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Subject[PATCH v4 04/12] arm64: Basic Branch Target Identification support
Date
From: Dave Martin <Dave.Martin@arm.com>

This patch adds the bare minimum required to expose the ARMv8.5
Branch Target Identification feature to userspace.

By itself, this does _not_ automatically enable BTI for any initial
executable pages mapped by execve(). This will come later, but for
now it should be possible to enable BTI manually on those pages by
using mprotect() from within the target process.

Other arches already using the generic mman.h are already using
0x10 for arch-specific prot flags, so we use that for PROT_BTI
here.

For consistency, signal handler entry points in BTI guarded pages
are required to be annotated as such, just like any other function.
This blocks a relatively minor attack vector, but comforming
userspace will have the annotations anyway, so we may as well
enforce them.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
---
Documentation/arm64/cpu-feature-registers.rst | 2 +
Documentation/arm64/elf_hwcaps.rst | 4 ++
arch/arm64/Kconfig | 26 +++++++++++++
arch/arm64/include/asm/cpucaps.h | 3 +-
arch/arm64/include/asm/cpufeature.h | 6 +++
arch/arm64/include/asm/esr.h | 2 +-
arch/arm64/include/asm/exception.h | 1 +
arch/arm64/include/asm/hwcap.h | 1 +
arch/arm64/include/asm/mman.h | 37 +++++++++++++++++++
arch/arm64/include/asm/pgtable-hwdef.h | 1 +
arch/arm64/include/asm/pgtable.h | 2 +-
arch/arm64/include/asm/ptrace.h | 8 ++++
arch/arm64/include/asm/sysreg.h | 4 ++
arch/arm64/include/uapi/asm/hwcap.h | 1 +
arch/arm64/include/uapi/asm/mman.h | 9 +++++
arch/arm64/include/uapi/asm/ptrace.h | 1 +
arch/arm64/kernel/cpufeature.c | 33 +++++++++++++++++
arch/arm64/kernel/cpuinfo.c | 1 +
arch/arm64/kernel/entry-common.c | 11 ++++++
arch/arm64/kernel/ptrace.c | 2 +-
arch/arm64/kernel/signal.c | 16 ++++++++
arch/arm64/kernel/syscall.c | 18 +++++++++
arch/arm64/kernel/traps.c | 8 ++++
include/linux/mm.h | 3 ++
24 files changed, 196 insertions(+), 4 deletions(-)
create mode 100644 arch/arm64/include/asm/mman.h
create mode 100644 arch/arm64/include/uapi/asm/mman.h

diff --git a/Documentation/arm64/cpu-feature-registers.rst b/Documentation/arm64/cpu-feature-registers.rst
index b6e44884e3ad..400e14009008 100644
--- a/Documentation/arm64/cpu-feature-registers.rst
+++ b/Documentation/arm64/cpu-feature-registers.rst
@@ -174,6 +174,8 @@ infrastructure:
+------------------------------+---------+---------+
| SSBS | [7-4] | y |
+------------------------------+---------+---------+
+ | BT | [3-0] | y |
+ +------------------------------+---------+---------+


4) MIDR_EL1 - Main ID Register
diff --git a/Documentation/arm64/elf_hwcaps.rst b/Documentation/arm64/elf_hwcaps.rst
index 7fa3d215ae6a..2997d7b398ce 100644
--- a/Documentation/arm64/elf_hwcaps.rst
+++ b/Documentation/arm64/elf_hwcaps.rst
@@ -204,6 +204,10 @@ HWCAP2_FRINT

Functionality implied by ID_AA64ISAR1_EL1.FRINTTS == 0b0001.

