);
> +	unsigned long end = PAGE_ALIGN(start + size);

We have aligned the start and the size, so the end is automatically aligned.

Christophe

> +	struct page_change_data data;
> +	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
> +	data.set_mask = set_mask;
> +	data.clear_mask = clear_mask;
> +
> +	ret = apply_to_page_range(&init_mm, start, size, change_page_range,
> +				  &data);
> +
> +	flush_tlb_kernel_range(start, end);
> +	return ret;
> +}
> +
> +int set_memory_ro(unsigned long addr, int numpages)
> +{
> +	return change_memory(addr, numpages,
> +			     __pgprot(0), __pgprot(_PAGE_WRITE));
> +}
> +
> +int set_memory_rw(unsigned long addr, int numpages)
> +{
> +	return change_memory(addr, numpages,
> +			     __pgprot(_PAGE_RW), __pgprot(0));
> +}
> +
> +int set_memory_nx(unsigned long addr, int numpages)
> +{
> +	return change_memory(addr, numpages,
> +			     __pgprot(0), __pgprot(_PAGE_EXEC));
> +}
> +
> +int set_memory_x(unsigned long addr, int numpages)
> +{
> +	return change_memory(addr, numpages,
> +			     __pgprot(_PAGE_EXEC), __pgprot(0));
> +}
>