arm64: Kconfig: Reword UNMAP_KERNEL_AT_EL0 kconfig entry (0617052d) · Commits · 戴 / test

Although CONFIG_UNMAP_KERNEL_AT_EL0 does make KASLR more robust, it's actually more useful as a mitigation against speculation attacks that can leak arbitrary kernel data to userspace through speculation. Reword the Kconfig help message to reflect this, and make the option depend on EXPERT so that it is on by default for the majority of users. Signed-off-by:

Will Deacon <will.deacon@arm.com> Signed-off-by:

Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/Kconfig

+6 −7

Original line number	Diff line number	Diff line
		@@ -863,15 +863,14 @@ config FORCE_MAX_ZONEORDER
		4M allocations matching the default size used by generic code.

		config UNMAP_KERNEL_AT_EL0
		bool "Unmap kernel when running in userspace (aka \"KAISER\")"
		bool "Unmap kernel when running in userspace (aka \"KAISER\")" if EXPERT
		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.
		Speculation attacks against some high-performance processors can
		be used to bypass MMU permission checks and leak kernel data to
		userspace. This can be defended against by unmapping the kernel
		when running in userspace, mapping it back in on exception entry
		via a trampoline page in the vector table.

		If unsure, say Y.

Admin message