arm64: kernel: Add support for User Access Override

'User Access Override' is a new ARMv8.2 feature which allows the
unprivileged load and store instructions to be overridden to behave in
the normal way.

This patch converts {get,put}_user() and friends to use ldtr*/sttr*
instructions - so that they can only access EL0 memory, then enables
UAO when fs==KERNEL_DS so that these functions can access kernel memory.

This allows user space's read/write permissions to be checked against the
page tables, instead of testing addr<USER_DS, then using the kernel's
read/write permissions.

Signed-off-by: James Morse <james.morse@arm.com>
[catalin.marinas@arm.com: move uao_thread_switch() above dsb()]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

1 files changed, 11 insertions, 0 deletions

diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 42918c797e8e..ae22edf9d3c9 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -677,6 +677,17 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
 		.capability = ARM64_HAS_NO_HW_PREFETCH,
 		.matches = has_no_hw_prefetch,
 	},
+#ifdef CONFIG_ARM64_UAO
+	{
+		.desc = "User Access Override",
+		.capability = ARM64_HAS_UAO,
+		.matches = has_cpuid_feature,
+		.sys_reg = SYS_ID_AA64MMFR2_EL1,
+		.field_pos = ID_AA64MMFR2_UAO_SHIFT,
+		.min_field_value = 1,
+		.enable = cpu_enable_uao,
+	},
+#endif /* CONFIG_ARM64_UAO */
 	{},
 };