[PATCH 1/4] ftrace: Fix function pid filter on instances

[Namhyung Kim]

When function tracer has a pid filter, it adds a probe to sched_switch
to track if current task can be ignored.  The probe checks the
ftrace_ignore_pid from current tr to filter tasks.  But it misses to
delete the probe when removing an instance so that it can cause a crash
due to the invalid tr pointer (use-after-free).

This is easily reproducible with the following:

  # cd /sys/kernel/debug/tracing
  # mkdir instances/buggy
  # echo $$ > instances/buggy/set_ftrace_pid
  # rmdir instances/buggy

  ============================================================================
  BUG: KASAN: use-after-free in ftrace_filter_pid_sched_switch_probe+0x3d/0x90
  Read of size 8 by task kworker/0:1/17
  CPU: 0 PID: 17 Comm: kworker/0:1 Tainted: G    B           4.11.0-rc3  #198
  Call Trace:
   dump_stack+0x68/0x9f
   kasan_object_err+0x21/0x70
   kasan_report.part.1+0x22b/0x500
   ? ftrace_filter_pid_sched_switch_probe+0x3d/0x90
   kasan_report+0x25/0x30
   __asan_load8+0x5e/0x70
   ftrace_filter_pid_sched_switch_probe+0x3d/0x90
   ? fpid_start+0x130/0x130
   __schedule+0x571/0xce0
   ...

To fix it, use ftrace_pid_reset() to unregister the probe.  As
instance_rmdir() already updated ftrace codes, it can just free the
filter safely.

Fixes: 0c8916c34203 ("tracing: Add rmdir to remove multibuffer instances")
Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
---
 kernel/trace/ftrace.c | 10 ++++++----
 kernel/trace/trace.c  |  1 +
 kernel/trace/trace.h  |  2 ++
 3 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 0556a202c055..b451a860e885 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -5598,13 +5598,15 @@ static void clear_ftrace_pids(struct trace_array *tr)
 	trace_free_pid_list(pid_list);
 }
 
-static void ftrace_pid_reset(struct trace_array *tr)
+void ftrace_pid_reset(struct trace_array *tr, bool update)
 {
 	mutex_lock(&ftrace_lock);
 	clear_ftrace_pids(tr);
 
-	ftrace_update_pid_func();
-	ftrace_startup_all(0);
+	if (update) {
+		ftrace_update_pid_func();
+		ftrace_startup_all(0);
+	}
 
 	mutex_unlock(&ftrace_lock);
 }
@@ -5676,7 +5678,7 @@ ftrace_pid_open(struct inode *inode, struct file *file)
 
 	if ((file->f_mode & FMODE_WRITE) &&
 	    (file->f_flags & O_TRUNC))
-		ftrace_pid_reset(tr);
+		ftrace_pid_reset(tr, true);
 
 	ret = seq_open(file, &ftrace_pid_sops);
 	if (ret < 0) {
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index b5d4b80f2d45..b92489dfa829 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -7485,6 +7485,7 @@ static int instance_rmdir(const char *name)
 
 	tracing_set_nop(tr);
 	event_trace_del_tracer(tr);
+	ftrace_pid_reset(tr, false);
 	ftrace_destroy_function_files(tr);
 	tracefs_remove_recursive(tr->dir);
 	free_trace_buffers(tr);
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index 571acee52a32..4d9804fd9a2d 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -897,6 +897,7 @@ void ftrace_init_tracefs(struct trace_array *tr, struct dentry *d_tracer);
 void ftrace_init_tracefs_toplevel(struct trace_array *tr,
 				  struct dentry *d_tracer);
 int init_function_trace(void);
+void ftrace_pid_reset(struct trace_array *tr, bool update);
 #else
 static inline int ftrace_trace_task(struct trace_array *tr)
 {
@@ -916,6 +917,7 @@ static inline void ftrace_reset_array_ops(struct trace_array *tr) { }
 static inline void ftrace_init_tracefs(struct trace_array *tr, struct dentry *d) { }
 static inline void ftrace_init_tracefs_toplevel(struct trace_array *tr, struct dentry *d) { }
 static inline int init_function_trace(void) { return 0; }
+static inline void ftrace_pid_reset(struct trace_array *tr, bool update) { }
 /* ftace_func_t type is not defined, use macro instead of static inline */
 #define ftrace_init_array_ops(tr, func) do { } while (0)
 #endif /* CONFIG_FUNCTION_TRACER */
-- 
2.12.0