Re: [PATH] Fix stack randomization on x86_64 bit

[Kees Cook]

On Wed, Jan 7, 2015 at 11:47 AM, Hector Marco Gisbert <hecmargi@xxxxxx> wrote:
>
> [PATH] Fix stack randomization on x86_64 bit

s/PATH/PATCH/

>
> The issue is that the stack for processes is not properly randomized on 64
> bit
> architectures due to an integer overflow.
>
>
> The affected function is randomize_stack_top() in file "fs/binfmt_elf.c":
>
> static unsigned long randomize_stack_top(unsigned long stack_top)
> {
>         unsigned int random_variable = 0;
>
>         if ((current->flags & PF_RANDOMIZE) &&
>                 !(current->personality & ADDR_NO_RANDOMIZE)) {
>                 random_variable = get_random_int() & 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
> }
>
>
> Note that, it declares the "random_variable" variable as "unsigned int".
> Since
> the result of the shifting operation between STACK_RND_MASK (which is
> 0x3fffff on
> x86_64, 22 bits) and PAGE_SHIFT (which is 12 on x86_64):
>
> random_variable <<= PAGE_SHIFT;
>
> then the two leftmost bits are dropped when storing the result in the
> "random_variable". This variable shall be at least 34 bits long to hold the
> (22+12) result.
>
> These two dropped bits have an impact on the entropy of process stack.
> Concretely, the total stack entropy is reduced by four: from 2^28 to 2^30
> (One
> fourth of expected entropy).
>
> This patch restores back the entropy by correcting the types involved in the
> operations in the functions randomize_stack_top() and
> stack_maxrandom_size().
>
>
> Signed-off-by: Hector Marco-Gisbert <hecmargi@xxxxxx>
> Signed-off-by: Ismael Ripoll <iripoll@xxxxxx>

Good catch!

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

>
>
> diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
> index 919b912..df4552b 100644
> --- a/arch/x86/mm/mmap.c
> +++ b/arch/x86/mm/mmap.c
> @@ -35,12 +35,12 @@ struct va_alignment __read_mostly va_align = {
>         .flags = -1,
>  };
>
> -static unsigned int stack_maxrandom_size(void)
> +static unsigned long stack_maxrandom_size(void)
>  {
> -       unsigned int max = 0;
> +       unsigned long max = 0;
>         if ((current->flags & PF_RANDOMIZE) &&
>                 !(current->personality & ADDR_NO_RANDOMIZE)) {
> -               max = ((-1U) & STACK_RND_MASK) << PAGE_SHIFT;
> +               max = ((-1UL) & STACK_RND_MASK) << PAGE_SHIFT;
>         }
>
>         return max;
> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> index d8fc060..ee668b4 100644
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -554,11 +554,11 @@ out:
>
>  static unsigned long randomize_stack_top(unsigned long stack_top)
>  {
> -       unsigned int random_variable = 0;
> +       unsigned long random_variable = 0;
>
>         if ((current->flags & PF_RANDOMIZE) &&
>                 !(current->personality & ADDR_NO_RANDOMIZE)) {
> -               random_variable = get_random_int() & STACK_RND_MASK;
> +               random_variable = (unsigned long) get_random_int() &
> STACK_RND_MASK;
>                 random_variable <<= PAGE_SHIFT;
>         }
>  #ifdef CONFIG_STACK_GROWSUP
>
>



-- 
Kees Cook
Chrome OS Security
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