4.9-stable review patch. If anyone has any objections, please let me know.
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From: Will Deacon will.deacon@arm.com
commit 0617052ddde3 upstream.
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 Signed-off-by: Alex Shi alex.shi@linaro.org [v4.9 backport] Signed-off-by: Mark Rutland mark.rutland@arm.com [v4.9 backport] Tested-by: Will Deacon will.deacon@arm.com Tested-by: Greg Hackmann ghackmann@google.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/arm64/Kconfig | 13 ++++++------- 1 file changed, 6 insertions(+), 7 deletions(-)
--- a/arch/arm64/Kconfig +++ b/arch/arm64/Kconfig @@ -734,15 +734,14 @@ 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.