[PATCH 07/11] arm64: add basic pointer authentication support

Mark Rutland
2017-07-19T16:01:28Z

This patch adds basic support for pointer authentication, allowing
userspace to make use of APIAKey. The kernel maintains an APIAKey value
for each process (shared by all threads within), which is initialised to
a random value at exec() time.

Instructions using other keys (APIBKey, APDAKey, APDBKey) are disabled,
and will behave as NOPs. These may be made use of in future patches.

No support is added for the generic key (APGAKey), though this cannot be
trapped or made to behave as a NOP. Its presence is not advertised with
a hwcap.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Suzuki K Poulose <suzuki.poulose@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
---
arch/arm64/Kconfig | 23 +++++++++
arch/arm64/include/asm/mmu.h | 5 ++
arch/arm64/include/asm/mmu_context.h | 25 +++++++++-
arch/arm64/include/asm/pointer_auth.h | 89 +++++++++++++++++++++++++++++++++++
arch/arm64/include/uapi/asm/hwcap.h | 1 +
arch/arm64/kernel/cpufeature.c | 11 +++++
arch/arm64/kernel/cpuinfo.c | 1 +
7 files changed, 153 insertions(+), 2 deletions(-)
create mode 100644 arch/arm64/include/asm/pointer_auth.h

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index dfd9086..15a9931 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -962,6 +962,29 @@ config ARM64_UAO

endmenu

+menu "ARMv8.3 architectural features"
+
+config ARM64_POINTER_AUTHENTICATION
+ bool "Enable support for pointer authentication"
+ default y
+ help
+ Pointer authentication (part of the ARMv8.3 Extensions) provides
+ instructions for signing and authenticating pointers against secret
+ keys, which can be used to mitigate Return Oriented Programming (ROP)
+ and other attacks.
+
+ This option enables these instructions at EL0 (i.e. for userspace).
+
+ Choosing this option will cause the kernel to initialise secret keys
+ for each process at exec() time, with these keys being
+ context-switched along with the process.
+
+ The feature is detected at runtime. If the feature is not present in
+ hardware it will not be advertised to userspace nor will it be
+ enabled.
+
+endmenu
+
config ARM64_MODULE_CMODEL_LARGE
bool

diff --git a/arch/arm64/include/asm/mmu.h b/arch/arm64/include/asm/mmu.h
index 5468c83..6a848f3 100644
--- a/arch/arm64/include/asm/mmu.h
+++ b/arch/arm64/include/asm/mmu.h
@@ -16,10 +16,15 @@
#ifndef __ASM_MMU_H
#define __ASM_MMU_H

+#include <asm/pointer_auth.h>
+
typedef struct {
atomic64_t id;
void *vdso;
unsigned long flags;
+#ifdef CONFIG_ARM64_POINTER_AUTHENTICATION
+ struct ptrauth_keys ptrauth_keys;
+#endif
} mm_context_t;

/*
diff --git a/arch/arm64/include/asm/mmu_context.h b/arch/arm64/include/asm/mmu_context.h
index 3257895a..06757a5 100644
--- a/arch/arm64/include/asm/mmu_context.h
+++ b/arch/arm64/include/asm/mmu_context.h
@@ -31,7 +31,6 @@
#include <asm/cacheflush.h>
#include <asm/cpufeature.h>
#include <asm/proc-fns.h>
-#include <asm-generic/mm_hooks.h>
#include <asm/cputype.h>
#include <asm/pgtable.h>
#include <asm/sysreg.h>
@@ -154,7 +153,14 @@ static inline void cpu_replace_ttbr1(pgd_t *pgd)
#define destroy_context(mm) do { } while(0)
void check_and_switch_context(struct mm_struct *mm, unsigned int cpu);

-#define init_new_context(tsk,mm) ({ atomic64_set(&(mm)->context.id, 0); 0; })
+static inline int init_new_context(struct task_struct *tsk,
+ struct mm_struct *mm)
+{
+ atomic64_set(&mm->context.id, 0);
+ mm_ctx_ptrauth_init(&mm->context);
+
+ return 0;
+}

/*
* This is called when "tsk" is about to enter lazy TLB mode.
@@ -200,6 +206,8 @@ static inline void __switch_mm(struct mm_struct *next)
return;
}

+ mm_ctx_ptrauth_switch(&next->context);
+
check_and_switch_context(next, cpu);
}

@@ -226,6 +234,19 @@ static inline void __switch_mm(struct mm_struct *next)

void verify_cpu_asid_bits(void);

+static inline void arch_dup_mmap(struct mm_struct *oldmm,
+ struct mm_struct *mm)
+{
+ mm_ctx_ptrauth_dup(&oldmm->context, &mm->context);
+}
+#define arch_dup_mmap arch_dup_mmap
+
+/*
+ * We need to override arch_dup_mmap before including the generic hooks, which
+ * are otherwise sufficient for us.
+ */
+#include <asm-generic/mm_hooks.h>
+
#endif /* !__ASSEMBLY__ */

