Re: [PATCH v3 01/11] mm, fs: Move randomize_stack_top from fs to mm

[Kees Cook]

On Wed, Apr 17, 2019 at 12:24 AM Alexandre Ghiti <alex@xxxxxxxx> wrote:
>
> This preparatory commit moves this function so that further introduction
> of generic topdown mmap layout is contained only in mm/util.c.
>
> Signed-off-by: Alexandre Ghiti <alex@xxxxxxxx>
> Reviewed-by: Christoph Hellwig <hch@xxxxxx>
> ---
>  fs/binfmt_elf.c    | 20 --------------------
>  include/linux/mm.h |  2 ++
>  mm/util.c          | 22 ++++++++++++++++++++++
>  3 files changed, 24 insertions(+), 20 deletions(-)
>
> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> index 7d09d125f148..045f3b29d264 100644
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -662,26 +662,6 @@ static unsigned long load_elf_interp(struct elfhdr *interp_elf_ex,
>   * libraries.  There is no binary dependent code anywhere else.
>   */
>
> -#ifndef STACK_RND_MASK
> -#define STACK_RND_MASK (0x7ff >> (PAGE_SHIFT - 12))    /* 8MB of VA */
> -#endif
> -
> -static unsigned long randomize_stack_top(unsigned long stack_top)
> -{
> -       unsigned long random_variable = 0;
> -
> -       if (current->flags & PF_RANDOMIZE) {
> -               random_variable = get_random_long();
> -               random_variable &= STACK_RND_MASK;
> -               random_variable <<= PAGE_SHIFT;
> -       }
> -#ifdef CONFIG_STACK_GROWSUP
> -       return PAGE_ALIGN(stack_top) + random_variable;
> -#else
> -       return PAGE_ALIGN(stack_top) - random_variable;
> -#endif
> -}
> -
>  static int load_elf_binary(struct linux_binprm *bprm)
>  {
>         struct file *interpreter = NULL; /* to shut gcc up */
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 76769749b5a5..087824a5059f 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2312,6 +2312,8 @@ extern int install_special_mapping(struct mm_struct *mm,
>                                    unsigned long addr, unsigned long len,
>                                    unsigned long flags, struct page **pages);
>
> +unsigned long randomize_stack_top(unsigned long stack_top);
> +
>  extern unsigned long get_unmapped_area(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
>
>  extern unsigned long mmap_region(struct file *file, unsigned long addr,
> diff --git a/mm/util.c b/mm/util.c
> index d559bde497a9..a54afb9b4faa 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -14,6 +14,8 @@
>  #include <linux/hugetlb.h>
>  #include <linux/vmalloc.h>
>  #include <linux/userfaultfd_k.h>
> +#include <linux/elf.h>
> +#include <linux/random.h>
>
>  #include <linux/uaccess.h>
>
> @@ -291,6 +293,26 @@ int vma_is_stack_for_current(struct vm_area_struct *vma)
>         return (vma->vm_start <= KSTK_ESP(t) && vma->vm_end >= KSTK_ESP(t));
>  }
>
> +#ifndef STACK_RND_MASK
> +#define STACK_RND_MASK (0x7ff >> (PAGE_SHIFT - 12))     /* 8MB of VA */
> +#endif

Oh right, here's the generic one... this should probably just copy
arm64's version instead. Then x86 can be tweaked (it uses
mmap_is_ia32() instead of is_compat_task() by default, but has a weird
override..)

Regardless, yes, this is a direct code move:

Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

> +
> +unsigned long randomize_stack_top(unsigned long stack_top)
> +{
> +       unsigned long random_variable = 0;
> +
> +       if (current->flags & PF_RANDOMIZE) {
> +               random_variable = get_random_long();
> +               random_variable &= STACK_RND_MASK;
> +               random_variable <<= PAGE_SHIFT;
> +       }
> +#ifdef CONFIG_STACK_GROWSUP
> +       return PAGE_ALIGN(stack_top) + random_variable;
> +#else
> +       return PAGE_ALIGN(stack_top) - random_variable;
> +#endif
> +}
> +
>  #if defined(CONFIG_MMU) && !defined(HAVE_ARCH_PICK_MMAP_LAYOUT)
>  void arch_pick_mmap_layout(struct mm_struct *mm, struct rlimit *rlim_stack)
>  {
> --
> 2.20.1
>


-- 
Kees Cook