[PATCH v2 0/5] arm64: kernel: Add support for User Access Override

On Friday 05 February 2016 14:58:45 James Morse wrote:

This series adds support for User Access Override (UAO; part of the ARMv8.2
Extensions[0]). When enabled, this causes the get_user() accessors to use
the unprivileged load/store instructions. When addr_limit is set to
KERNEL_DS, we set the override bit allowing privileged access.

Because the unprivileged instructions don't trip PAN, the last patch changes
which 'alternative' values are swapped in, allowing PAN to be left enabled
during get_user() and friends.

This series can be retrieved from:
git://linux-arm.org/linux-jm.git -b uao/v2

Looks very nice. I have no complaints about the implementations, but two questions:

* There was recently some work in reducing the number of set_fs() calls in
  the compat_ioctl, which further reduces the attack surface. Should we try to
  continue that effort in other syscalls?

* Do we expect to handle this using live patching indefinitely? I can imagine
  that at some point in the future, ARMv8.2+ systems will be the vast majority,
  so it might be nice to support enabling it unconditionally (same for any
  of the alternative bits really). Is there a long-term strategy?

	Arnd