+HWCAP2_BTI
+
+ Functionality implied by ID_AA64PFR0_EL1.BT == 0b0001.
+

4. Unused AT_HWCAP bits
-----------------------
diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index b1b4476ddb83..39292a0fc603 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -1484,6 +1484,32 @@ config ARM64_PTR_AUTH

endmenu

+menu "ARMv8.5 architectural features"
+
+config ARM64_BTI
+ bool "Branch Target Identification support"
+ default y
+ help
+ Branch Target Identification (part of the ARMv8.5 Extensions)
+ provides a mechanism to limit the set of locations to which computed
+ branch instructions such as BR or BLR can jump.
+
+ To make use of BTI on CPUs that support it, say Y.
+
+ BTI is intended to provide complementary protection to other control
+ flow integrity protection mechanisms, such as the Pointer
+ authentication mechanism provided as part of the ARMv8.3 Extensions.
+ For this reason, it does not make sense to enable this option without
+ also enabling support for pointer authentication. Thus, when
+ enabling this option you should also select ARM64_PTR_AUTH=y.
+
+ Userspace binaries must also be specifically compiled to make use of
+ this mechanism. If you say N here or the hardware does not support
+ BTI, such binaries can still run, but you get no additional
+ enforcement of branch destinations.
+
+endmenu
+
config ARM64_SVE
bool "ARM Scalable Vector Extension support"
default y
diff --git a/arch/arm64/include/asm/cpucaps.h b/arch/arm64/include/asm/cpucaps.h
index b92683871119..23518d70e304 100644
--- a/arch/arm64/include/asm/cpucaps.h
+++ b/arch/arm64/include/asm/cpucaps.h
@@ -56,7 +56,8 @@
#define ARM64_WORKAROUND_CAVIUM_TX2_219_PRFM 46
#define ARM64_WORKAROUND_1542419 47
#define ARM64_WORKAROUND_1319367 48
+#define ARM64_BTI 49

-#define ARM64_NCAPS 49
+#define ARM64_NCAPS 50

#endif /* __ASM_CPUCAPS_H */
diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
index 4261d55e8506..dc39ca83de19 100644
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -613,6 +613,12 @@ static inline bool system_has_prio_mask_debugging(void)
system_uses_irq_prio_masking();
}

+static inline bool system_supports_bti(void)
+{
+ return IS_ENABLED(CONFIG_ARM64_BTI) &&
+ cpus_have_const_cap(ARM64_BTI);
+}
+
#define ARM64_BP_HARDEN_UNKNOWN -1
#define ARM64_BP_HARDEN_WA_NEEDED 0
#define ARM64_BP_HARDEN_NOT_REQUIRED 1
diff --git a/arch/arm64/include/asm/esr.h b/arch/arm64/include/asm/esr.h
index cb29253ae86b..390b8ba67830 100644
--- a/arch/arm64/include/asm/esr.h
+++ b/arch/arm64/include/asm/esr.h
@@ -22,7 +22,7 @@
#define ESR_ELx_EC_PAC (0x09) /* EL2 and above */
/* Unallocated EC: 0x0A - 0x0B */
#define ESR_ELx_EC_CP14_64 (0x0C)
-/* Unallocated EC: 0x0d */
+#define ESR_ELx_EC_BTI (0x0D)
#define ESR_ELx_EC_ILL (0x0E)
/* Unallocated EC: 0x0F - 0x10 */
#define ESR_ELx_EC_SVC32 (0x11)
diff --git a/arch/arm64/include/asm/exception.h b/arch/arm64/include/asm/exception.h
index 4d5f3b5f50cd..3384cbe2f89c 100644
--- a/arch/arm64/include/asm/exception.h
+++ b/arch/arm64/include/asm/exception.h
@@ -35,6 +35,7 @@ asmlinkage void enter_from_user_mode(void);
void do_mem_abort(unsigned long addr, unsigned int esr, struct pt_regs *regs);
void do_sp_pc_abort(unsigned long addr, unsigned int esr, struct pt_regs *regs);
void do_undefinstr(struct pt_regs *regs);
+void do_bti(struct pt_regs *regs);
asmlinkage void bad_mode(struct pt_regs *regs, int reason, unsigned int esr);
void do_debug_exception(unsigned long addr_if_watchpoint, unsigned int esr,
struct pt_regs *regs);
diff --git a/arch/arm64/include/asm/hwcap.h b/arch/arm64/include/asm/hwcap.h
index 3d2f2472a36c..f9e681df33d0 100644
--- a/arch/arm64/include/asm/hwcap.h
+++ b/arch/arm64/include/asm/hwcap.h
@@ -86,6 +86,7 @@
#define KERNEL_HWCAP_SVESM4 __khwcap2_feature(SVESM4)
#define KERNEL_HWCAP_FLAGM2 __khwcap2_feature(FLAGM2)
#define KERNEL_HWCAP_FRINT __khwcap2_feature(FRINT)
+#define KERNEL_HWCAP_BTI __khwcap2_feature(BTI)

