arm64: add basic pointer authentication support

This patch adds basic support for pointer authentication, allowing
userspace to make use of APIAKey, APIBKey, APDAKey, APDBKey, and
APGAKey. The kernel maintains key values for each process (shared by all
threads within), which are initialised to random values at exec() time.

The ID_AA64ISAR1_EL1.{APA,API,GPA,GPI} fields are exposed to userspace,
to describe that pointer authentication instructions are available and
that the kernel is managing the keys. Two new hwcaps are added for the
same reason: PACA (for address authentication) and PACG (for generic
authentication).
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Kristina Martsenko <kristina.martsenko@arm.com>
Tested-by: Adam Wallis <awallis@codeaurora.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Ramana Radhakrishnan <ramana.radhakrishnan@arm.com>
Cc: Suzuki K Poulose <suzuki.poulose@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
[will: Fix sizeof() usage and unroll address key initialisation]
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/include/asm/pointer_auth.h

+// SPDX-License-Identifier: GPL-2.0
+#ifndef __ASM_POINTER_AUTH_H
+#define __ASM_POINTER_AUTH_H
+
+#include <linux/random.h>
+
+#include <asm/cpufeature.h>
+#include <asm/sysreg.h>
+
+#ifdef CONFIG_ARM64_PTR_AUTH
+/*
+ * Each key is a 128-bit quantity which is split across a pair of 64-bit
+ * registers (Lo and Hi).
+ */
+struct ptrauth_key {
+	unsigned long lo, hi;
+};
+
+/*
+ * We give each process its own keys, which are shared by all threads. The keys
+ * are inherited upon fork(), and reinitialised upon exec*().
+ */
+struct ptrauth_keys {
+	struct ptrauth_key apia;
+	struct ptrauth_key apib;
+	struct ptrauth_key apda;
+	struct ptrauth_key apdb;
+	struct ptrauth_key apga;
+};
+
+static inline void ptrauth_keys_init(struct ptrauth_keys *keys)
+{
+	if (system_supports_address_auth()) {
+		get_random_bytes(&keys->apia, sizeof(keys->apia));
+		get_random_bytes(&keys->apib, sizeof(keys->apib));
+		get_random_bytes(&keys->apda, sizeof(keys->apda));
+		get_random_bytes(&keys->apdb, sizeof(keys->apdb));
+	}
+
+	if (system_supports_generic_auth())
+		get_random_bytes(&keys->apga, sizeof(keys->apga));
+}
+
+#define __ptrauth_key_install(k, v)				\
+do {								\
+	struct ptrauth_key __pki_v = (v);			\
+	write_sysreg_s(__pki_v.lo, SYS_ ## k ## KEYLO_EL1);	\
+	write_sysreg_s(__pki_v.hi, SYS_ ## k ## KEYHI_EL1);	\
+} while (0)
+
+static inline void ptrauth_keys_switch(struct ptrauth_keys *keys)
+{
+	if (system_supports_address_auth()) {
+		__ptrauth_key_install(APIA, keys->apia);
+		__ptrauth_key_install(APIB, keys->apib);
+		__ptrauth_key_install(APDA, keys->apda);
+		__ptrauth_key_install(APDB, keys->apdb);
+	}
+
+	if (system_supports_generic_auth())
+		__ptrauth_key_install(APGA, keys->apga);
+}
+
+#define ptrauth_thread_init_user(tsk)					\
+do {									\
+	struct task_struct *__ptiu_tsk = (tsk);				\
+	ptrauth_keys_init(&__ptiu_tsk->thread_info.keys_user);		\
+	ptrauth_keys_switch(&__ptiu_tsk->thread_info.keys_user);	\
+} while (0)
+
+#define ptrauth_thread_switch(tsk)	\
+	ptrauth_keys_switch(&(tsk)->thread_info.keys_user)
+
+#else /* CONFIG_ARM64_PTR_AUTH */
+#define ptrauth_thread_init_user(tsk)
+#define ptrauth_thread_switch(tsk)
+#endif /* CONFIG_ARM64_PTR_AUTH */
+
+#endif /* __ASM_POINTER_AUTH_H */

arch/arm64/include/asm/thread_info.h

@@ -28,6 +28,7 @@
 struct task_struct;
 
 #include <asm/memory.h>
+#include <asm/pointer_auth.h>
 #include <asm/stack_pointer.h>
 #include <asm/types.h>
 
