Re: [PATCH v4 08/14] arm: Use generic mmap top-down layout and brk randomization

[Alex Ghiti <alex@ghiti.fr>]

On 5/29/19 3:26 PM, Kees Cook wrote:
> On Sun, May 26, 2019 at 09:47:40AM -0400, Alexandre Ghiti wrote:
>> arm uses a top-down mmap layout by default that exactly fits the generic
>> functions, so get rid of arch specific code and use the generic version
>> by selecting ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT.
>> As ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT selects ARCH_HAS_ELF_RANDOMIZE,
>> use the generic version of arch_randomize_brk since it also fits.
>> Note that this commit also removes the possibility for arm to have elf
>> randomization and no MMU: without MMU, the security added by randomization
>> is worth nothing.
>>
>> Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
> Acked-by: Kees Cook <keescook@chromium.org>
>
> It may be worth noting that STACK_RND_MASK is safe to remove here
> because it matches the default that now exists in mm/util.c.


Yes, thanks for pointing that.


Thanks,


Alex


>
> -Kees
>
>> ---
>>   arch/arm/Kconfig                 |  2 +-
>>   arch/arm/include/asm/processor.h |  2 --
>>   arch/arm/kernel/process.c        |  5 ---
>>   arch/arm/mm/mmap.c               | 62 --------------------------------
>>   4 files changed, 1 insertion(+), 70 deletions(-)
>>
>> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
>> index 8869742a85df..27687a8c9fb5 100644
>> --- a/arch/arm/Kconfig
>> +++ b/arch/arm/Kconfig
>> @@ -6,7 +6,6 @@ config ARM
>>   	select ARCH_CLOCKSOURCE_DATA
>>   	select ARCH_HAS_DEBUG_VIRTUAL if MMU
>>   	select ARCH_HAS_DEVMEM_IS_ALLOWED
>> -	select ARCH_HAS_ELF_RANDOMIZE
>>   	select ARCH_HAS_FORTIFY_SOURCE
>>   	select ARCH_HAS_KEEPINITRD
>>   	select ARCH_HAS_KCOV
>> @@ -29,6 +28,7 @@ config ARM
>>   	select ARCH_SUPPORTS_ATOMIC_RMW
>>   	select ARCH_USE_BUILTIN_BSWAP
>>   	select ARCH_USE_CMPXCHG_LOCKREF
>> +	select ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT if MMU
>>   	select ARCH_WANT_IPC_PARSE_VERSION
>>   	select BUILDTIME_EXTABLE_SORT if MMU
>>   	select CLONE_BACKWARDS
>> diff --git a/arch/arm/include/asm/processor.h b/arch/arm/include/asm/processor.h
>> index 5d06f75ffad4..95b7688341c5 100644
>> --- a/arch/arm/include/asm/processor.h
>> +++ b/arch/arm/include/asm/processor.h
>> @@ -143,8 +143,6 @@ static inline void prefetchw(const void *ptr)
>>   #endif
>>   #endif
>>   
>> -#define HAVE_ARCH_PICK_MMAP_LAYOUT
>> -
>>   #endif
>>   
>>   #endif /* __ASM_ARM_PROCESSOR_H */
>> diff --git a/arch/arm/kernel/process.c b/arch/arm/kernel/process.c
>> index 72cc0862a30e..19a765db5f7f 100644
>> --- a/arch/arm/kernel/process.c
>> +++ b/arch/arm/kernel/process.c
>> @@ -322,11 +322,6 @@ unsigned long get_wchan(struct task_struct *p)
>>   	return 0;
>>   }
>>   
>> -unsigned long arch_randomize_brk(struct mm_struct *mm)
>> -{
>> -	return randomize_page(mm->brk, 0x02000000);
>> -}
>> -
>>   #ifdef CONFIG_MMU
>>   #ifdef CONFIG_KUSER_HELPERS
>>   /*
>> diff --git a/arch/arm/mm/mmap.c b/arch/arm/mm/mmap.c
>> index 0b94b674aa91..b8d912ac9e61 100644
>> --- a/arch/arm/mm/mmap.c
>> +++ b/arch/arm/mm/mmap.c
>> @@ -17,43 +17,6 @@
>>   	((((addr)+SHMLBA-1)&~(SHMLBA-1)) +	\
>>   	 (((pgoff)<<PAGE_SHIFT) & (SHMLBA-1)))
>>   
>> -/* gap between mmap and stack */
>> -#define MIN_GAP		(128*1024*1024UL)
>> -#define MAX_GAP		((STACK_TOP)/6*5)
>> -#define STACK_RND_MASK	(0x7ff >> (PAGE_SHIFT - 12))
>> -
>> -static int mmap_is_legacy(struct rlimit *rlim_stack)
>> -{
>> -	if (current->personality & ADDR_COMPAT_LAYOUT)
>> -		return 1;
>> -
>> -	if (rlim_stack->rlim_cur == RLIM_INFINITY)
>> -		return 1;
>> -
>> -	return sysctl_legacy_va_layout;
>> -}
>> -
>> -static unsigned long mmap_base(unsigned long rnd, struct rlimit *rlim_stack)
>> -{
>> -	unsigned long gap = rlim_stack->rlim_cur;
>> -	unsigned long pad = stack_guard_gap;
>> -
>> -	/* Account for stack randomization if necessary */
>> -	if (current->flags & PF_RANDOMIZE)
>> -		pad += (STACK_RND_MASK << PAGE_SHIFT);
>> -
>> -	/* Values close to RLIM_INFINITY can overflow. */
>> -	if (gap + pad > gap)
>> -		gap += pad;
>> -
>> -	if (gap < MIN_GAP)
>> -		gap = MIN_GAP;
>> -	else if (gap > MAX_GAP)
>> -		gap = MAX_GAP;
>> -
>> -	return PAGE_ALIGN(STACK_TOP - gap - rnd);
>> -}
>> -
>>   /*
>>    * We need to ensure that shared mappings are correctly aligned to
>>    * avoid aliasing issues with VIPT caches.  We need to ensure that
>> @@ -181,31 +144,6 @@ arch_get_unmapped_area_topdown(struct file *filp, const unsigned long addr0,
>>   	return addr;
>>   }
>>   
>> -unsigned long arch_mmap_rnd(void)
>> -{
>> -	unsigned long rnd;
>> -
>> -	rnd = get_random_long() & ((1UL << mmap_rnd_bits) - 1);
>> -
>> -	return rnd << PAGE_SHIFT;
>> -}
>> -
>> -void arch_pick_mmap_layout(struct mm_struct *mm, struct rlimit *rlim_stack)
>> -{
>> -	unsigned long random_factor = 0UL;
>> -
>> -	if (current->flags & PF_RANDOMIZE)
>> -		random_factor = arch_mmap_rnd();
>> -
>> -	if (mmap_is_legacy(rlim_stack)) {
>> -		mm->mmap_base = TASK_UNMAPPED_BASE + random_factor;
>> -		mm->get_unmapped_area = arch_get_unmapped_area;
>> -	} else {
>> -		mm->mmap_base = mmap_base(random_factor, rlim_stack);
>> -		mm->get_unmapped_area = arch_get_unmapped_area_topdown;
>> -	}
>> -}
>> -
>>   /*
>>    * You really shouldn't be using read() or write() on /dev/mem.  This
>>    * might go away in the future.
>> -- 
>> 2.20.1
>>