/*
* This yields a mask that user programs can use to figure out what
diff --git a/arch/arm64/include/asm/mman.h b/arch/arm64/include/asm/mman.h
new file mode 100644
index 000000000000..081ec8de9ea6
--- /dev/null
+++ b/arch/arm64/include/asm/mman.h
@@ -0,0 +1,37 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __ASM_MMAN_H__
+#define __ASM_MMAN_H__
+
+#include <linux/compiler.h>
+#include <linux/types.h>
+#include <uapi/asm/mman.h>
+
+static inline unsigned long arch_calc_vm_prot_bits(unsigned long prot,
+ unsigned long pkey __always_unused)
+{
+ if (system_supports_bti() && (prot & PROT_BTI))
+ return VM_ARM64_BTI;
+
+ return 0;
+}
+#define arch_calc_vm_prot_bits(prot, pkey) arch_calc_vm_prot_bits(prot, pkey)
+
+static inline pgprot_t arch_vm_get_page_prot(unsigned long vm_flags)
+{
+ return (vm_flags & VM_ARM64_BTI) ? __pgprot(PTE_GP) : __pgprot(0);
+}
+#define arch_vm_get_page_prot(vm_flags) arch_vm_get_page_prot(vm_flags)
+
+static inline bool arch_validate_prot(unsigned long prot,
+ unsigned long addr __always_unused)
+{
+ unsigned long supported = PROT_READ | PROT_WRITE | PROT_EXEC | PROT_SEM;
+
+ if (system_supports_bti())
+ supported |= PROT_BTI;
+
+ return (prot & ~supported) == 0;
+}
+#define arch_validate_prot(prot, addr) arch_validate_prot(prot, addr)
+
+#endif /* ! __ASM_MMAN_H__ */
diff --git a/arch/arm64/include/asm/pgtable-hwdef.h b/arch/arm64/include/asm/pgtable-hwdef.h
index d9fbd433cc17..c422f03ad411 100644
--- a/arch/arm64/include/asm/pgtable-hwdef.h
+++ b/arch/arm64/include/asm/pgtable-hwdef.h
@@ -150,6 +150,7 @@
#define PTE_SHARED (_AT(pteval_t, 3) << 8) /* SH[1:0], inner shareable */
#define PTE_AF (_AT(pteval_t, 1) << 10) /* Access Flag */
#define PTE_NG (_AT(pteval_t, 1) << 11) /* nG */
+#define PTE_GP (_AT(pteval_t, 1) << 50) /* BTI guarded */
#define PTE_DBM (_AT(pteval_t, 1) << 51) /* Dirty Bit Management */
#define PTE_CONT (_AT(pteval_t, 1) << 52) /* Contiguous range */
#define PTE_PXN (_AT(pteval_t, 1) << 53) /* Privileged XN */
diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index 5d15b4735a0e..3e13e415119e 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -662,7 +662,7 @@ static inline phys_addr_t pgd_page_paddr(pgd_t pgd)
static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)
{
const pteval_t mask = PTE_USER | PTE_PXN | PTE_UXN | PTE_RDONLY |
- PTE_PROT_NONE | PTE_VALID | PTE_WRITE;
+ PTE_PROT_NONE | PTE_VALID | PTE_WRITE | PTE_GP;
/* preserve the hardware dirty information */
if (pte_hw_dirty(pte))
pte = pte_mkdirty(pte);
diff --git a/arch/arm64/include/asm/ptrace.h b/arch/arm64/include/asm/ptrace.h
index fbebb411ae20..212bba1f8d84 100644
--- a/arch/arm64/include/asm/ptrace.h
+++ b/arch/arm64/include/asm/ptrace.h
@@ -35,8 +35,16 @@
#define GIC_PRIO_PSR_I_SET (1 << 4)

