linux-kernel - Re: [RFC PATCH v2 1/2] Randomization of address chosen by mmap.

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Message-Id: <20180322135314.61efce938293e051e118fa46@linux-foundation.org>
Date:   Thu, 22 Mar 2018 13:53:14 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Ilya Smith <blackzert@...il.com>
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Subject: Re: [RFC PATCH v2 1/2] Randomization of address chosen by mmap.

On Thu, 22 Mar 2018 19:36:37 +0300 Ilya Smith <blackzert@...il.com> wrote:

>  include/linux/mm.h |  16 ++++--
>  mm/mmap.c          | 164 +++++++++++++++++++++++++++++++++++++++++++++++++++++

You'll be wanting to update the documentation. 
Documentation/sysctl/kernel.txt and
Documentation/admin-guide/kernel-parameters.txt.

> ...
>
> @@ -2268,6 +2276,9 @@ extern unsigned long unmapped_area_topdown(struct vm_unmapped_area_info *info);
>  static inline unsigned long
>  vm_unmapped_area(struct vm_unmapped_area_info *info)
>  {
> +	/* How about 32 bit process?? */
> +	if ((current->flags & PF_RANDOMIZE) && randomize_va_space > 3)
> +		return unmapped_area_random(info);

The handling of randomize_va_space is peculiar.  Rather than being a
bitfield which independently selects different modes, it is treated as
a scalar: the larger the value, the more stuff we randomize.

I can see the sense in that (and I wonder what randomize_va_space=5
will do).  But it is...  odd.

Why did you select randomize_va_space=4 for this?  Is there a mode 3
already and we forgot to document it?  Or did you leave a gap for
something?  If the former, please feel free to fix the documentation
(in a separate, preceding patch) while you're in there ;)

>  	if (info->flags & VM_UNMAPPED_AREA_TOPDOWN)
>  		return unmapped_area_topdown(info);
>  	else
> @@ -2529,11 +2540,6 @@ int drop_caches_sysctl_handler(struct ctl_table *, int,
>  void drop_slab(void);
>  void drop_slab_node(int nid);
>  
>
> ...
>
> @@ -1780,6 +1781,169 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>  	return error;
>  }
>  
> +unsigned long unmapped_area_random(struct vm_unmapped_area_info *info)
> +{

This function is just dead code if CONFIG_MMU=n, yes?  Let's add the
ifdefs to make it go away in that case.

> +	struct mm_struct *mm = current->mm;
> +	struct vm_area_struct *vma = NULL;
> +	struct vm_area_struct *visited_vma = NULL;
> +	unsigned long entropy[2];
> +	unsigned long length, low_limit, high_limit, gap_start, gap_end;
> +	unsigned long addr = 0;
> +
> +	/* get entropy with prng */
> +	prandom_bytes(&entropy, sizeof(entropy));
> +	/* small hack to prevent EPERM result */
> +	info->low_limit = max(info->low_limit, mmap_min_addr);
> +
>
> ...
>
> +found:
> +	/* We found a suitable gap. Clip it with the original high_limit. */
> +	if (gap_end > info->high_limit)
> +		gap_end = info->high_limit;
> +	gap_end -= info->length;
> +	gap_end -= (gap_end - info->align_offset) & info->align_mask;
> +	/* only one suitable page */
> +	if (gap_end ==  gap_start)
> +		return gap_start;
> +	addr = entropy[1] % (min((gap_end - gap_start) >> PAGE_SHIFT,
> +							 0x10000UL));

What does the magic 10000 mean?  Isn't a comment needed explaining this?

> +	addr = gap_end - (addr << PAGE_SHIFT);
> +	addr += (info->align_offset - addr) & info->align_mask;
> +	return addr;
> +}
>
> ...
>