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

[Kees Cook]

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@xxxxxxxx>

Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>

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.

-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
> 

-- 
Kees Cook