[PATCH v2 4/6] riscv: Implement Shadow Call Stack

Sami Tolvanen
2023-08-15T20:35:06Z

Implement CONFIG_SHADOW_CALL_STACK for RISC-V. When enabled, the
compiler injects instructions to all non-leaf C functions to
store the return address to the shadow stack and unconditionally
load it again before returning, which makes it harder to corrupt
the return address through a stack overflow, for example.

The active shadow call stack pointer is stored in the gp
register, which makes SCS incompatible with gp relaxation. Use
--no-relax-gp to ensure gp relaxation is disabled and disable
global pointer loading. Add SCS pointers to struct thread_info,
implement SCS initialization, and task switching

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
---
arch/riscv/Kconfig | 6 ++++
arch/riscv/Makefile | 4 +++
arch/riscv/include/asm/asm.h | 6 ++++
arch/riscv/include/asm/scs.h | 47 ++++++++++++++++++++++++++++
arch/riscv/include/asm/thread_info.h | 13 ++++++++
arch/riscv/kernel/asm-offsets.c | 3 ++
arch/riscv/kernel/entry.S | 11 +++++++
arch/riscv/kernel/head.S | 4 +++
arch/riscv/kernel/vdso/Makefile | 2 +-
arch/riscv/purgatory/Makefile | 4 +++
10 files changed, 99 insertions(+), 1 deletion(-)
create mode 100644 arch/riscv/include/asm/scs.h

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 4c07b9189c86..8fe31ec59da4 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -46,6 +46,7 @@ config RISCV
select ARCH_SUPPORTS_HUGETLBFS if MMU
select ARCH_SUPPORTS_PAGE_TABLE_CHECK if MMU
select ARCH_SUPPORTS_PER_VMA_LOCK if MMU
+ select ARCH_SUPPORTS_SHADOW_CALL_STACK if HAVE_SHADOW_CALL_STACK
select ARCH_USE_MEMTEST
select ARCH_USE_QUEUED_RWLOCKS
select ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT if MMU
@@ -169,6 +170,11 @@ config GCC_SUPPORTS_DYNAMIC_FTRACE
def_bool CC_IS_GCC
depends on $(cc-option,-fpatchable-function-entry=8)

+config HAVE_SHADOW_CALL_STACK
+ def_bool $(cc-option,-fsanitize=shadow-call-stack)
+ # https://github.com/riscv-non-isa/riscv-elf-psabi-doc/commit/a484e843e6eeb51f0cb7b8819e50da6d2444d769
+ depends on $(ld-option,--no-relax-gp)
+
config ARCH_MMAP_RND_BITS_MIN
default 18 if 64BIT
default 8
diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
index 6ec6d52a4180..e518a74640fb 100644
--- a/arch/riscv/Makefile
+++ b/arch/riscv/Makefile
@@ -55,6 +55,10 @@ endif
endif
endif

+ifeq ($(CONFIG_SHADOW_CALL_STACK),y)
+ KBUILD_LDFLAGS += --no-relax-gp
+endif
+
# ISA string setting
riscv-march-$(CONFIG_ARCH_RV32I) := rv32ima
riscv-march-$(CONFIG_ARCH_RV64I) := rv64ima
diff --git a/arch/riscv/include/asm/asm.h b/arch/riscv/include/asm/asm.h
index f34dd1a526a1..b0487b39e674 100644
--- a/arch/riscv/include/asm/asm.h
+++ b/arch/riscv/include/asm/asm.h
@@ -109,6 +109,11 @@
REG_L \dst, 0(\dst)
.endm

+#ifdef CONFIG_SHADOW_CALL_STACK
+/* gp is used as the shadow call stack pointer instead */
+.macro load_global_pointer
+.endm
+#else
/* load __global_pointer to gp */
.macro load_global_pointer
.option push
@@ -116,6 +121,7 @@
la gp, __global_pointer$
.option pop
.endm
+#endif /* CONFIG_SHADOW_CALL_STACK */

