[RFC PATCH 08/12] riscv: dynamic (zicfiss) shadow call stack support

Deepak Gupta
2024-04-09T06:12:14Z

Adding support for dynamic shadow call stack on riscv. zicfiss ISA extn.
enables protection for shadow stack against stray writes. This patch
enables scs_* macros to use zicfiss shadow stack pointer (CSR_SSP) instead
of relying on `gp`.

Since zicfiss based shadow stack needs to have correct encoding set in PTE
init shadow stack can't be established too early. It has to be setup after
`setup_vm` is called. Thus `scs_load_init_stack` is noped out if
CONFIG_DYNAMIC_SCS is selected.

Adds `arch_scs_store` that can be used in generic scs magic store routine.

Signed-off-by: Deepak Gupta <debug@rivosinc.com>
---
arch/riscv/include/asm/asm.h | 2 +-
arch/riscv/include/asm/scs.h | 47 +++++++++++++++++++++++++++++-------
arch/riscv/kernel/entry.S | 14 +++++------
arch/riscv/kernel/head.S | 4 +--
4 files changed, 48 insertions(+), 19 deletions(-)

diff --git a/arch/riscv/include/asm/asm.h b/arch/riscv/include/asm/asm.h
index 776354895b81..0304978ea4e4 100644
--- a/arch/riscv/include/asm/asm.h
+++ b/arch/riscv/include/asm/asm.h
@@ -109,7 +109,7 @@
REG_L \dst, 0(\dst)
.endm

-#ifdef CONFIG_SHADOW_CALL_STACK
+#if defined(CONFIG_SHADOW_CALL_STACK) && !defined(CONFIG_DYNAMIC_SCS)
/* gp is used as the shadow call stack pointer instead */
.macro load_global_pointer
.endm
diff --git a/arch/riscv/include/asm/scs.h b/arch/riscv/include/asm/scs.h
index 0e45db78b24b..14ef539922c2 100644
--- a/arch/riscv/include/asm/scs.h
+++ b/arch/riscv/include/asm/scs.h
@@ -9,46 +9,75 @@

/* Load init_shadow_call_stack to gp. */
.macro scs_load_init_stack
+#ifndef CONFIG_DYNAMIC_SCS
la gp, init_shadow_call_stack
XIP_FIXUP_OFFSET gp
+#endif
.endm

/* Load the per-CPU IRQ shadow call stack to gp. */
-.macro scs_load_irq_stack tmp
+.macro scs_load_irq_stack tmp tmp1
+#ifdef CONFIG_DYNAMIC_SCS
+ load_per_cpu \tmp1, irq_shadow_call_stack_ptr, \tmp
+ li \tmp, 4096
+ add \tmp, \tmp, \tmp1
+ csrw CSR_SSP, \tmp
+#else
load_per_cpu gp, irq_shadow_call_stack_ptr, \tmp
+#endif
.endm

/* Load task_scs_sp(current) to gp. */
-.macro scs_load_current
+.macro scs_load_current tmp
+#ifdef CONFIG_DYNAMIC_SCS
+ REG_L \tmp, TASK_TI_SCS_SP(tp)
+ csrw CSR_SSP, \tmp
+#else
REG_L gp, TASK_TI_SCS_SP(tp)
+#endif
.endm

/* Load task_scs_sp(current) to gp, but only if tp has changed. */
-.macro scs_load_current_if_task_changed prev
+.macro scs_load_current_if_task_changed prev tmp
beq \prev, tp, _skip_scs
- scs_load_current
+ scs_load_current \tmp
_skip_scs:
.endm

/* Save gp to task_scs_sp(current). */
-.macro scs_save_current
+.macro scs_save_current tmp
+#ifdef CONFIG_DYNAMIC_SCS
+ csrr \tmp, CSR_SSP
+ REG_S \tmp, TASK_TI_SCS_SP(tp)
+#else
REG_S gp, TASK_TI_SCS_SP(tp)
+#endif
.endm

