[RFC PATCH v1 07/28] riscv: kernel handling on trap entry/exit for user cfi

[debug@rivosinc.com]

From: Deepak Gupta <debug@rivosinc.com>

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)

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     | 24 ++++++++++++++++++++++++
 arch/riscv/kernel/asm-offsets.c      |  5 ++++-
 arch/riscv/kernel/entry.S            | 25 +++++++++++++++++++++++++
 5 files changed, 57 insertions(+), 1 deletion(-)
 create mode 100644 arch/riscv/include/asm/usercfi.h

diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h
index ee2f51787ff8..d4dc298880fc 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>
 
 #ifdef CONFIG_64BIT
 #define DEFAULT_MAP_WINDOW	(UL(1) << (MMAP_VA_BITS - 1))
diff --git a/arch/riscv/include/asm/thread_info.h b/arch/riscv/include/asm/thread_info.h
index 320bc899a63b..6a2acecec546 100644
--- a/arch/riscv/include/asm/thread_info.h
+++ b/arch/riscv/include/asm/thread_info.h
@@ -58,6 +58,9 @@ struct thread_info {
 	int			cpu;
 	unsigned long		syscall_work;	/* SYSCALL_WORK_ flags */
 	unsigned long envcfg;
+#ifdef CONFIG_RISCV_USER_CFI
+	struct cfi_status       user_cfi_state;
+#endif
 #ifdef CONFIG_SHADOW_CALL_STACK
 	void			*scs_base;
 	void			*scs_sp;
diff --git a/arch/riscv/include/asm/usercfi.h b/arch/riscv/include/asm/usercfi.h
new file mode 100644
index 000000000000..080d7077d12c
--- /dev/null
+++ b/arch/riscv/include/asm/usercfi.h
@@ -0,0 +1,24 @@
+/* SPDX-License-Identifier: GPL-2.0
+ * Copyright (C) 2023 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_status {
+	unsigned long ubcfi_en : 1; /* Enable for backward cfi. */
+	unsigned long rsvd : ((sizeof(unsigned long)*8) - 1);
+	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 cdd8f095c30c..5e1f412e96ba 100644
--- a/arch/riscv/kernel/asm-offsets.c
+++ b/arch/riscv/kernel/asm-offsets.c
@@ -43,8 +43,11 @@ void asm_offsets(void)
 #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);
+#ifdef CONFIG_RISCV_USER_CFI
+	OFFSET(TASK_TI_CFI_STATUS, 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 63c3855ba80d..410659e2eadb 100644
--- a/arch/riscv/kernel/entry.S
+++ b/arch/riscv/kernel/entry.S
@@ -49,6 +49,21 @@ SYM_CODE_START(handle_exception)
 	REG_S x5,  PT_T0(sp)
 	save_from_x6_to_x31
 
+#ifdef CONFIG_RISCV_USER_CFI
+	/*
+	* we need to save cfi status only when previous mode was U
+	*/
+	csrr s2, CSR_STATUS
+	andi s2, s2, SR_SPP
+	bnez s2, skip_bcfi_save
+	/* load cfi status word */
+	lw s3, TASK_TI_CFI_STATUS(tp)
+	andi s3, s3, 1
+	beqz s3, skip_bcfi_save
+	csrr s3, CSR_SSP
+	REG_S s3, TASK_TI_USER_SSP(tp) /* save user ssp in thread_info */
+skip_bcfi_save:
+#endif
 	/*
 	 * Disable user-mode memory access as it should only be set in the
 	 * actual user copy routines.
@@ -141,6 +156,16 @@ SYM_CODE_START_NOALIGN(ret_from_exception)
 	 * structures again.
 	 */
 	csrw CSR_SCRATCH, tp
+
+#ifdef CONFIG_RISCV_USER_CFI
+	lw s3, TASK_TI_CFI_STATUS(tp)
+	andi s3, s3, 1
+	beqz s3, skip_bcfi_resume
+	REG_L s3, TASK_TI_USER_SSP(tp) /* restore user ssp from thread struct */
+	csrw CSR_SSP, s3
+skip_bcfi_resume:
+#endif
+
 1:
 	REG_L a0, PT_STATUS(sp)
 	/*
-- 
2.43.0