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Message-ID: <c9946c229f6f53379deeef00fbdee88fe2fdd96e.camel@linux.intel.com>
Date: Thu, 06 Feb 2020 09:36:59 -0800
From: Kristen Carlson Accardi <kristen@...ux.intel.com>
To: Kees Cook <keescook@...omium.org>, Andy Lutomirski <luto@...nel.org>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
Borislav Petkov <bp@...en8.de>, "H. Peter Anvin" <hpa@...or.com>, Arjan van
de Ven <arjan@...ux.intel.com>, Rick Edgecombe
<rick.p.edgecombe@...el.com>, X86 ML <x86@...nel.org>, LKML
<linux-kernel@...r.kernel.org>, Kernel Hardening
<kernel-hardening@...ts.openwall.com>
Subject: Re: [RFC PATCH 08/11] x86: Add support for finer grained KASLR
On Thu, 2020-02-06 at 03:56 -0800, Kees Cook wrote:
> 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@...ux.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
>
Yes - that was a huge issue (that free() doesn't actually...). Having
to do the copy really caused me to need to bump up the boot heap.
Thankfully, this is a readily solvable problem.
I think there's a temptation to focus too hard on the boot latency.
While I measured this on a reasonably fast system, we aren't talking
minutes of latency here, just a second or a second and a half. I know
there are those who sweat the milliseconds on booting vms, but I expect
they might just turn this feature off anyway. That said, there are
absolutely a lot of great ideas for improving things here that I am
excited to try should people be interested enough in this feature for
me to take it to the next stage.
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