/* Additional SPSR bits not exposed in the UABI */
+#define PSR_BTYPE_SHIFT 10
+
#define PSR_IL_BIT (1 << 20)

+/* Convenience names for the values of PSTATE.BTYPE */
+#define PSR_BTYPE_NONE (0b00 << PSR_BTYPE_SHIFT)
+#define PSR_BTYPE_JC (0b01 << PSR_BTYPE_SHIFT)
+#define PSR_BTYPE_C (0b10 << PSR_BTYPE_SHIFT)
+#define PSR_BTYPE_J (0b11 << PSR_BTYPE_SHIFT)
+
/* AArch32-specific ptrace requests */
#define COMPAT_PTRACE_GETREGS 12
#define COMPAT_PTRACE_SETREGS 13
diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
index 6e919fafb43d..3f4710f23277 100644
--- a/arch/arm64/include/asm/sysreg.h
+++ b/arch/arm64/include/asm/sysreg.h
@@ -510,6 +510,8 @@
#endif

/* SCTLR_EL1 specific flags. */
+#define SCTLR_EL1_BT1 (BIT(36))
+#define SCTLR_EL1_BT0 (BIT(35))
#define SCTLR_EL1_UCI (BIT(26))
#define SCTLR_EL1_E0E (BIT(24))
#define SCTLR_EL1_SPAN (BIT(23))
@@ -599,10 +601,12 @@

/* id_aa64pfr1 */
#define ID_AA64PFR1_SSBS_SHIFT 4
+#define ID_AA64PFR1_BT_SHIFT 0

#define ID_AA64PFR1_SSBS_PSTATE_NI 0
#define ID_AA64PFR1_SSBS_PSTATE_ONLY 1
#define ID_AA64PFR1_SSBS_PSTATE_INSNS 2
+#define ID_AA64PFR1_BT_BTI 0x1

/* id_aa64zfr0 */
#define ID_AA64ZFR0_SM4_SHIFT 40
diff --git a/arch/arm64/include/uapi/asm/hwcap.h b/arch/arm64/include/uapi/asm/hwcap.h
index a1e72886b30c..363f569cf599 100644
--- a/arch/arm64/include/uapi/asm/hwcap.h
+++ b/arch/arm64/include/uapi/asm/hwcap.h
@@ -65,5 +65,6 @@
#define HWCAP2_SVESM4 (1 << 6)
#define HWCAP2_FLAGM2 (1 << 7)
#define HWCAP2_FRINT (1 << 8)
+#define HWCAP2_BTI (1 << 9)

