Re: [RFC PATCH 08/11] x86: Add support for finer grained KASLR

Kees Cook
2020-02-06T11:56:57Z

On Wed, Feb 05, 2020 at 05:17:11PM -0800, Andy Lutomirski wrote:
> On Wed, Feb 5, 2020 at 2:39 PM Kristen Carlson Accardi
> <kristen@linux.intel.com> wrote:
> >
> > At boot time, find all the function sections that have separate .text
> > sections, shuffle them, and then copy them to new locations. Adjust
> > any relocations accordingly.
> >
>
> > + sort(base, num_syms, sizeof(int), kallsyms_cmp, kallsyms_swp);
>
> Hah, here's a huge bottleneck. Unless you are severely
> memory-constrained, never do a sort with an expensive swap function
> like this. Instead allocate an array of indices that starts out as
> [0, 1, 2, ...]. Sort *that* where the swap function just swaps the
> indices. Then use the sorted list of indices to permute the actual
> data. The result is exactly one expensive swap per item instead of
> one expensive swap per swap.

I think there are few places where memory-vs-speed need to be examined.
I remain surprised about how much memory the entire series already uses
(58MB in my local tests), but I suspect this is likely dominated by the
two factors: a full copy of the decompressed kernel, and that the
"allocator" in the image doesn't really implement free():
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/linux/decompress/mm.h#n55

--
Kees Cook