#endif /* !__ASM_MMU_CONTEXT_H */
diff --git a/arch/arm64/include/asm/pointer_auth.h b/arch/arm64/include/asm/pointer_auth.h
new file mode 100644
index 0000000..964da0c
--- /dev/null
+++ b/arch/arm64/include/asm/pointer_auth.h
@@ -0,0 +1,89 @@
+/*
+ * Copyright (C) 2016 ARM Ltd.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+#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_POINTER_AUTHENTICATION
+/*
+ * Each key is a 128-bit quantity which is split accross a pair of 64-bit
+ * registers (Lo and Hi).
+ */
+struct ptrauth_key {
+ unsigned long lo, hi;
+};
+
+/*
+ * We give each process its own instruction A key (APIAKey), which is shared by
+ * all threads. This is inherited upon fork(), and reinitialised upon exec*().
+ * All other keys are currently unused, with APIBKey, APDAKey, and APBAKey
+ * instructions behaving as NOPs.
+ */
+struct ptrauth_keys {
+ struct ptrauth_key apia;
+};
+
+static inline void ptrauth_keys_init(struct ptrauth_keys *keys)
+{
+ if (!cpus_have_const_cap(ARM64_HAS_ADDRESS_AUTH))
+ return;
+
+ get_random_bytes(keys, sizeof(*keys));
+}
+
+#define __ptrauth_key_install(k, v) \
+do { \
+ write_sysreg_s(v.lo, SYS_ ## k ## KEYLO_EL1); \
+ write_sysreg_s(v.hi, SYS_ ## k ## KEYHI_EL1); \
+} while (0)
+
+static inline void ptrauth_keys_switch(struct ptrauth_keys *keys)
+{
+ if (!cpus_have_const_cap(ARM64_HAS_ADDRESS_AUTH))
+ return;
+
+ __ptrauth_key_install(APIA, keys->apia);
+}
+
+static inline void ptrauth_keys_dup(struct ptrauth_keys *old,
+ struct ptrauth_keys *new)
+{
+ if (!cpus_have_const_cap(ARM64_HAS_ADDRESS_AUTH))
+ return;
+
+ *new = *old;
+}
+
+#define mm_ctx_ptrauth_init(ctx) \
+ ptrauth_keys_init(&(ctx)->ptrauth_keys)
+
+#define mm_ctx_ptrauth_switch(ctx) \
+ ptrauth_keys_switch(&(ctx)->ptrauth_keys)
+
+#define mm_ctx_ptrauth_dup(oldctx, newctx) \
+ ptrauth_keys_dup(&(oldctx)->ptrauth_keys, &(newctx)->ptrauth_keys)
+
+#else
+#define mm_ctx_ptrauth_init(ctx)
+#define mm_ctx_ptrauth_switch(ctx)
+#define mm_ctx_ptrauth_dup(oldctx, newctx)
+#endif
+
+#endif /* __ASM_POINTER_AUTH_H */
diff --git a/arch/arm64/include/uapi/asm/hwcap.h b/arch/arm64/include/uapi/asm/hwcap.h
index 4e187ce..0481c73 100644
--- a/arch/arm64/include/uapi/asm/hwcap.h
+++ b/arch/arm64/include/uapi/asm/hwcap.h
@@ -35,5 +35,6 @@
#define HWCAP_JSCVT (1 << 13)
#define HWCAP_FCMA (1 << 14)
#define HWCAP_LRCPC (1 << 15)
+#define HWCAP_APIA (1 << 16)

#endif /* _UAPI__ASM_HWCAP_H */
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 4016b1e7..7e2885e 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -778,6 +778,15 @@ static bool runs_at_el2(const struct arm64_cpu_capabilities *entry, int __unused
return is_kernel_in_hyp_mode();
}

+#ifdef CONFIG_ARM64_POINTER_AUTHENTICATION
+static int cpu_enable_address_auth(void *__unused)
+{
+ config_sctlr_el1(0, SCTLR_ELx_ENIA);
+
+ return 0;
+}
+#endif /* CONFIG_ARM64_POINTER_AUTHENTICATION */
+
static bool hyp_offset_low(const struct arm64_cpu_capabilities *entry,
int __unused)
{
@@ -902,6 +911,7 @@ static bool has_no_fpsimd(const struct arm64_cpu_capabilities *entry, int __unus
.field_pos = ID_AA64ISAR1_APA_SHIFT,
.min_field_value = ID_AA64ISAR1_APA_ARCHITECTED,
.matches = has_cpuid_feature,
+ .enable = cpu_enable_address_auth,
},
{
.desc = "Generic authentication (architected algorithm)",
@@ -945,6 +955,7 @@ static bool has_no_fpsimd(const struct arm64_cpu_capabilities *entry, int __unus
HWCAP_CAP(SYS_ID_AA64ISAR1_EL1, ID_AA64ISAR1_JSCVT_SHIFT, FTR_UNSIGNED, 1, CAP_HWCAP, HWCAP_JSCVT),
HWCAP_CAP(SYS_ID_AA64ISAR1_EL1, ID_AA64ISAR1_FCMA_SHIFT, FTR_UNSIGNED, 1, CAP_HWCAP, HWCAP_FCMA),
HWCAP_CAP(SYS_ID_AA64ISAR1_EL1, ID_AA64ISAR1_LRCPC_SHIFT, FTR_UNSIGNED, 1, CAP_HWCAP, HWCAP_LRCPC),
+ HWCAP_CAP(SYS_ID_AA64ISAR1_EL1, ID_AA64ISAR1_APA_SHIFT, FTR_UNSIGNED, 1, CAP_HWCAP, HWCAP_APIA),
{},
};

diff --git a/arch/arm64/kernel/cpuinfo.c b/arch/arm64/kernel/cpuinfo.c
index f495ee5..b5bd2d3 100644
--- a/arch/arm64/kernel/cpuinfo.c
+++ b/arch/arm64/kernel/cpuinfo.c
@@ -68,6 +68,7 @@
"jscvt",
"fcma",
"lrcpc",
+ "apia",
NULL
};

--
1.9.1