[PATCH v16 05/27] riscv: usercfi state for task and save/restore of CSR_SSP on trap entry/exit

Deepak Gupta
2025-05-23T05:31:33Z

Carves out space in arch specific thread struct for cfi status and shadow
stack in usermode on riscv.

This patch does following
- defines a new structure cfi_status with status bit for cfi feature
- defines shadow stack pointer, base and size in cfi_status structure
- defines offsets to new member fields in thread in asm-offsets.c
- Saves and restore shadow stack pointer on trap entry (U --> S) and exit
(S --> U)

Shadow stack save/restore is gated on feature availiblity and implemented
using alternative. CSR can be context switched in `switch_to` as well but
soon as kernel shadow stack support gets rolled in, shadow stack pointer
will need to be switched at trap entry/exit point (much like `sp`). It can
be argued that kernel using shadow stack deployment scenario may not be as
prevalant as user mode using this feature. But even if there is some
minimal deployment of kernel shadow stack, that means that it needs to be
supported. And thus save/restore of shadow stack pointer in entry.S instead
of in `switch_to.h`.

Reviewed-by: Charlie Jenkins <charlie@rivosinc.com>
Reviewed-by: Zong Li <zong.li@sifive.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Signed-off-by: Deepak Gupta <debug@rivosinc.com>
---
arch/riscv/include/asm/processor.h | 1 +
arch/riscv/include/asm/thread_info.h | 3 +++
arch/riscv/include/asm/usercfi.h | 23 +++++++++++++++++++++++
arch/riscv/kernel/asm-offsets.c | 4 ++++
arch/riscv/kernel/entry.S | 28 ++++++++++++++++++++++++++++
5 files changed, 59 insertions(+)

diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h
index e3aba3336e63..d851bb5c6da0 100644
--- a/arch/riscv/include/asm/processor.h
+++ b/arch/riscv/include/asm/processor.h
@@ -14,6 +14,7 @@

#include <asm/ptrace.h>
#include <asm/hwcap.h>
+#include <asm/usercfi.h>

#define arch_get_mmap_end(addr, len, flags) \
({ \
diff --git a/arch/riscv/include/asm/thread_info.h b/arch/riscv/include/asm/thread_info.h
index f5916a70879a..e066f41176ca 100644
--- a/arch/riscv/include/asm/thread_info.h
+++ b/arch/riscv/include/asm/thread_info.h
@@ -73,6 +73,9 @@ struct thread_info {
*/
unsigned long a0, a1, a2;
#endif
+#ifdef CONFIG_RISCV_USER_CFI
+ struct cfi_state user_cfi_state;
+#endif
};

#ifdef CONFIG_SHADOW_CALL_STACK
diff --git a/arch/riscv/include/asm/usercfi.h b/arch/riscv/include/asm/usercfi.h
new file mode 100644
index 000000000000..94b214c295c0
--- /dev/null
+++ b/arch/riscv/include/asm/usercfi.h
@@ -0,0 +1,23 @@
+/* SPDX-License-Identifier: GPL-2.0
+ * Copyright (C) 2024 Rivos, Inc.
+ * Deepak Gupta <debug@rivosinc.com>
+ */
+#ifndef _ASM_RISCV_USERCFI_H
+#define _ASM_RISCV_USERCFI_H
+
+#ifndef __ASSEMBLY__
+#include <linux/types.h>
+
+#ifdef CONFIG_RISCV_USER_CFI
+struct cfi_state {
+ unsigned long ubcfi_en : 1; /* Enable for backward cfi. */
+ unsigned long user_shdw_stk; /* Current user shadow stack pointer */
+ unsigned long shdw_stk_base; /* Base address of shadow stack */
+ unsigned long shdw_stk_size; /* size of shadow stack */
+};
+
+#endif /* CONFIG_RISCV_USER_CFI */
+
+#endif /* __ASSEMBLY__ */
+
+#endif /* _ASM_RISCV_USERCFI_H */
diff --git a/arch/riscv/kernel/asm-offsets.c b/arch/riscv/kernel/asm-offsets.c
index 16490755304e..f33945432f8f 100644
--- a/arch/riscv/kernel/asm-offsets.c
+++ b/arch/riscv/kernel/asm-offsets.c
@@ -49,6 +49,10 @@ void asm_offsets(void)
#endif

OFFSET(TASK_TI_CPU_NUM, task_struct, thread_info.cpu);
+#ifdef CONFIG_RISCV_USER_CFI
+ OFFSET(TASK_TI_CFI_STATE, task_struct, thread_info.user_cfi_state);
+ OFFSET(TASK_TI_USER_SSP, task_struct, thread_info.user_cfi_state.user_shdw_stk);
+#endif
OFFSET(TASK_THREAD_F0, task_struct, thread.fstate.f[0]);
OFFSET(TASK_THREAD_F1, task_struct, thread.fstate.f[1]);
OFFSET(TASK_THREAD_F2, task_struct, thread.fstate.f[2]);
diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
index 33a5a9f2a0d4..c4bfe2085c41 100644
--- a/arch/riscv/kernel/entry.S
+++ b/arch/riscv/kernel/entry.S
@@ -91,6 +91,32 @@
REG_L a0, TASK_TI_A0(tp)
.endm

+/*
+ * If previous mode was U, capture shadow stack pointer and save it away
+ * Zero CSR_SSP at the same time for sanitization.
+ */
+.macro save_userssp tmp, status
+ ALTERNATIVE("nops(4)",
+ __stringify( \
+ andi \tmp, \status, SR_SPP; \
+ bnez \tmp, skip_ssp_save; \
+ csrrw \tmp, CSR_SSP, x0; \
+ REG_S \tmp, TASK_TI_USER_SSP(tp); \
+ skip_ssp_save:),
+ 0,
+ RISCV_ISA_EXT_ZICFISS,
+ CONFIG_RISCV_USER_CFI)
+.endm
+
+.macro restore_userssp tmp
+ ALTERNATIVE("nops(2)",
+ __stringify( \
+ REG_L \tmp, TASK_TI_USER_SSP(tp); \
+ csrw CSR_SSP, \tmp),
+ 0,
+ RISCV_ISA_EXT_ZICFISS,
+ CONFIG_RISCV_USER_CFI)
+.endm

SYM_CODE_START(handle_exception)
/*
@@ -147,6 +173,7 @@ SYM_CODE_START(handle_exception)

REG_L s0, TASK_TI_USER_SP(tp)
csrrc s1, CSR_STATUS, t0
+ save_userssp s2, s1
csrr s2, CSR_EPC
csrr s3, CSR_TVAL
csrr s4, CSR_CAUSE
@@ -236,6 +263,7 @@ SYM_CODE_START_NOALIGN(ret_from_exception)
* structures again.
*/
csrw CSR_SCRATCH, tp
+ restore_userssp s3
1:
#ifdef CONFIG_RISCV_ISA_V_PREEMPTIVE
move a0, sp

--
2.43.0