Re: [PATCH v3 07/11] arm: Use generic mmap top-down layout

[Kees Cook]

On Wed, Apr 17, 2019 at 12:30 AM Alexandre Ghiti <alex@xxxxxxxx> 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.
>
> Signed-off-by: Alexandre Ghiti <alex@xxxxxxxx>

Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

> ---
>  arch/arm/Kconfig                 |  1 +
>  arch/arm/include/asm/processor.h |  2 --
>  arch/arm/mm/mmap.c               | 62 --------------------------------
>  3 files changed, 1 insertion(+), 64 deletions(-)
>
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index 850b4805e2d1..f8f603da181f 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -28,6 +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 57fe73ea0f72..944ef1fb1237 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/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