@@ -54,6 +55,9 @@ struct thread_info {
 #endif
 		} preempt;
 	};
+#ifdef CONFIG_ARM64_PTR_AUTH
+	struct ptrauth_keys	keys_user;
+#endif
 };
 
 #define thread_saved_pc(tsk)	\

arch/arm64/include/uapi/asm/hwcap.h

@@ -50,5 +50,7 @@
 #define HWCAP_FLAGM		(1 << 27)
 #define HWCAP_SSBS		(1 << 28)
 #define HWCAP_SB		(1 << 29)
+#define HWCAP_PACA		(1 << 30)
+#define HWCAP_PACG		(1UL << 31)
 
 #endif /* _UAPI__ASM_HWCAP_H */

arch/arm64/kernel/cpufeature.c

@@ -1191,6 +1191,12 @@ static void cpu_clear_disr(const struct arm64_cpu_capabilities *__unused)
 #endif /* CONFIG_ARM64_RAS_EXTN */
 
 #ifdef CONFIG_ARM64_PTR_AUTH
+static void cpu_enable_address_auth(struct arm64_cpu_capabilities const *cap)
+{
+	sysreg_clear_set(sctlr_el1, 0, SCTLR_ELx_ENIA | SCTLR_ELx_ENIB |
+				       SCTLR_ELx_ENDA | SCTLR_ELx_ENDB);
+}
+
 static bool has_address_auth(const struct arm64_cpu_capabilities *entry,
 			     int __unused)
 {
@@ -1478,6 +1484,7 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
 		.capability = ARM64_HAS_ADDRESS_AUTH,
 		.type = ARM64_CPUCAP_SYSTEM_FEATURE,
 		.matches = has_address_auth,
+		.cpu_enable = cpu_enable_address_auth,
 	},
 	{
 		.desc = "Generic authentication (architected algorithm)",
@@ -1552,6 +1559,12 @@ static const struct arm64_cpu_capabilities arm64_elf_hwcaps[] = {
 	HWCAP_CAP(SYS_ID_AA64PFR0_EL1, ID_AA64PFR0_SVE_SHIFT, FTR_UNSIGNED, ID_AA64PFR0_SVE, CAP_HWCAP, HWCAP_SVE),
 #endif
 	HWCAP_CAP(SYS_ID_AA64PFR1_EL1, ID_AA64PFR1_SSBS_SHIFT, FTR_UNSIGNED, ID_AA64PFR1_SSBS_PSTATE_INSNS, CAP_HWCAP, HWCAP_SSBS),
+#ifdef CONFIG_ARM64_PTR_AUTH
+	{ .desc = "HWCAP_PACA", .type = ARM64_CPUCAP_SYSTEM_FEATURE, .matches = has_address_auth,
+		.hwcap_type = CAP_HWCAP, .hwcap = HWCAP_PACA },
+	{ .desc = "HWCAP_PACG", .type = ARM64_CPUCAP_SYSTEM_FEATURE, .matches = has_generic_auth,
+		.hwcap_type = CAP_HWCAP, .hwcap = HWCAP_PACG },
+#endif
 	{},
 };
 

arch/arm64/kernel/cpuinfo.c

@@ -83,6 +83,8 @@ static const char *const hwcap_str[] = {
 	"flagm",
 	"ssbs",
 	"sb",
+	"paca",
+	"pacg",
 	NULL
 };
 

arch/arm64/kernel/process.c

@@ -57,6 +57,7 @@
 #include <asm/fpsimd.h>
 #include <asm/mmu_context.h>
 #include <asm/processor.h>
+#include <asm/pointer_auth.h>
 #include <asm/stacktrace.h>
 
 #if defined(CONFIG_STACKPROTECTOR) && !defined(CONFIG_STACKPROTECTOR_PER_TASK)
@@ -429,6 +430,7 @@ __notrace_funcgraph struct task_struct *__switch_to(struct task_struct *prev,
 	contextidr_thread_switch(next);
 	entry_task_switch(next);
 	uao_thread_switch(next);
+	ptrauth_thread_switch(next);
 
 	/*
 	 * Complete any pending TLB or cache maintenance on this CPU in case
@@ -496,4 +498,6 @@ unsigned long arch_randomize_brk(struct mm_struct *mm)
 void arch_setup_new_exec(void)
 {
 	current->mm->context.flags = is_compat_task() ? MMCF_AARCH32 : 0;
+
+	ptrauth_thread_init_user(current);
 }