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    Subject[PATCH v13 10/12] arm64: implement Shadow Call Stack
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    This change implements shadow stack switching, initial SCS set-up,
    and interrupt shadow stacks for arm64.

    Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
    Reviewed-by: Kees Cook <keescook@chromium.org>
    ---
    arch/arm64/Kconfig | 5 +++
    arch/arm64/include/asm/scs.h | 46 ++++++++++++++++++++++++++++
    arch/arm64/include/asm/thread_info.h | 13 ++++++++
    arch/arm64/kernel/Makefile | 1 +
    arch/arm64/kernel/asm-offsets.c | 4 +++
    arch/arm64/kernel/entry.S | 24 +++++++++++++--
    arch/arm64/kernel/head.S | 6 ++++
    arch/arm64/kernel/process.c | 2 ++
    arch/arm64/kernel/scs.c | 15 +++++++++
    9 files changed, 114 insertions(+), 2 deletions(-)
    create mode 100644 arch/arm64/include/asm/scs.h
    create mode 100644 arch/arm64/kernel/scs.c

    diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
    index 40fb05d96c60..c380a16533f6 100644
    --- a/arch/arm64/Kconfig
    +++ b/arch/arm64/Kconfig
    @@ -64,6 +64,7 @@ config ARM64
    select ARCH_USE_QUEUED_RWLOCKS
    select ARCH_USE_QUEUED_SPINLOCKS
    select ARCH_SUPPORTS_MEMORY_FAILURE
    + select ARCH_SUPPORTS_SHADOW_CALL_STACK if CC_HAVE_SHADOW_CALL_STACK
    select ARCH_SUPPORTS_ATOMIC_RMW
    select ARCH_SUPPORTS_INT128 if CC_HAS_INT128 && (GCC_VERSION >= 50000 || CC_IS_CLANG)
    select ARCH_SUPPORTS_NUMA_BALANCING
    @@ -1025,6 +1026,10 @@ config ARCH_HAS_CACHE_LINE_SIZE
    config ARCH_ENABLE_SPLIT_PMD_PTLOCK
    def_bool y if PGTABLE_LEVELS > 2

    +# Supported by clang >= 7.0
    +config CC_HAVE_SHADOW_CALL_STACK
    + def_bool $(cc-option, -fsanitize=shadow-call-stack -ffixed-x18)
    +
    config SECCOMP
    bool "Enable seccomp to safely compute untrusted bytecode"
    ---help---
    diff --git a/arch/arm64/include/asm/scs.h b/arch/arm64/include/asm/scs.h
    new file mode 100644
    index 000000000000..96549353b0cb
    --- /dev/null
    +++ b/arch/arm64/include/asm/scs.h
    @@ -0,0 +1,46 @@
    +/* SPDX-License-Identifier: GPL-2.0 */
    +#ifndef _ASM_SCS_H
    +#define _ASM_SCS_H
    +
    +#ifdef __ASSEMBLY__
    +
    +#ifdef CONFIG_SHADOW_CALL_STACK
    + .macro scs_load tsk, tmp
    + ldp x18, \tmp, [\tsk, #TSK_TI_SCS_BASE]
    + add x18, x18, \tmp
    + .endm
    +
    + .macro scs_save tsk, tmp
    + ldr \tmp, [\tsk, #TSK_TI_SCS_BASE]
    + sub \tmp, x18, \tmp
    + str \tmp, [\tsk, #TSK_TI_SCS_OFFSET]
    + .endm
    +#else
    + .macro scs_load tsk, tmp
    + .endm
    +
    + .macro scs_save tsk, tmp
    + .endm
    +#endif /* CONFIG_SHADOW_CALL_STACK */
    +
    +#else /* __ASSEMBLY__ */
    +
    +#include <linux/scs.h>
    +
    +#ifdef CONFIG_SHADOW_CALL_STACK
    +
    +static inline void scs_overflow_check(struct task_struct *tsk)
    +{
    + if (unlikely(scs_corrupted(tsk)))
    + panic("corrupted shadow stack detected inside scheduler\n");
    +}
    +
    +#else /* CONFIG_SHADOW_CALL_STACK */
    +
    +static inline void scs_overflow_check(struct task_struct *tsk) {}
    +
    +#endif /* CONFIG_SHADOW_CALL_STACK */
    +
    +#endif /* __ASSEMBLY __ */
    +
    +#endif /* _ASM_SCS_H */
    diff --git a/arch/arm64/include/asm/thread_info.h b/arch/arm64/include/asm/thread_info.h
    index 512174a8e789..9df79c0a4c43 100644
    --- a/arch/arm64/include/asm/thread_info.h
    +++ b/arch/arm64/include/asm/thread_info.h
    @@ -41,6 +41,10 @@ struct thread_info {
    #endif
    } preempt;
    };
    +#ifdef CONFIG_SHADOW_CALL_STACK
    + void *scs_base;
    + unsigned long scs_offset;
    +#endif
    };

