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Date: Wed, 8 Jan 2020 13:52:31 +0100
From: Christophe Leroy <christophe.leroy@....fr>
To: Russell Currey <ruscur@...sell.cc>, linuxppc-dev@...ts.ozlabs.org
Cc: joel@....id.au, mpe@...erman.id.au, ajd@...ux.ibm.com, dja@...ens.net,
 npiggin@...il.com, kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH v6 1/5] powerpc/mm: Implement set_memory() routines



Le 24/12/2019 à 06:55, Russell Currey a écrit :
> 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 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.
> 
> Signed-off-by: Russell Currey <ruscur@...sell.cc>
> ---
>   arch/powerpc/Kconfig                  |  1 +
>   arch/powerpc/include/asm/set_memory.h | 32 +++++++++++
>   arch/powerpc/mm/Makefile              |  1 +
>   arch/powerpc/mm/pageattr.c            | 83 +++++++++++++++++++++++++++
>   4 files changed, 117 insertions(+)
>   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 1ec34e16ed65..f0b9b47b5353 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -133,6 +133,7 @@ config PPC
>   	select ARCH_HAS_PTE_SPECIAL
>   	select ARCH_HAS_MEMBARRIER_CALLBACKS
>   	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) && !RELOCATABLE && !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..5230ddb2fefd
> --- /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

Maybe going from 0 to 3 would be better than 1 to 4

> +
> +int change_memory_attr(unsigned long addr, int numpages, int action);

action could be unsigned.

> +
> +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 5e147986400d..d0a0bcbc9289 100644
> --- a/arch/powerpc/mm/Makefile
> +++ b/arch/powerpc/mm/Makefile
> @@ -20,3 +20,4 @@ obj-$(CONFIG_HIGHMEM)		+= highmem.o
>   obj-$(CONFIG_PPC_COPRO_BASE)	+= copro_fault.o
>   obj-$(CONFIG_PPC_PTDUMP)	+= ptdump/
>   obj-$(CONFIG_KASAN)		+= kasan/
> +obj-$(CONFIG_ARCH_HAS_SET_MEMORY) += pageattr.o

CONFIG_ARCH_HAS_SET_MEMORY is set inconditionnally, I think you should 
add pageattr.o to obj-y instead. CONFIG_ARCH_HAS_XXX are almost never 
used in Makefiles

> diff --git a/arch/powerpc/mm/pageattr.c b/arch/powerpc/mm/pageattr.c
> new file mode 100644
> index 000000000000..15d5fb04f531
> --- /dev/null
> +++ b/arch/powerpc/mm/pageattr.c
> @@ -0,0 +1,83 @@
> +// 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.
> + */
> +static int __change_page_attr(pte_t *ptep, unsigned long addr, void *data)
> +{
> +	int action = *((int *)data);

Don't use pointers for so simple things, pointers forces the compiler to 
setup a stack frame and save the data into stack. Instead do:

	int action = (int)data;

> +	pte_t pte_val;
> +
> +	// invalidate the PTE so it's safe to modify
> +	pte_val = ptep_get_and_clear(&init_mm, addr, ptep);
> +	flush_tlb_kernel_range(addr, addr + PAGE_SIZE);

Why flush a range for a single page ? On most targets this will do a 
tlbia which is heavy, while a tlbie would suffice.

I think flush_tlb_kernel_range() should be replaced by something 
flushing only a single page.

> +
> +	// modify the PTE bits as desired, then apply
> +	switch (action) {
> +	case SET_MEMORY_RO:
> +		pte_val = pte_wrprotect(pte_val);
> +		break;
> +	case SET_MEMORY_RW:
> +		pte_val = pte_mkwrite(pte_val);
> +		break;
> +	case SET_MEMORY_NX:
> +		pte_val = pte_exprotect(pte_val);
> +		break;
> +	case SET_MEMORY_X:
> +		pte_val = pte_mkexec(pte_val);
> +		break;
> +	default:
> +		WARN_ON(true);
> +		return -EINVAL;

Is it worth checking that the action is valid for each page ? I think 
validity of action should be checked in change_memory_attr(). All other 
functions are static so you know they won't be called from outside.

Once done, you can squash __change_page_attr() into change_page_attr(), 
remove the ret var and return 0 all the time.

> +	}
> +
> +	set_pte_at(&init_mm, addr, ptep, pte_val);
> +
> +	return 0;
> +}
> +
> +static int change_page_attr(pte_t *ptep, unsigned long addr, void *data)
> +{
> +	int ret;
> +
> +	spin_lock(&init_mm.page_table_lock);
> +	ret = __change_page_attr(ptep, addr, data);
> +	spin_unlock(&init_mm.page_table_lock);
> +
> +	return ret;
> +}
> +
> +int change_memory_attr(unsigned long addr, int numpages, int action)
> +{
> +	unsigned long start = ALIGN_DOWN(addr, PAGE_SIZE);
> +	unsigned long size = numpages * PAGE_SIZE;
> +
> +	if (!numpages)
> +		return 0;
> +
> +	return apply_to_page_range(&init_mm, start, size, change_page_attr, &action);

Use (void*)action instead of &action (see upper comment)

> +}
> 

Christophe

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