[PATCH v3 19/29] riscv: Implements arch agnostic indirect branch tracking prctls

[Deepak Gupta]

prctls implemented are:
PR_SET_INDIR_BR_LP_STATUS, PR_GET_INDIR_BR_LP_STATUS and
PR_LOCK_INDIR_BR_LP_STATUS.

Signed-off-by: Deepak Gupta <debug at rivosinc.com>
---
 arch/riscv/include/asm/usercfi.h | 22 ++++++++-
 arch/riscv/kernel/process.c      |  5 +++
 arch/riscv/kernel/usercfi.c      | 76 ++++++++++++++++++++++++++++++++
 3 files changed, 102 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/include/asm/usercfi.h b/arch/riscv/include/asm/usercfi.h
index a168ae0fa5d8..8accdc8ec164 100644
--- a/arch/riscv/include/asm/usercfi.h
+++ b/arch/riscv/include/asm/usercfi.h
@@ -16,7 +16,9 @@ struct kernel_clone_args;
 struct cfi_status {
 	unsigned long ubcfi_en : 1; /* Enable for backward cfi. */
 	unsigned long ubcfi_locked : 1;
-	unsigned long rsvd : ((sizeof(unsigned long)*8) - 2);
+	unsigned long ufcfi_en : 1; /* Enable for forward cfi. Note that ELP goes in sstatus */
+	unsigned long ufcfi_locked : 1;
+	unsigned long rsvd : ((sizeof(unsigned long)*8) - 4);
 	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 */
@@ -30,6 +32,9 @@ void set_active_shstk(struct task_struct *task, unsigned long shstk_addr);
 bool is_shstk_enabled(struct task_struct *task);
 bool is_shstk_locked(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);
 
 #define PR_SHADOW_STACK_SUPPORTED_STATUS_MASK (PR_SHADOW_STACK_ENABLE)
 
@@ -72,6 +77,21 @@ static inline void set_shstk_status(struct task_struct *task, bool enable)
 
 }
 
+static inline bool is_indir_lp_enabled(struct task_struct *task)
+{
+	return false;
+}
+
+static inline bool is_indir_lp_locked(struct task_struct *task)
+{
+	return false;
+}
+
+static inline void set_indir_lp_status(struct task_struct *task, bool enable)
+{
+
+}
+
 #endif /* CONFIG_RISCV_USER_CFI */
 
 #endif /* __ASSEMBLY__ */
diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
index 3fb8b23f629b..ebed7589c51a 100644
--- a/arch/riscv/kernel/process.c
+++ b/arch/riscv/kernel/process.c
@@ -152,6 +152,11 @@ 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 cdedf1f78b3e..13920b9d86f3 100644
--- a/arch/riscv/kernel/usercfi.c
+++ b/arch/riscv/kernel/usercfi.c
@@ -69,6 +69,32 @@ 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 ? true : false;
+}
+
+bool is_indir_lp_locked(struct task_struct *task)
+{
+	return task->thread_info.user_cfi_state.ufcfi_locked ? true : false;
+}
+
+void set_indir_lp_status(struct task_struct *task, bool enable)
+{
+	task->thread_info.user_cfi_state.ufcfi_en = enable ? 1 : 0;
+
+	if (enable)
+		task->thread_info.envcfg |= ENVCFG_LPE;
+	else
+		task->thread_info.envcfg &= ~ENVCFG_LPE;
+
+	csr_write(CSR_ENVCFG, task->thread_info.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
@@ -375,3 +401,53 @@ 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) ? true : false;
+	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))
+		return -EINVAL;
+
+	set_indir_lp_lock(task);
+
+	return 0;
+}
-- 
2.43.2