    #define thread_saved_pc(tsk) \
    @@ -100,11 +104,20 @@ void arch_release_task_struct(struct task_struct *tsk);
    _TIF_SYSCALL_TRACEPOINT | _TIF_SECCOMP | \
    _TIF_SYSCALL_EMU)

    +#ifdef CONFIG_SHADOW_CALL_STACK
    +#define INIT_SCS \
    + .scs_base = init_shadow_call_stack, \
    + .scs_offset = 0,
    +#else
    +#define INIT_SCS
    +#endif
    +
    #define INIT_THREAD_INFO(tsk) \
    { \
    .flags = _TIF_FOREIGN_FPSTATE, \
    .preempt_count = INIT_PREEMPT_COUNT, \
    .addr_limit = KERNEL_DS, \
    + INIT_SCS \
    }

    #endif /* __ASM_THREAD_INFO_H */
    diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
    index 4e5b8ee31442..151f28521f1e 100644
    --- a/arch/arm64/kernel/Makefile
    +++ b/arch/arm64/kernel/Makefile
    @@ -63,6 +63,7 @@ obj-$(CONFIG_CRASH_CORE) += crash_core.o
    obj-$(CONFIG_ARM_SDE_INTERFACE) += sdei.o
    obj-$(CONFIG_ARM64_SSBD) += ssbd.o
    obj-$(CONFIG_ARM64_PTR_AUTH) += pointer_auth.o
    +obj-$(CONFIG_SHADOW_CALL_STACK) += scs.o

    obj-y += vdso/ probes/
    obj-$(CONFIG_COMPAT_VDSO) += vdso32/
    diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c
    index 9981a0a5a87f..d7934250b68c 100644
    --- a/arch/arm64/kernel/asm-offsets.c
    +++ b/arch/arm64/kernel/asm-offsets.c
    @@ -33,6 +33,10 @@ int main(void)
    DEFINE(TSK_TI_ADDR_LIMIT, offsetof(struct task_struct, thread_info.addr_limit));
    #ifdef CONFIG_ARM64_SW_TTBR0_PAN
    DEFINE(TSK_TI_TTBR0, offsetof(struct task_struct, thread_info.ttbr0));
    +#endif
    +#ifdef CONFIG_SHADOW_CALL_STACK
    + DEFINE(TSK_TI_SCS_BASE, offsetof(struct task_struct, thread_info.scs_base));
    + DEFINE(TSK_TI_SCS_OFFSET, offsetof(struct task_struct, thread_info.scs_offset));
    #endif
    DEFINE(TSK_STACK, offsetof(struct task_struct, stack));
    #ifdef CONFIG_STACKPROTECTOR
    diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
    index ddcde093c433..244268d5ae47 100644
    --- a/arch/arm64/kernel/entry.S
    +++ b/arch/arm64/kernel/entry.S
    @@ -23,6 +23,7 @@
    #include <asm/mmu.h>
    #include <asm/processor.h>
    #include <asm/ptrace.h>
    +#include <asm/scs.h>
    #include <asm/thread_info.h>
    #include <asm/asm-uaccess.h>
    #include <asm/unistd.h>
    @@ -179,6 +180,8 @@ alternative_cb_end
    apply_ssbd 1, x22, x23

    ptrauth_keys_install_kernel tsk, 1, x20, x22, x23
    +
    + scs_load tsk, x20
    .else
    add x21, sp, #S_FRAME_SIZE
    get_current_task tsk
    @@ -343,6 +346,8 @@ alternative_else_nop_endif
    msr cntkctl_el1, x1
    4:
    #endif
    + scs_save tsk, x0
    +
    /* No kernel C function calls after this as user keys are set. */
    ptrauth_keys_install_user tsk, x0, x1, x2

    @@ -388,6 +393,9 @@ alternative_insn eret, nop, ARM64_UNMAP_KERNEL_AT_EL0

    .macro irq_stack_entry
    mov x19, sp // preserve the original sp
    +#ifdef CONFIG_SHADOW_CALL_STACK
    + mov x24, x18 // preserve the original shadow stack
    +#endif

