[PATCH v9 1/8] powerpc/mm: Implement set_memory() routines

[Michael Ellerman]

Jordan Niethe <jniethe5 at gmail.com> writes:
> From: Russell Currey <ruscur at russell.cc>
>
> 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 completely generic across powerpc's many MMUs.
>
> It's possible that this could be optimised to be faster for specific
> MMUs, but the focus is on having a generic and safe implementation for
> now.

This won't work for the linear mapping with HPT on book3s 64. Because
the linear mapping is not in the kernel page tables.

apply_to_existing_page_range() should work that out and return an error.
But I'm not sure if callers handle that well or at all.

We might want to add a WARN_ON_ONCE() in change_memory_attr(), at least
to begin with, to report those errors, so we know when we are failing to
set permissions. Rather than silently failing and then crashing some
time later due to the permissions being wrong for some mapping.

cheers


> 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, so that is left as a TODO for now.
>
> These functions do nothing if STRICT_KERNEL_RWX is not enabled.
>
> Reviewed-by: Daniel Axtens <dja at axtens.net>
> Signed-off-by: Russell Currey <ruscur at russell.cc>
> Signed-off-by: Christophe Leroy <christophe.leroy at c-s.fr>
> [jpn: rebase on next plus "powerpc/mm/64s: Allow STRICT_KERNEL_RWX again"]
> Signed-off-by: Jordan Niethe <jniethe5 at gmail.com>
> ---
>  arch/powerpc/Kconfig                  |  1 +
>  arch/powerpc/include/asm/set_memory.h | 32 +++++++++++
>  arch/powerpc/mm/Makefile              |  2 +-
>  arch/powerpc/mm/pageattr.c            | 81 +++++++++++++++++++++++++++
>  4 files changed, 115 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 fc7f5c5933e6..4498a27ac9db 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -135,6 +135,7 @@ config PPC
>  	select ARCH_HAS_MEMBARRIER_CALLBACKS
>  	select ARCH_HAS_MEMBARRIER_SYNC_CORE
>  	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
> diff --git a/arch/powerpc/include/asm/set_memory.h b/arch/powerpc/include/asm/set_memory.h
> new file mode 100644
> index 000000000000..64011ea444b4
> --- /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	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
> diff --git a/arch/powerpc/mm/Makefile b/arch/powerpc/mm/Makefile
> index 3b4e9e4e25ea..d8a08abde1ae 100644
> --- a/arch/powerpc/mm/Makefile
> +++ b/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
> diff --git a/arch/powerpc/mm/pageattr.c b/arch/powerpc/mm/pageattr.c
> new file mode 100644
> index 000000000000..2da3fbab6ff7
> --- /dev/null
> +++ b/arch/powerpc/mm/pageattr.c
> @@ -0,0 +1,81 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +/*
> + * MMU-generic set_memory implementation for powerpc
> + *
> + * Copyright 2019, IBM Corporation.
> + */
> +
> +#include <linux/mm.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
> + *
> + * This is unsafe if 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.
> + *
> + * TODO make the implementation resistant to this.
> + *
> + * NOTE: can be dangerous to call without STRICT_KERNEL_RWX
> + */
> +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);
> +		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);
> +	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 sz = numpages * PAGE_SIZE;
> +
> +	if (!IS_ENABLED(CONFIG_STRICT_KERNEL_RWX))
> +		return 0;
> +
> +	if (numpages <= 0)
> +		return 0;
> +
> +	return apply_to_existing_page_range(&init_mm, start, sz,
> +					    change_page_attr, (void *)action);
> +}
> -- 
> 2.25.1