#endif /* _UAPI__ASM_HWCAP_H */
diff --git a/arch/arm64/include/uapi/asm/mman.h b/arch/arm64/include/uapi/asm/mman.h
new file mode 100644
index 000000000000..6fdd71eb644f
--- /dev/null
+++ b/arch/arm64/include/uapi/asm/mman.h
@@ -0,0 +1,9 @@
+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+#ifndef _UAPI__ASM_MMAN_H
+#define _UAPI__ASM_MMAN_H
+
+#include <asm-generic/mman.h>
+
+#define PROT_BTI 0x10 /* BTI guarded page */
+
+#endif /* ! _UAPI__ASM_MMAN_H */
diff --git a/arch/arm64/include/uapi/asm/ptrace.h b/arch/arm64/include/uapi/asm/ptrace.h
index 7ed9294e2004..09e66fa5f13a 100644
--- a/arch/arm64/include/uapi/asm/ptrace.h
+++ b/arch/arm64/include/uapi/asm/ptrace.h
@@ -46,6 +46,7 @@
#define PSR_I_BIT 0x00000080
#define PSR_A_BIT 0x00000100
#define PSR_D_BIT 0x00000200
+#define PSR_BTYPE_MASK 0x00000c00
#define PSR_SSBS_BIT 0x00001000
#define PSR_PAN_BIT 0x00400000
#define PSR_UAO_BIT 0x00800000
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 04cf64e9f0c9..bb95963ea90c 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -172,6 +172,8 @@ static const struct arm64_ftr_bits ftr_id_aa64pfr0[] = {

static const struct arm64_ftr_bits ftr_id_aa64pfr1[] = {
ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR1_SSBS_SHIFT, 4, ID_AA64PFR1_SSBS_PSTATE_NI),
+ ARM64_FTR_BITS(FTR_VISIBLE_IF_IS_ENABLED(CONFIG_ARM64_BTI),
+ FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR1_BT_SHIFT, 4, 0),
ARM64_FTR_END,
};

@@ -1267,6 +1269,21 @@ static bool can_use_gic_priorities(const struct arm64_cpu_capabilities *entry,
}
#endif

+#ifdef CONFIG_ARM64_BTI
+static void bti_enable(const struct arm64_cpu_capabilities *__unused)
+{
+ /*
+ * Use of X16/X17 for tail-calls and trampolines that jump to
+ * function entry points using BR is a requirement for
+ * marking binaries with GNU_PROPERTY_AARCH64_FEATURE_1_BTI.
+ * So, be strict and forbid other BRs using other registers to
+ * jump onto a PACIxSP instruction:
+ */
+ sysreg_clear_set(sctlr_el1, 0, SCTLR_EL1_BT0 | SCTLR_EL1_BT1);
+ isb();
+}
+#endif /* CONFIG_ARM64_BTI */
+
static const struct arm64_cpu_capabilities arm64_features[] = {
{
.desc = "GIC system register CPU interface",
@@ -1566,6 +1583,19 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
.sign = FTR_UNSIGNED,
.min_field_value = 1,
},
+#endif
+#ifdef CONFIG_ARM64_BTI
+ {
+ .desc = "Branch Target Identification",
+ .capability = ARM64_BTI,
+ .type = ARM64_CPUCAP_SYSTEM_FEATURE,
+ .matches = has_cpuid_feature,
+ .cpu_enable = bti_enable,
+ .sys_reg = SYS_ID_AA64PFR1_EL1,
+ .field_pos = ID_AA64PFR1_BT_SHIFT,
+ .min_field_value = ID_AA64PFR1_BT_BTI,
+ .sign = FTR_UNSIGNED,
+ },
#endif
{},
};
@@ -1662,6 +1692,9 @@ static const struct arm64_cpu_capabilities arm64_elf_hwcaps[] = {
HWCAP_CAP(SYS_ID_AA64ZFR0_EL1, ID_AA64ZFR0_SM4_SHIFT, FTR_UNSIGNED, ID_AA64ZFR0_SM4, CAP_HWCAP, KERNEL_HWCAP_SVESM4),
#endif
HWCAP_CAP(SYS_ID_AA64PFR1_EL1, ID_AA64PFR1_SSBS_SHIFT, FTR_UNSIGNED, ID_AA64PFR1_SSBS_PSTATE_INSNS, CAP_HWCAP, KERNEL_HWCAP_SSBS),
+#ifdef CONFIG_ARM64_BTI
+ HWCAP_CAP(SYS_ID_AA64PFR1_EL1, ID_AA64PFR1_BT_SHIFT, FTR_UNSIGNED, ID_AA64PFR1_BT_BTI, CAP_HWCAP, KERNEL_HWCAP_BTI),
+#endif
#ifdef CONFIG_ARM64_PTR_AUTH
HWCAP_MULTI_CAP(ptr_auth_hwcap_addr_matches, CAP_HWCAP, KERNEL_HWCAP_PACA),
HWCAP_MULTI_CAP(ptr_auth_hwcap_gen_matches, CAP_HWCAP, KERNEL_HWCAP_PACG),
diff --git a/arch/arm64/kernel/cpuinfo.c b/arch/arm64/kernel/cpuinfo.c
index 56bba746da1c..2c67ce0a7eb6 100644
--- a/arch/arm64/kernel/cpuinfo.c
+++ b/arch/arm64/kernel/cpuinfo.c
@@ -84,6 +84,7 @@ static const char *const hwcap_str[] = {
"svesm4",
"flagm2",
"frint",
+ "bti",
NULL
};

diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
index 5dce5e56995a..d64358a9794f 100644
--- a/arch/arm64/kernel/entry-common.c
+++ b/arch/arm64/kernel/entry-common.c
@@ -188,6 +188,14 @@ static void notrace el0_undef(struct pt_regs *regs)
}
NOKPROBE_SYMBOL(el0_undef);

+static void notrace el0_bti(struct pt_regs *regs)
+{
+ user_exit_irqoff();
+ local_daif_restore(DAIF_PROCCTX);
+ do_bti(regs);
+}
+NOKPROBE_SYMBOL(el0_bti);
+
static void notrace el0_inv(struct pt_regs *regs, unsigned long esr)
{
user_exit_irqoff();
@@ -255,6 +263,9 @@ asmlinkage void notrace el0_sync_handler(struct pt_regs *regs)
case ESR_ELx_EC_UNKNOWN:
el0_undef(regs);
break;
+ case ESR_ELx_EC_BTI:
+ el0_bti(regs);
+ break;
case ESR_ELx_EC_BREAKPT_LOW:
case ESR_ELx_EC_SOFTSTP_LOW:
case ESR_ELx_EC_WATCHPT_LOW:
diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index 6771c399d40c..a7d00e335fc6 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -1853,7 +1853,7 @@ void syscall_trace_exit(struct pt_regs *regs)
*/
#define SPSR_EL1_AARCH64_RES0_BITS \
(GENMASK_ULL(63, 32) | GENMASK_ULL(27, 25) | GENMASK_ULL(23, 22) | \
- GENMASK_ULL(20, 13) | GENMASK_ULL(11, 10) | GENMASK_ULL(5, 5))
+ GENMASK_ULL(20, 13) | GENMASK_ULL(5, 5))
#define SPSR_EL1_AARCH32_RES0_BITS \
(GENMASK_ULL(63, 32) | GENMASK_ULL(22, 22) | GENMASK_ULL(20, 20))

diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
index dd2cdc0d5be2..b089eff158e5 100644
--- a/arch/arm64/kernel/signal.c
+++ b/arch/arm64/kernel/signal.c
@@ -730,6 +730,22 @@ static void setup_return(struct pt_regs *regs, struct k_sigaction *ka,
regs->regs[29] = (unsigned long)&user->next_frame->fp;
regs->pc = (unsigned long)ka->sa.sa_handler;

