[PATCH v1 RFC Zisslpcfi 04/20] riscv: kernel enabling user code for shadow stack and landing pad

Deepak Gupta
2023-02-13T04:54:20Z

Enables architectural support for shadow stack and landing pad instr
for user mode on riscv.

This patch does following
- Defines a new structure cfi_status
- Includes cfi_status in thread_info
- Defines offsets to new member fields in thread_info in asm-offsets.c
- Saves and restore cfi state on trap entry (U --> S) and exit (S --> U)

Signed-off-by: Deepak Gupta <debug@rivosinc.com>
---
arch/riscv/include/asm/processor.h | 11 ++++++++
arch/riscv/include/asm/thread_info.h | 5 ++++
arch/riscv/kernel/asm-offsets.c | 5 ++++
arch/riscv/kernel/entry.S | 40 ++++++++++++++++++++++++++++
4 files changed, 61 insertions(+)

diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h
index bdebce2cc323..f065309927b1 100644
--- a/arch/riscv/include/asm/processor.h
+++ b/arch/riscv/include/asm/processor.h
@@ -41,6 +41,17 @@ struct thread_struct {
unsigned long bad_cause;
};

+#if defined(CONFIG_USER_SHADOW_STACK) || defined(CONFIG_USER_INDIRECT_BR_LP)
+struct cfi_status {
+ unsigned int ufcfi_en : 1; /* Enable for forward cfi. Note that ELP goes in sstatus */
+ unsigned int ubcfi_en : 1; /* Enable for backward cfi. */
+ unsigned int rsvd1 : 30;
+ unsigned int lp_label; /* saved label value (25bit) */
+ long user_shdw_stk; /* Current user shadow stack pointer */
+ long shdw_stk_base; /* Base address of shadow stack */
+};
+#endif
+
/* Whitelist the fstate from the task_struct for hardened usercopy */
static inline void arch_thread_struct_whitelist(unsigned long *offset,
unsigned long *size)
diff --git a/arch/riscv/include/asm/thread_info.h b/arch/riscv/include/asm/thread_info.h
index 67322f878e0d..f74b8bd55d5b 100644
--- a/arch/riscv/include/asm/thread_info.h
+++ b/arch/riscv/include/asm/thread_info.h
@@ -65,6 +65,11 @@ struct thread_info {
*/
long kernel_sp; /* Kernel stack pointer */
long user_sp; /* User stack pointer */
+#if defined(CONFIG_USER_SHADOW_STACK) || defined(CONFIG_USER_INDIRECT_BR_LP)
+ /* cfi_state only if config is defined */
+ /* state of user cfi state. note this includes LPLR and SSP as well */
+ struct cfi_status user_cfi_state;
+#endif
int cpu;
};

diff --git a/arch/riscv/kernel/asm-offsets.c b/arch/riscv/kernel/asm-offsets.c
index df9444397908..340e6413cf3c 100644
--- a/arch/riscv/kernel/asm-offsets.c
+++ b/arch/riscv/kernel/asm-offsets.c
@@ -38,6 +38,11 @@ void asm_offsets(void)
OFFSET(TASK_TI_KERNEL_SP, task_struct, thread_info.kernel_sp);
OFFSET(TASK_TI_USER_SP, task_struct, thread_info.user_sp);

+#if defined(CONFIG_USER_SHADOW_STACK) || defined(CONFIG_USER_INDIRECT_BR_LP)
+ OFFSET(TASK_TI_USER_CFI_STATUS, task_struct, thread_info.user_cfi_state);
+ OFFSET(TASK_TI_USER_LPLR, task_struct, thread_info.user_cfi_state.lp_label);
+ 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 99d38fdf8b18..f283130c81ec 100644
--- a/arch/riscv/kernel/entry.S
+++ b/arch/riscv/kernel/entry.S
@@ -73,6 +73,31 @@ _save_context:
REG_S x30, PT_T5(sp)
REG_S x31, PT_T6(sp)

+#if defined(CONFIG_USER_SHADOW_STACK) || defined(CONFIG_USER_INDIRECT_BR_LP)
+ /*
+ * If U --> S, CSR_SCRATCH should be holding U TP
+ * If S --> S, CSR_SCRATCH should be holding S TP
+ * s2 == tp means, previous mode was S
+ * else previous mode U
+ * we need to save cfi status only when previous mode was U
+ */
+ csrr s2, CSR_SCRATCH
+ xor s2, s2, tp
+ beqz s2, skip_bcfi_save
+ /* load cfi status word */
+ lw s2, TASK_TI_USER_CFI_STATUS(tp)
+ andi s3, s2, 1
+ beqz s3, skip_fcfi_save
+ /* fcfi is enabled, capture ELP and LPLR state and record it */
+ csrr s3, CSR_LPLR /* record label register */
+ sw s3, TASK_TI_USER_LPLR(tp) /* save it back in thread_info structure */
+skip_fcfi_save:
+ andi s3, s2, 2
+ 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.
@@ -283,6 +308,21 @@ resume_userspace:
*/
csrw CSR_SCRATCH, tp

+#if defined(CONFIG_USER_SHADOW_STACK) || defined(CONFIG_USER_INDIRECT_BR_LP)
+ lw s2, TASK_TI_USER_CFI_STATUS(tp)
+ andi s3, s2, 1
+ beqz s3, skip_fcfi_resume
+ xor s3, s3, s3
+ lw s3, TASK_TI_USER_LPLR(tp)
+ csrw CSR_LPLR, s3
+skip_fcfi_resume:
+ andi s3, s2, 2
+ beqz s3, skip_bcfi_resume
+ REG_L s3, TASK_TI_USER_SSP(tp) /* save user ssp in thread_info */
+ csrw CSR_SSP, s3
+skip_bcfi_resume:
+#endif
+
restore_all:
#ifdef CONFIG_TRACE_IRQFLAGS
REG_L s1, PT_STATUS(sp)
--
2.25.1