[PATCH 18/18] arm64: Kconfig: Add CONFIG_UNMAP_KERNEL_AT_EL0

From: Will Deacon
Date: Fri Nov 17 2017 - 13:23:08 EST

Add a Kconfig entry to control use of the entry trampoline, which allows
us to unmap the kernel whilst running in userspace and improve the
robustness of KASLR.

Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
arch/arm64/Kconfig | 13 +++++++++++++
1 file changed, 13 insertions(+)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index f0fcbfc2262e..f99ffb88843a 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -796,6 +796,19 @@ config FORCE_MAX_ZONEORDER
However for 4K, we choose a higher default value, 11 as opposed to 10, giving us
4M allocations matching the default size used by generic code.

+ bool "Unmap kernel when running in userspace (aka \"KAISER\")"
+ default y
+ help
+ Some attacks against KASLR make use of the timing difference between
+ a permission fault which could arise from a page table entry that is
+ present in the TLB, and a translation fault which always requires a
+ page table walk. This option defends against these attacks by unmapping
+ the kernel whilst running in userspace, therefore forcing translation
+ faults for all of kernel space.
+ If unsure, say Y.
bool "Emulate deprecated/obsolete ARMv8 instructions"
depends on COMPAT