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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>
Signed-off-by: Will Deacon <will@kernel.org>
This commit is contained in:
Sami Tolvanen 2020-04-27 09:00:16 -07:00 committed by Will Deacon
parent 9654736891
commit 5287569a79
9 changed files with 114 additions and 2 deletions
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5
arch/arm64/Kconfig
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@ -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---

46
arch/arm64/include/asm/scs.h Normal file
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@ -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 */

13
arch/arm64/include/asm/thread_info.h
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@ -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 */

1
arch/arm64/kernel/Makefile
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@ -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/

4
arch/arm64/kernel/asm-offsets.c
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@ -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

24
arch/arm64/kernel/entry.S
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@ -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)

6
arch/arm64/kernel/head.S
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@ -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

2
arch/arm64/kernel/process.c
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@ -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

15
arch/arm64/kernel/scs.c Normal file
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@ -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);