[PATCH v16 14/27] riscv: Implements arch agnostic indirect branch tracking prctls

Deepak Gupta
2025-05-23T05:32:03Z

prctls implemented are:
PR_SET_INDIR_BR_LP_STATUS, PR_GET_INDIR_BR_LP_STATUS and
PR_LOCK_INDIR_BR_LP_STATUS

Reviewed-by: Zong Li <zong.li@sifive.com>
Signed-off-by: Deepak Gupta <debug@rivosinc.com>
---
arch/riscv/include/asm/usercfi.h | 14 +++++++
arch/riscv/kernel/entry.S | 2 +-
arch/riscv/kernel/process.c | 5 +++
arch/riscv/kernel/usercfi.c | 79 ++++++++++++++++++++++++++++++++++++++++
4 files changed, 99 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/include/asm/usercfi.h b/arch/riscv/include/asm/usercfi.h
index b530ff5baa6e..cea7908cdb3a 100644
--- a/arch/riscv/include/asm/usercfi.h
+++ b/arch/riscv/include/asm/usercfi.h
@@ -16,6 +16,8 @@ struct kernel_clone_args;
struct cfi_state {
unsigned long ubcfi_en : 1; /* Enable for backward cfi. */
unsigned long ubcfi_locked : 1;
+ unsigned long ufcfi_en : 1; /* Enable for forward cfi. Note that ELP goes in sstatus */
+ unsigned long ufcfi_locked : 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 */
@@ -32,6 +34,10 @@ bool is_shstk_locked(struct task_struct *task);
bool is_shstk_allocated(struct task_struct *task);
void set_shstk_lock(struct task_struct *task);
void set_shstk_status(struct task_struct *task, bool enable);
+bool is_indir_lp_enabled(struct task_struct *task);
+bool is_indir_lp_locked(struct task_struct *task);
+void set_indir_lp_status(struct task_struct *task, bool enable);
+void set_indir_lp_lock(struct task_struct *task);

#define PR_SHADOW_STACK_SUPPORTED_STATUS_MASK (PR_SHADOW_STACK_ENABLE)

@@ -57,6 +63,14 @@ void set_shstk_status(struct task_struct *task, bool enable);

#define set_shstk_status(task, enable)

+#define is_indir_lp_enabled(task) false
+
+#define is_indir_lp_locked(task) false
+
+#define set_indir_lp_status(task, enable)
+
+#define set_indir_lp_lock(task)
+
#endif /* CONFIG_RISCV_USER_CFI */

#endif /* __ASSEMBLY__ */
diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
index c4bfe2085c41..978115567bca 100644
--- a/arch/riscv/kernel/entry.S
+++ b/arch/riscv/kernel/entry.S
@@ -169,7 +169,7 @@ SYM_CODE_START(handle_exception)
* Disable the FPU/Vector to detect illegal usage of floating point
* or vector in kernel space.
*/
- li t0, SR_SUM | SR_FS_VS
+ li t0, SR_SUM | SR_FS_VS | SR_ELP

REG_L s0, TASK_TI_USER_SP(tp)
csrrc s1, CSR_STATUS, t0
diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
index cd11667593fe..4587201dd81d 100644
--- a/arch/riscv/kernel/process.c
+++ b/arch/riscv/kernel/process.c
@@ -160,6 +160,11 @@ void start_thread(struct pt_regs *regs, unsigned long pc,
set_shstk_status(current, false);
set_shstk_base(current, 0, 0);
set_active_shstk(current, 0);
+ /*
+ * disable indirect branch tracking on exec.
+ * libc will enable it later via prctl.
+ */
+ set_indir_lp_status(current, false);

#ifdef CONFIG_64BIT
regs->status &= ~SR_UXL;
diff --git a/arch/riscv/kernel/usercfi.c b/arch/riscv/kernel/usercfi.c
index 08620bdae696..2ebe789caa6b 100644
--- a/arch/riscv/kernel/usercfi.c
+++ b/arch/riscv/kernel/usercfi.c
@@ -72,6 +72,35 @@ void set_shstk_lock(struct task_struct *task)
task->thread_info.user_cfi_state.ubcfi_locked = 1;
}

+bool is_indir_lp_enabled(struct task_struct *task)
+{
+ return task->thread_info.user_cfi_state.ufcfi_en;
+}
+
+bool is_indir_lp_locked(struct task_struct *task)
+{
+ return task->thread_info.user_cfi_state.ufcfi_locked;
+}
+
+void set_indir_lp_status(struct task_struct *task, bool enable)
+{
+ if (!cpu_supports_indirect_br_lp_instr())
+ return;
+
+ task->thread_info.user_cfi_state.ufcfi_en = enable ? 1 : 0;
+
+ if (enable)
+ task->thread.envcfg |= ENVCFG_LPE;
+ else
+ task->thread.envcfg &= ~ENVCFG_LPE;
+
+ csr_write(CSR_ENVCFG, task->thread.envcfg);
+}
+
+void set_indir_lp_lock(struct task_struct *task)
+{
+ task->thread_info.user_cfi_state.ufcfi_locked = 1;
+}
/*
* If size is 0, then to be compatible with regular stack we want it to be as big as
* regular stack. Else PAGE_ALIGN it and return back
@@ -371,3 +400,53 @@ int arch_lock_shadow_stack_status(struct task_struct *task,

return 0;
}
+
+int arch_get_indir_br_lp_status(struct task_struct *t, unsigned long __user *status)
+{
+ unsigned long fcfi_status = 0;
+
+ if (!cpu_supports_indirect_br_lp_instr())
+ return -EINVAL;
+
+ /* indirect branch tracking is enabled on the task or not */
+ fcfi_status |= (is_indir_lp_enabled(t) ? PR_INDIR_BR_LP_ENABLE : 0);
+
+ return copy_to_user(status, &fcfi_status, sizeof(fcfi_status)) ? -EFAULT : 0;
+}
+
+int arch_set_indir_br_lp_status(struct task_struct *t, unsigned long status)
+{
+ bool enable_indir_lp = false;
+
+ if (!cpu_supports_indirect_br_lp_instr())
+ return -EINVAL;
+
+ /* indirect branch tracking is locked and further can't be modified by user */
+ if (is_indir_lp_locked(t))
+ return -EINVAL;
+
+ /* Reject unknown flags */
+ if (status & ~PR_INDIR_BR_LP_ENABLE)
+ return -EINVAL;
+
+ enable_indir_lp = (status & PR_INDIR_BR_LP_ENABLE);
+ set_indir_lp_status(t, enable_indir_lp);
+
+ return 0;
+}
+
+int arch_lock_indir_br_lp_status(struct task_struct *task,
+ unsigned long arg)
+{
+ /*
+ * If indirect branch tracking is not supported or not enabled on task,
+ * nothing to lock here
+ */
+ if (!cpu_supports_indirect_br_lp_instr() ||
+ !is_indir_lp_enabled(task) || arg != 0)
+ return -EINVAL;
+
+ set_indir_lp_lock(task);
+
+ return 0;
+}

--
2.43.0