+ /*
+ * Signal delivery is a (wacky) indirect function call in
+ * userspace, so simulate the same setting of BTYPE as a BLR
+ * <register containing the signal handler entry point>.
+ * Signal delivery to a location in a PROT_BTI guarded page
+ * that is not a function entry point will now trigger a
+ * SIGILL in userspace.
+ *
+ * If the signal handler entry point is not in a PROT_BTI
+ * guarded page, this is harmless.
+ */
+ if (system_supports_bti()) {
+ regs->pstate &= ~PSR_BTYPE_MASK;
+ regs->pstate |= PSR_BTYPE_C;
+ }
+
if (ka->sa.sa_flags & SA_RESTORER)
sigtramp = ka->sa.sa_restorer;
else
diff --git a/arch/arm64/kernel/syscall.c b/arch/arm64/kernel/syscall.c
index 9a9d98a443fc..ef80ecbd6eaf 100644
--- a/arch/arm64/kernel/syscall.c
+++ b/arch/arm64/kernel/syscall.c
@@ -98,6 +98,24 @@ static void el0_svc_common(struct pt_regs *regs, int scno, int sc_nr,
regs->orig_x0 = regs->regs[0];
regs->syscallno = scno;

+ /*
+ * BTI note:
+ * The architecture does not guarantee that SPSR.BTYPE is zero
+ * on taking an SVC, so we could return to userspace with a
+ * non-zero BTYPE after the syscall.
+ *
+ * This shouldn't matter except when userspace is explicitly
+ * doing something stupid, such as setting PROT_BTI on a page
+ * that lacks conforming BTI/PACIxSP instructions, falling
+ * through from one executable page to another with differing
+ * PROT_BTI, or messing with BYTPE via ptrace: in such cases,
+ * userspace should not be surprised if a SIGILL occurs on
+ * syscall return.
+ *
+ * So, don't touch regs->pstate & PSR_BTYPE_MASK here.
+ * (Similarly for HVC and SMC elsewhere.)
+ */
+
cortex_a76_erratum_1463225_svc_handler();
local_daif_restore(DAIF_PROCCTX);
user_exit();
diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
index 73caf35c2262..84c7a88dd617 100644
--- a/arch/arm64/kernel/traps.c
+++ b/arch/arm64/kernel/traps.c
@@ -408,6 +408,13 @@ void do_undefinstr(struct pt_regs *regs)
}
NOKPROBE_SYMBOL(do_undefinstr);

+void do_bti(struct pt_regs *regs)
+{
+ BUG_ON(!user_mode(regs));
+ force_signal_inject(SIGILL, ILL_ILLOPC, regs->pc);
+}
+NOKPROBE_SYMBOL(do_bti);
+
#define __user_cache_maint(insn, address, res) \
if (address >= user_addr_max()) { \
res = -EFAULT; \
@@ -750,6 +757,7 @@ static const char *esr_class_str[] = {
[ESR_ELx_EC_CP10_ID] = "CP10 MRC/VMRS",
[ESR_ELx_EC_PAC] = "PAC",
[ESR_ELx_EC_CP14_64] = "CP14 MCRR/MRRC",
+ [ESR_ELx_EC_BTI] = "BTI",
[ESR_ELx_EC_ILL] = "PSTATE.IL",
[ESR_ELx_EC_SVC32] = "SVC (AArch32)",
[ESR_ELx_EC_HVC32] = "HVC (AArch32)",
diff --git a/include/linux/mm.h b/include/linux/mm.h
index c97ea3b694e6..0a2db242fd56 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -329,6 +329,9 @@ extern unsigned int kobjsize(const void *objp);
#elif defined(CONFIG_SPARC64)
# define VM_SPARC_ADI VM_ARCH_1 /* Uses ADI tag for access control */
# define VM_ARCH_CLEAR VM_SPARC_ADI
+#elif defined(CONFIG_ARM64)
+# define VM_ARM64_BTI VM_ARCH_1 /* BTI guarded page, a.k.a. GP bit */
+# define VM_ARCH_CLEAR VM_ARM64_BTI
#elif !defined(CONFIG_MMU)
# define VM_MAPPED_COPY VM_ARCH_1 /* T if mapped copy of data (nommu mmap) */
#endif
--
2.20.1
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