/* save all GPs except x1 ~ x5 */
.macro save_from_x6_to_x31
diff --git a/arch/riscv/include/asm/scs.h b/arch/riscv/include/asm/scs.h
new file mode 100644
index 000000000000..94726ea773e3
--- /dev/null
+++ b/arch/riscv/include/asm/scs.h
@@ -0,0 +1,47 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_SCS_H
+#define _ASM_SCS_H
+
+#ifdef __ASSEMBLY__
+#include <asm/asm-offsets.h>
+
+#ifdef CONFIG_SHADOW_CALL_STACK
+
+/* Load init_shadow_call_stack to gp. */
+.macro scs_load_init_stack
+ la gp, init_shadow_call_stack
+ XIP_FIXUP_OFFSET gp
+.endm
+
+/* Load task_scs_sp(current) to gp. */
+.macro scs_load_current
+ REG_L gp, TASK_TI_SCS_SP(tp)
+.endm
+
+/* Load task_scs_sp(current) to gp, but only if tp has changed. */
+.macro scs_load_current_if_task_changed prev
+ beq \prev, tp, _skip_scs
+ scs_load_current
+_skip_scs:
+.endm
+
+/* Save gp to task_scs_sp(current). */
+.macro scs_save_current
+ REG_S gp, TASK_TI_SCS_SP(tp)
+.endm
+
+#else /* CONFIG_SHADOW_CALL_STACK */
+
+.macro scs_load_init_stack
+.endm
+.macro scs_load_current
+.endm
+.macro scs_load_current_if_task_changed prev
+.endm
+.macro scs_save_current
+.endm
+
+#endif /* CONFIG_SHADOW_CALL_STACK */
+#endif /* __ASSEMBLY__ */
+
+#endif /* _ASM_SCS_H */
diff --git a/arch/riscv/include/asm/thread_info.h b/arch/riscv/include/asm/thread_info.h
index d18ce0113ca1..574779900bfb 100644
--- a/arch/riscv/include/asm/thread_info.h
+++ b/arch/riscv/include/asm/thread_info.h
@@ -57,8 +57,20 @@ struct thread_info {
long user_sp; /* User stack pointer */
int cpu;
unsigned long syscall_work; /* SYSCALL_WORK_ flags */
+#ifdef CONFIG_SHADOW_CALL_STACK
+ void *scs_base;
+ void *scs_sp;
+#endif
};

+#ifdef CONFIG_SHADOW_CALL_STACK
+#define INIT_SCS \
+ .scs_base = init_shadow_call_stack, \
+ .scs_sp = init_shadow_call_stack,
+#else
+#define INIT_SCS
+#endif
+
/*
* macros/functions for gaining access to the thread information structure
*
@@ -68,6 +80,7 @@ struct thread_info {
{ \
.flags = 0, \
.preempt_count = INIT_PREEMPT_COUNT, \
+ INIT_SCS \
}

void arch_release_task_struct(struct task_struct *tsk);
diff --git a/arch/riscv/kernel/asm-offsets.c b/arch/riscv/kernel/asm-offsets.c
index 9f535d5de33f..177cef43a2ee 100644
--- a/arch/riscv/kernel/asm-offsets.c
+++ b/arch/riscv/kernel/asm-offsets.c
@@ -38,6 +38,9 @@ void asm_offsets(void)
OFFSET(TASK_TI_PREEMPT_COUNT, task_struct, thread_info.preempt_count);
OFFSET(TASK_TI_KERNEL_SP, task_struct, thread_info.kernel_sp);
OFFSET(TASK_TI_USER_SP, task_struct, thread_info.user_sp);
+#ifdef CONFIG_SHADOW_CALL_STACK
+ OFFSET(TASK_TI_SCS_SP, task_struct, thread_info.scs_sp);
+#endif

OFFSET(TASK_TI_CPU_NUM, task_struct, thread_info.cpu);
OFFSET(TASK_THREAD_F0, task_struct, thread.fstate.f[0]);
diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
index 2b4248c6b0a9..ad34507d3c96 100644
--- a/arch/riscv/kernel/entry.S
+++ b/arch/riscv/kernel/entry.S
@@ -9,6 +9,7 @@

#include <asm/asm.h>
#include <asm/csr.h>
+#include <asm/scs.h>
#include <asm/unistd.h>
#include <asm/page.h>
#include <asm/thread_info.h>
@@ -77,6 +78,9 @@ _save_context:
/* Load the global pointer */
load_global_pointer