#else /* CONFIG_SHADOW_CALL_STACK */

.macro scs_load_init_stack
.endm
-.macro scs_load_irq_stack tmp
+.macro scs_load_irq_stack tmp tmp1
.endm
-.macro scs_load_current
+.macro scs_load_current tmp
.endm
-.macro scs_load_current_if_task_changed prev
+.macro scs_load_current_if_task_changed prev tmp
.endm
-.macro scs_save_current
+.macro scs_save_current tmp
.endm

#endif /* CONFIG_SHADOW_CALL_STACK */
#endif /* __ASSEMBLY__ */

+#ifdef CONFIG_DYNAMIC_SCS
+#define arch_scs_store(ss_addr, magic_val) \
+ asm volatile ("ssamoswap.d %0, %2, %1" \
+ : "=r" (magic_val), "+A" (*ss_addr) \
+ : "r" (magic_val) \
+ : "memory")
+#else
+#define arch_scs_store(ss_addr, magic_val)
+#endif
+
#endif /* _ASM_SCS_H */
diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
index a35050a3e0ea..0262b46ab064 100644
--- a/arch/riscv/kernel/entry.S
+++ b/arch/riscv/kernel/entry.S
@@ -81,7 +81,7 @@ SYM_CODE_START(handle_exception)
load_global_pointer

/* Load the kernel shadow call stack pointer if coming from userspace */
- scs_load_current_if_task_changed s5
+ scs_load_current_if_task_changed s5 t0

#ifdef CONFIG_RISCV_ISA_V_PREEMPTIVE
move a0, sp
@@ -135,7 +135,7 @@ SYM_CODE_START_NOALIGN(ret_from_exception)
REG_S s0, TASK_TI_KERNEL_SP(tp)

/* Save the kernel shadow call stack pointer */
- scs_save_current
+ scs_save_current t0

/*
* Save TP into the scratch register , so we can find the kernel data
@@ -252,8 +252,8 @@ SYM_FUNC_START(call_on_irq_stack)
addi s0, sp, STACKFRAME_SIZE_ON_STACK

/* Switch to the per-CPU shadow call stack */
- scs_save_current
- scs_load_irq_stack t0
+ scs_save_current t0
+ scs_load_irq_stack t0 t1

/* Switch to the per-CPU IRQ stack and call the handler */
load_per_cpu t0, irq_stack_ptr, t1
@@ -263,7 +263,7 @@ SYM_FUNC_START(call_on_irq_stack)
jalr a1

/* Switch back to the thread shadow call stack */
- scs_load_current
+ scs_load_current t0

/* Switch back to the thread stack and restore ra and s0 */
addi sp, s0, -STACKFRAME_SIZE_ON_STACK
@@ -305,7 +305,7 @@ SYM_FUNC_START(__switch_to)
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
+ scs_save_current t0
/* Restore context from next->thread */
REG_L ra, TASK_THREAD_RA_RA(a4)
REG_L sp, TASK_THREAD_SP_RA(a4)
@@ -324,7 +324,7 @@ SYM_FUNC_START(__switch_to)
/* The offset of thread_info in task_struct is zero. */
move tp, a1
/* Switch to the next shadow call stack */
- scs_load_current
+ scs_load_current t0
ret
SYM_FUNC_END(__switch_to)

diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
index 6c311517c3b5..bc248c137c90 100644
--- a/arch/riscv/kernel/head.S
+++ b/arch/riscv/kernel/head.S
@@ -164,7 +164,7 @@ secondary_start_sbi:
call relocate_enable_mmu
#endif
call .Lsetup_trap_vector
- scs_load_current
+ scs_load_current t0
lui t2, 0x1
tail smp_callin
#endif /* CONFIG_SMP */
@@ -313,7 +313,7 @@ SYM_CODE_START(_start_kernel)
la tp, init_task
la sp, init_thread_union + THREAD_SIZE
addi sp, sp, -PT_SIZE_ON_STACK
- scs_load_current
+ scs_load_current t0

#ifdef CONFIG_KASAN
call kasan_early_init
--
2.43.2