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

Andrew Morton
2018-03-22T20:53:14Z

On Thu, 22 Mar 2018 19:36:37 +0300 Ilya Smith <blackzert@gmail.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;
> +}
>
> ...
>