riscv: Implement indirect branch tracking prctls

This patch adds a RISC-V implementation of the following prctls:
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>
Tested-by: Andreas Korb <andreas.korb@aisec.fraunhofer.de>
Tested-by: Valentin Haudiquet <valentin.haudiquet@canonical.com>
Link: https://patch.msgid.link/20251112-v5_user_cfi_series-v23-14-b55691eacf4f@rivosinc.com
[pjw@kernel.org: clean up patch description]
Signed-off-by: Paul Walmsley <pjw@kernel.org>

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) do {} while (0)
 
+#define is_indir_lp_enabled(task) false
+
+#define is_indir_lp_locked(task) false
+
+#define set_indir_lp_status(task, enable) do {} while (0)
+
+#define set_indir_lp_lock(task) do {} while (0)
+
 #endif /* CONFIG_RISCV_USER_CFI */
 
 #endif /* __ASSEMBLER__ */

arch/riscv/kernel/entry.S

@@ -174,6 +174,10 @@ SYM_CODE_START(handle_exception)
 	 * or vector in kernel space.
 	 */
 	li t0, SR_SUM | SR_FS_VS
+#ifdef CONFIG_64BIT
+	li t1, SR_ELP
+	or t0, t0, t1
+#endif
 
 	REG_L s0, TASK_TI_USER_SP(tp)
 	csrrc s1, CSR_STATUS, t0

arch/riscv/kernel/process.c

@@ -163,6 +163,11 @@ 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;

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
@@ -368,3 +397,53 @@ 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;
+}