arm64: Kconfig: Add CONFIG_UNMAP_KERNEL_AT_EL0

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: NMark Rutland <mark.rutland@arm.com>
Tested-by: NLaura Abbott <labbott@redhat.com>
Tested-by: NShanker Donthineni <shankerd@codeaurora.org>
Signed-off-by: NWill Deacon <will.deacon@arm.com>

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