    /*
    * Compare sp with the base of the task stack.
    @@ -405,15 +413,25 @@ alternative_insn eret, nop, ARM64_UNMAP_KERNEL_AT_EL0

    /* switch to the irq stack */
    mov sp, x26
    +
    +#ifdef CONFIG_SHADOW_CALL_STACK
    + /* also switch to the irq shadow stack */
    + adr_this_cpu x18, irq_shadow_call_stack, x26
    +#endif
    +
    9998:
    .endm

    /*
    - * x19 should be preserved between irq_stack_entry and
    - * irq_stack_exit.
    + * The callee-saved regs (x19-x29) should be preserved between
    + * irq_stack_entry and irq_stack_exit, but note that kernel_entry
    + * uses x20-x23 to store data for later use.
    */
    .macro irq_stack_exit
    mov sp, x19
    +#ifdef CONFIG_SHADOW_CALL_STACK
    + mov x18, x24
    +#endif
    .endm

    /* GPRs used by entry code */
    @@ -901,6 +919,8 @@ SYM_FUNC_START(cpu_switch_to)
    mov sp, x9
    msr sp_el0, x1
    ptrauth_keys_install_kernel x1, 1, x8, x9, x10
    + scs_save x0, x8
    + scs_load x1, x8
    ret
    SYM_FUNC_END(cpu_switch_to)
    NOKPROBE(cpu_switch_to)
    diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
    index 57a91032b4c2..2b01c19c5483 100644
    --- a/arch/arm64/kernel/head.S
    +++ b/arch/arm64/kernel/head.S
    @@ -27,6 +27,7 @@
    #include <asm/pgtable-hwdef.h>
    #include <asm/pgtable.h>
    #include <asm/page.h>
    +#include <asm/scs.h>
    #include <asm/smp.h>
    #include <asm/sysreg.h>
    #include <asm/thread_info.h>
    @@ -424,6 +425,10 @@ SYM_FUNC_START_LOCAL(__primary_switched)
    stp xzr, x30, [sp, #-16]!
    mov x29, sp

    +#ifdef CONFIG_SHADOW_CALL_STACK
    + adr_l x18, init_shadow_call_stack // Set shadow call stack
    +#endif
    +
    str_l x21, __fdt_pointer, x5 // Save FDT pointer

    ldr_l x4, kimage_vaddr // Save the offset between
    @@ -737,6 +742,7 @@ SYM_FUNC_START_LOCAL(__secondary_switched)
    ldr x2, [x0, #CPU_BOOT_TASK]
    cbz x2, __secondary_too_slow
    msr sp_el0, x2
    + scs_load x2, x3
    mov x29, #0
    mov x30, #0
    b secondary_start_kernel
    diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
    index 56be4cbf771f..a35d3318492c 100644
    --- a/arch/arm64/kernel/process.c
    +++ b/arch/arm64/kernel/process.c
    @@ -52,6 +52,7 @@
    #include <asm/mmu_context.h>
    #include <asm/processor.h>
    #include <asm/pointer_auth.h>
    +#include <asm/scs.h>
    #include <asm/stacktrace.h>

    #if defined(CONFIG_STACKPROTECTOR) && !defined(CONFIG_STACKPROTECTOR_PER_TASK)
    @@ -515,6 +516,7 @@ __notrace_funcgraph struct task_struct *__switch_to(struct task_struct *prev,
    entry_task_switch(next);
    uao_thread_switch(next);
    ssbs_thread_switch(next);
    + scs_overflow_check(next);

    /*
    * Complete any pending TLB or cache maintenance on this CPU in case
    diff --git a/arch/arm64/kernel/scs.c b/arch/arm64/kernel/scs.c
    new file mode 100644
    index 000000000000..acc6741d1a40
    --- /dev/null
    +++ b/arch/arm64/kernel/scs.c
    @@ -0,0 +1,15 @@
    +// SPDX-License-Identifier: GPL-2.0
    +/*
    + * Shadow Call Stack support.
    + *
    + * Copyright (C) 2019 Google LLC
    + */
    +
    +#include <linux/percpu.h>
    +#include <asm/scs.h>
    +
    +/* Allocate a static per-CPU shadow stack */
    +#define DEFINE_SCS(name) \
    + DEFINE_PER_CPU(unsigned long [SCS_SIZE/sizeof(long)], name) \
    +
    +DEFINE_SCS(irq_shadow_call_stack);
    --
    2.26.2.303.gf8c07b1a785-goog
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