ptrace: Move setting/clearing ptrace_message into ptrace_stop

Today ptrace_message is easy to overlook as it not a core part of
ptrace_stop.  It has been overlooked so much that there are places
that set ptrace_message and don't clear it, and places that never set
it.  So if you get an unlucky sequence of events the ptracer may be
able to read a ptrace_message that does not apply to the current
ptrace stop.

Move setting of ptrace_message into ptrace_stop so that it always gets
set before the stop, and always gets cleared after the stop.  This
prevents non-sense from being reported to userspace and makes
ptrace_message more visible in the ptrace helper functions so that
kernel developers can see it.

Link: https://lkml.kernel.org/r/87bky67qfv.fsf_-_@email.froward.int.ebiederm.org
Acked-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

include/linux/ptrace.h

@@ -60,7 +60,7 @@ extern int ptrace_writedata(struct task_struct *tsk, char __user *src, unsigned
 extern void ptrace_disable(struct task_struct *);
 extern int ptrace_request(struct task_struct *child, long request,
 			  unsigned long addr, unsigned long data);
-extern void ptrace_notify(int exit_code);
+extern void ptrace_notify(int exit_code, unsigned long message);
 extern void __ptrace_link(struct task_struct *child,
 			  struct task_struct *new_parent,
 			  const struct cred *ptracer_cred);
@@ -155,8 +155,7 @@ static inline bool ptrace_event_enabled(struct task_struct *task, int event)
 static inline void ptrace_event(int event, unsigned long message)
 {
 	if (unlikely(ptrace_event_enabled(current, event))) {
-		current->ptrace_message = message;
-		ptrace_notify((event << 8) | SIGTRAP);
+		ptrace_notify((event << 8) | SIGTRAP, message);
 	} else if (event == PTRACE_EVENT_EXEC) {
 		/* legacy EXEC report via SIGTRAP */
 		if ((current->ptrace & (PT_PTRACED|PT_SEIZED)) == PT_PTRACED)
@@ -424,8 +423,7 @@ static inline int ptrace_report_syscall(unsigned long message)
 	if (!(ptrace & PT_PTRACED))
 		return 0;
 
-	current->ptrace_message = message;
-	ptrace_notify(SIGTRAP | ((ptrace & PT_TRACESYSGOOD) ? 0x80 : 0));
+	ptrace_notify(SIGTRAP | ((ptrace & PT_TRACESYSGOOD) ? 0x80 : 0), message);
 
 	/*
 	 * this isn't the same as continuing with a signal, but it will do
@@ -437,7 +435,6 @@ static inline int ptrace_report_syscall(unsigned long message)
 		current->exit_code = 0;
 	}
 
-	current->ptrace_message = 0;
 	return fatal_signal_pending(current);
 }
 

include/uapi/linux/ptrace.h

@@ -114,7 +114,7 @@ struct ptrace_rseq_configuration {
 
 /*
  * These values are stored in task->ptrace_message
- * by ptrace_report_syscall_* to describe the current syscall-stop.
+ * by ptrace_stop to describe the current syscall-stop.
  */
 #define PTRACE_EVENTMSG_SYSCALL_ENTRY	1
 #define PTRACE_EVENTMSG_SYSCALL_EXIT	2

kernel/signal.c

@@ -2191,7 +2191,8 @@ static void do_notify_parent_cldstop(struct task_struct *tsk,
  * If we actually decide not to stop at all because the tracer
  * is gone, we keep current->exit_code unless clear_code.
  */
-static void ptrace_stop(int exit_code, int why, int clear_code, kernel_siginfo_t *info)
+static void ptrace_stop(int exit_code, int why, int clear_code,
+			unsigned long message, kernel_siginfo_t *info)
 	__releases(&current->sighand->siglock)
 	__acquires(&current->sighand->siglock)
 {
@@ -2237,6 +2238,7 @@ static void ptrace_stop(int exit_code, int why, int clear_code, kernel_siginfo_t
 	 */
 	smp_wmb();
 
+	current->ptrace_message = message;
 	current->last_siginfo = info;
 	current->exit_code = exit_code;
 
@@ -2315,6 +2317,7 @@ static void ptrace_stop(int exit_code, int why, int clear_code, kernel_siginfo_t
 	 */
 	spin_lock_irq(&current->sighand->siglock);
 	current->last_siginfo = NULL;
+	current->ptrace_message = 0;
 
 	/* LISTENING can be set only during STOP traps, clear it */
 	current->jobctl &= ~JOBCTL_LISTENING;
@@ -2327,7 +2330,7 @@ static void ptrace_stop(int exit_code, int why, int clear_code, kernel_siginfo_t
 	recalc_sigpending_tsk(current);
 }
 
-static void ptrace_do_notify(int signr, int exit_code, int why)
+static void ptrace_do_notify(int signr, int exit_code, int why, unsigned long message)
 {
 	kernel_siginfo_t info;
 
@@ -2338,17 +2341,17 @@ static void ptrace_do_notify(int signr, int exit_code, int why)
 	info.si_uid = from_kuid_munged(current_user_ns(), current_uid());
 
 	/* Let the debugger run.  */
-	ptrace_stop(exit_code, why, 1, &info);
+	ptrace_stop(exit_code, why, 1, message, &info);
 }
 
-void ptrace_notify(int exit_code)
+void ptrace_notify(int exit_code, unsigned long message)
 {
 	BUG_ON((exit_code & (0x7f | ~0xffff)) != SIGTRAP);
 	if (unlikely(task_work_pending(current)))
 		task_work_run();
 
 	spin_lock_irq(&current->sighand->siglock);
-	ptrace_do_notify(SIGTRAP, exit_code, CLD_TRAPPED);
+	ptrace_do_notify(SIGTRAP, exit_code, CLD_TRAPPED, message);
 	spin_unlock_irq(&current->sighand->siglock);
 }
 
@@ -2504,10 +2507,10 @@ static void do_jobctl_trap(void)
 			signr = SIGTRAP;
 		WARN_ON_ONCE(!signr);
 		ptrace_do_notify(signr, signr | (PTRACE_EVENT_STOP << 8),
-				 CLD_STOPPED);
+				 CLD_STOPPED, 0);
 	} else {
 		WARN_ON_ONCE(!signr);
-		ptrace_stop(signr, CLD_STOPPED, 0, NULL);
+		ptrace_stop(signr, CLD_STOPPED, 0, 0, NULL);
 		current->exit_code = 0;
 	}
 }
@@ -2561,7 +2564,7 @@ static int ptrace_signal(int signr, kernel_siginfo_t *info, enum pid_type type)
 	 * comment in dequeue_signal().
 	 */
 	current->jobctl |= JOBCTL_STOP_DEQUEUED;
-	ptrace_stop(signr, CLD_TRAPPED, 0, info);
+	ptrace_stop(signr, CLD_TRAPPED, 0, 0, info);
 
 	/* We're back.  Did the debugger cancel the sig?  */
 	signr = current->exit_code;
@@ -2891,7 +2894,7 @@ static void signal_delivered(struct ksignal *ksig, int stepping)
 	if (current->sas_ss_flags & SS_AUTODISARM)
 		sas_ss_reset(current);
 	if (stepping)
-		ptrace_notify(SIGTRAP);
+		ptrace_notify(SIGTRAP, 0);
 }
 
 void signal_setup_done(int failed, struct ksignal *ksig, int stepping)