arm64: Set GP bit in kernel page tables to enable BTI for the kernel
Now that the kernel is built with BTI annotations enable the feature by setting the GP bit in the stage 1 translation tables. This is done based on the features supported by the boot CPU so that we do not need to rewrite the translation tables. In order to avoid potential issues on big.LITTLE systems when there are a mix of BTI and non-BTI capable CPUs in the system when we have enabled kernel mode BTI we change BTI to be a _STRICT_BOOT_CPU_FEATURE when we have kernel BTI. This will prevent any CPUs that don't support BTI being started if the boot CPU supports BTI rather than simply not using BTI as we do when supporting BTI only in userspace. The main concern is the possibility of BTYPE being preserved by a CPU that does not implement BTI when a thread is migrated to it resulting in an incorrect state which could generate an exception when the thread migrates back to a CPU that does support BTI. If we encounter practical systems which mix BTI and non-BTI CPUs we will need to revisit this implementation. Since we currently do not generate landing pads in the BPF JIT we only map the base kernel text in this way. Signed-off-by: NMark Brown <broonie@kernel.org> Reviewed-by: NCatalin Marinas <catalin.marinas@arm.com> Link: https://lore.kernel.org/r/20200506195138.22086-5-broonie@kernel.orgSigned-off-by: NWill Deacon <will@kernel.org>
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