+ /* Load the kernel shadow call stack pointer if coming from userspace */
+ scs_load_current_if_task_changed s5
+
move a0, sp /* pt_regs */
la ra, ret_from_exception

@@ -123,6 +127,9 @@ SYM_CODE_START_NOALIGN(ret_from_exception)
addi s0, sp, PT_SIZE_ON_STACK
REG_S s0, TASK_TI_KERNEL_SP(tp)

+ /* Save the kernel shadow call stack pointer */
+ scs_save_current
+
/*
* Save TP into the scratch register , so we can find the kernel data
* structures again.
@@ -277,6 +284,8 @@ SYM_FUNC_START(__switch_to)
REG_S s9, TASK_THREAD_S9_RA(a3)
REG_S s10, TASK_THREAD_S10_RA(a3)
REG_S s11, TASK_THREAD_S11_RA(a3)
+ /* Save the kernel shadow call stack pointer */
+ scs_save_current
/* Restore context from next->thread */
REG_L ra, TASK_THREAD_RA_RA(a4)
REG_L sp, TASK_THREAD_SP_RA(a4)
@@ -294,6 +303,8 @@ SYM_FUNC_START(__switch_to)
REG_L s11, TASK_THREAD_S11_RA(a4)
/* The offset of thread_info in task_struct is zero. */
move tp, a1
+ /* Switch to the next shadow call stack */
+ scs_load_current
ret
SYM_FUNC_END(__switch_to)

diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
index 79b5a863c782..c3d0ee77483b 100644
--- a/arch/riscv/kernel/head.S
+++ b/arch/riscv/kernel/head.S
@@ -14,6 +14,7 @@
#include <asm/cpu_ops_sbi.h>
#include <asm/hwcap.h>
#include <asm/image.h>
+#include <asm/scs.h>
#include <asm/xip_fixup.h>
#include "efi-header.S"

@@ -153,6 +154,7 @@ secondary_start_sbi:
XIP_FIXUP_OFFSET a3
add a3, a3, a1
REG_L sp, (a3)
+ scs_load_current

.Lsecondary_start_common:

@@ -293,6 +295,7 @@ clear_bss_done:
la sp, init_thread_union + THREAD_SIZE
XIP_FIXUP_OFFSET sp
addi sp, sp, -PT_SIZE_ON_STACK
+ scs_load_init_stack
#ifdef CONFIG_BUILTIN_DTB
la a0, __dtb_start
XIP_FIXUP_OFFSET a0
@@ -311,6 +314,7 @@ clear_bss_done:
la tp, init_task
la sp, init_thread_union + THREAD_SIZE
addi sp, sp, -PT_SIZE_ON_STACK
+ scs_load_init_stack

#ifdef CONFIG_KASAN
call kasan_early_init
diff --git a/arch/riscv/kernel/vdso/Makefile b/arch/riscv/kernel/vdso/Makefile
index 6b1dba11bf6d..48c362c0cb3d 100644
--- a/arch/riscv/kernel/vdso/Makefile
+++ b/arch/riscv/kernel/vdso/Makefile
@@ -36,7 +36,7 @@ CPPFLAGS_vdso.lds += -DHAS_VGETTIMEOFDAY
endif

# Disable -pg to prevent insert call site
-CFLAGS_REMOVE_vgettimeofday.o = $(CC_FLAGS_FTRACE)
+CFLAGS_REMOVE_vgettimeofday.o = $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS)

# Disable profiling and instrumentation for VDSO code
GCOV_PROFILE := n
diff --git a/arch/riscv/purgatory/Makefile b/arch/riscv/purgatory/Makefile
index dc20e166983e..d5d60c040560 100644
--- a/arch/riscv/purgatory/Makefile
+++ b/arch/riscv/purgatory/Makefile
@@ -77,6 +77,10 @@ ifdef CONFIG_STACKPROTECTOR_STRONG
PURGATORY_CFLAGS_REMOVE += -fstack-protector-strong
endif

+ifdef CONFIG_SHADOW_CALL_STACK
+PURGATORY_CFLAGS_REMOVE += $(CC_FLAGS_SCS)
+endif
+
CFLAGS_REMOVE_purgatory.o += $(PURGATORY_CFLAGS_REMOVE)
CFLAGS_purgatory.o += $(PURGATORY_CFLAGS)