[PATCH] arm64: trap implementation defined functionality in userspace

[Kristina Martsenko]

The Arm v8.8 extension adds a new control FEAT_TIDCP1 that allows the
kernel to disable all implementation-defined system registers and
instructions in userspace. This can improve robustness against covert
channels between processes, for example in cases where the firmware or
hardware didn't disable that functionality by default.

The kernel does not currently support any implementation-defined
features, as there are no hwcaps for any such features, so disable all
imp-def features unconditionally. Any use of imp-def instructions will
result in a SIGILL being delivered to the process (same as for undefined
instructions).

Signed-off-by: Kristina Martsenko <kristina.martsenko at arm.com>
---
 arch/arm64/include/asm/sysreg.h |  4 ++++
 arch/arm64/kernel/cpufeature.c  | 18 ++++++++++++++++++
 arch/arm64/tools/cpucaps        |  1 +
 3 files changed, 23 insertions(+)

diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
index 55f998c3dc28..0696ef9f156e 100644
--- a/arch/arm64/include/asm/sysreg.h
+++ b/arch/arm64/include/asm/sysreg.h
@@ -902,6 +902,7 @@
 
 /* id_aa64mmfr1 */
 #define ID_AA64MMFR1_ECBHB_SHIFT	60
+#define ID_AA64MMFR1_TIDCP1_SHIFT	52
 #define ID_AA64MMFR1_HCX_SHIFT		40
 #define ID_AA64MMFR1_AFP_SHIFT		44
 #define ID_AA64MMFR1_ETS_SHIFT		36
@@ -918,6 +919,9 @@
 #define ID_AA64MMFR1_VMIDBITS_8		0
 #define ID_AA64MMFR1_VMIDBITS_16	2
 
+#define ID_AA64MMFR1_TIDCP1_NI		0
+#define ID_AA64MMFR1_TIDCP1_IMP		1
+
 /* id_aa64mmfr2 */
 #define ID_AA64MMFR2_E0PD_SHIFT		60
 #define ID_AA64MMFR2_EVT_SHIFT		56
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 42ea2bd856c6..a1df0e6349f0 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -361,6 +361,7 @@ static const struct arm64_ftr_bits ftr_id_aa64mmfr0[] = {
 };
 
 static const struct arm64_ftr_bits ftr_id_aa64mmfr1[] = {
+	ARM64_FTR_BITS(FTR_HIDDEN, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64MMFR1_TIDCP1_SHIFT, 4, 0),
 	ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64MMFR1_AFP_SHIFT, 4, 0),
 	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64MMFR1_ETS_SHIFT, 4, 0),
 	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64MMFR1_TWED_SHIFT, 4, 0),
@@ -1986,6 +1987,11 @@ static bool is_kvm_protected_mode(const struct arm64_cpu_capabilities *entry, in
 }
 #endif /* CONFIG_KVM */
 
+static void cpu_trap_el0_impdef(const struct arm64_cpu_capabilities *__unused)
+{
+	sysreg_clear_set(sctlr_el1, 0, SCTLR_EL1_TIDCP);
+}
+
 /* Internal helper functions to match cpu capability type */
 static bool
 cpucap_late_cpu_optional(const struct arm64_cpu_capabilities *cap)
@@ -2529,6 +2535,18 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
 		.matches = has_cpuid_feature,
 		.min_field_value = ID_AA64ISAR2_WFXT_SUPPORTED,
 	},
+	{
+		.desc = "Trap EL0 IMPLEMENTATION DEFINED functionality",
+		.capability = ARM64_HAS_TIDCP1,
+		.type = ARM64_CPUCAP_SYSTEM_FEATURE,
+		.sys_reg = SYS_ID_AA64MMFR1_EL1,
+		.sign = FTR_UNSIGNED,
+		.field_pos = ID_AA64MMFR1_TIDCP1_SHIFT,
+		.field_width = 4,
+		.min_field_value = ID_AA64MMFR1_TIDCP1_IMP,
+		.matches = has_cpuid_feature,
+		.cpu_enable = cpu_trap_el0_impdef,
+	},
 	{},
 };
 
diff --git a/arch/arm64/tools/cpucaps b/arch/arm64/tools/cpucaps
index 507b20373953..e491d89913c2 100644
--- a/arch/arm64/tools/cpucaps
+++ b/arch/arm64/tools/cpucaps
@@ -36,6 +36,7 @@ HAS_RNG
 HAS_SB
 HAS_STAGE2_FWB
 HAS_SYSREG_GIC_CPUIF
+HAS_TIDCP1
 HAS_TLB_RANGE
 HAS_VIRT_HOST_EXTN
 HAS_WFXT
-- 
2.25.1