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Subject[PATCH v12 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 | 34 ++++++++++++++++++++++++++++
arch/arm64/include/asm/thread_info.h | 3 +++
arch/arm64/kernel/Makefile | 1 +
arch/arm64/kernel/asm-offsets.c | 3 +++
arch/arm64/kernel/entry.S | 33 +++++++++++++++++++++++++--
arch/arm64/kernel/head.S | 8 +++++++
arch/arm64/kernel/process.c | 2 ++
arch/arm64/kernel/scs.c | 16 +++++++++++++
arch/arm64/kernel/smp.c | 4 ++++
10 files changed, 107 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..27dced9cbaf7
--- /dev/null
+++ b/arch/arm64/include/asm/scs.h
@@ -0,0 +1,34 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_SCS_H
+#define _ASM_SCS_H
+
+#ifndef __ASSEMBLY__
+
+#include <linux/scs.h>
+
+#ifdef CONFIG_SHADOW_CALL_STACK
+
+static __always_inline void scs_save(struct task_struct *tsk)
+{
+ void *s;
+
+ asm volatile("mov %0, x18" : "=r" (s));
+ task_set_scs(tsk, s);
+}
+
+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_save(struct task_struct *tsk) {}
+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..1fb651f73da3 100644
--- a/arch/arm64/include/asm/thread_info.h
+++ b/arch/arm64/include/asm/thread_info.h
@@ -41,6 +41,9 @@ struct thread_info {
#endif
} preempt;
};
+#ifdef CONFIG_SHADOW_CALL_STACK
+ void *shadow_call_stack;
+#endif
};

#define thread_saved_pc(tsk) \
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..777a662888ec 100644
--- a/arch/arm64/kernel/asm-offsets.c
+++ b/arch/arm64/kernel/asm-offsets.c
@@ -33,6 +33,9 @@ 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, offsetof(struct task_struct, thread_info.shadow_call_stack));
#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..14f0ff763b39 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -179,6 +179,11 @@ alternative_cb_end
apply_ssbd 1, x22, x23

ptrauth_keys_install_kernel tsk, 1, x20, x22, x23
+
+#ifdef CONFIG_SHADOW_CALL_STACK
+ ldr x18, [tsk, #TSK_TI_SCS] // Restore shadow call stack
+ str xzr, [tsk, #TSK_TI_SCS] // Limit visibility of saved SCS
+#endif
.else
add x21, sp, #S_FRAME_SIZE
get_current_task tsk
@@ -280,6 +285,12 @@ alternative_else_nop_endif
ct_user_enter
.endif

+#ifdef CONFIG_SHADOW_CALL_STACK
+ .if \el == 0
+ str x18, [tsk, #TSK_TI_SCS] // Save shadow call stack
+ .endif
+#endif
+
#ifdef CONFIG_ARM64_SW_TTBR0_PAN
/*
* Restore access to TTBR0_EL1. If returning to EL0, no need for SPSR
@@ -388,6 +399,9 @@ 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 +419,25 @@ 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 +925,11 @@ SYM_FUNC_START(cpu_switch_to)
mov sp, x9
msr sp_el0, x1
ptrauth_keys_install_kernel x1, 1, x8, x9, x10
+#ifdef CONFIG_SHADOW_CALL_STACK
+ str x18, [x0, #TSK_TI_SCS]
+ ldr x18, [x1, #TSK_TI_SCS]
+ str xzr, [x1, #TSK_TI_SCS] // limit visibility of saved SCS
+#endif
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..1514445bbccb 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -424,6 +424,10 @@ 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 +741,10 @@ SYM_FUNC_START_LOCAL(__secondary_switched)
ldr x2, [x0, #CPU_BOOT_TASK]
cbz x2, __secondary_too_slow
msr sp_el0, x2
+#ifdef CONFIG_SHADOW_CALL_STACK
+ ldr x18, [x2, #TSK_TI_SCS] // set shadow call stack
+ str xzr, [x2, #TSK_TI_SCS] // limit visibility of saved SCS
+#endif
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..086ad97bba86
--- /dev/null
+++ b/arch/arm64/kernel/scs.c
@@ -0,0 +1,16 @@
+// 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) \
+ __aligned(SCS_SIZE)
+
+DEFINE_SCS(irq_shadow_call_stack);
diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
index 061f60fe452f..1d112e34a636 100644
--- a/arch/arm64/kernel/smp.c
+++ b/arch/arm64/kernel/smp.c
@@ -46,6 +46,7 @@
#include <asm/pgtable.h>
#include <asm/pgalloc.h>
#include <asm/processor.h>
+#include <asm/scs.h>
#include <asm/smp_plat.h>
#include <asm/sections.h>
#include <asm/tlbflush.h>
@@ -370,6 +371,9 @@ void cpu_die(void)
unsigned int cpu = smp_processor_id();
const struct cpu_operations *ops = get_cpu_ops(cpu);

+ /* Save the shadow stack pointer before exiting the idle task */
+ scs_save(current);
+
idle_task_exit();

local_daif_mask();
--
2.26.1.301.g55bc3eb7cb9-goog
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