[PATCH v3 17/20] arm64: Kconfig: Add CONFIG_UNMAP_KERNEL_AT_EL0

Will Deacon
2017-12-06T12:37:50Z

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.

Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
---
arch/arm64/Kconfig | 13 +++++++++++++
1 file changed, 13 insertions(+)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index fdcc7b9bb15d..3af1657fcac3 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -833,6 +833,19 @@ 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.

+config UNMAP_KERNEL_AT_EL0
+ 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.
+
menuconfig ARMV8_DEPRECATED
bool "Emulate deprecated/obsolete ARMv8 instructions"
depends on COMPAT
--
2.1.4