Re: [PATCH 00/18] arm64: Unmap the kernel whilst running in userspace (KAISER)

[Laura Abbott]

On 11/17/2017 10:21 AM, Will Deacon wrote:
> Hi all,
>
> This patch series implements something along the lines of KAISER for arm64:
>
>    https://gruss.cc/files/kaiser.pdf
>
> although I wrote this from scratch because the paper has some funny
> assumptions about how the architecture works. There is a patch series
> in review for x86, which follows a similar approach:
>
>    http://lkml.kernel.org/r/<20171110193058.BECA7D88@xxxxxxxxxxxxxxxxxx>
>
> and the topic was recently covered by LWN (currently subscriber-only):
>
>    https://lwn.net/Articles/738975/
>
> The basic idea is that transitions to and from userspace are proxied
> through a trampoline page which is mapped into a separate page table and
> can switch the full kernel mapping in and out on exception entry and
> exit respectively. This is a valuable defence against various KASLR and
> timing attacks, particularly as the trampoline page is at a fixed virtual
> address and therefore the kernel text can be randomized independently.
>
> The major consequences of the trampoline are:
>
>    * We can no longer make use of global mappings for kernel space, so
>      each task is assigned two ASIDs: one for user mappings and one for
>      kernel mappings
>
>    * Our ASID moves into TTBR1 so that we can quickly switch between the
>      trampoline and kernel page tables
>
>    * Switching TTBR0 always requires use of the zero page, so we can
>      dispense with some of our errata workaround code.
>
>    * entry.S gets more complicated to read
>
> The performance hit from this series isn't as bad as I feared: things
> like cyclictest and kernbench seem to be largely unaffected, although
> syscall micro-benchmarks appear to show that syscall overhead is roughly
> doubled, and this has an impact on things like hackbench which exhibits
> a ~10% hit due to its heavy context-switching.
>
> Patches based on 4.14 and also pushed here:
>
>    git://git.kernel.org/pub/scm/linux/kernel/git/will/linux.git kaiser
>
> Feedback welcome,
>
> Will

Passed some basic tests on Hikey Android and my Mustang box. I'll
leave the Mustang building kernels for a few days. You're welcome
to add Tested-by or I can re-test on v2.

Thanks,
Laura