tracing: Enforce passing in filter=NULL to create_filter()

There's some inconsistency with what to set the output parameter filterp
when passing to create_filter(..., struct event_filter **filterp).

Whatever filterp points to, should be NULL when calling this function. The
create_filter() calls create_filter_start() with a pointer to a local
"filter" variable that is set to NULL. The create_filter_start() has a
WARN_ON() if the passed in pointer isn't pointing to a value set to NULL.

Ideally, create_filter() should pass the filterp variable it received to
create_filter_start() and not hide it as with a local variable, this allowed
create_filter() to fail, and not update the passed in filter, and the caller
of create_filter() then tried to free filter, which was never initialized to
anything, causing memory corruption.

Link: http://lkml.kernel.org/r/00000000000032a0c30569916870@google.com
Fixes: 80765597bc587 ("tracing: Rewrite filter logic to be simpler and faster")
Reported-by: syzbot+dadcc936587643d7f568@syzkaller.appspotmail.com
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

diff --git a/kernel/trace/trace_events_filter.c b/kernel/trace/trace_events_filter.c
index 5eba1ce..9b4716b 100644 (file)
--- a/kernel/trace/trace_events_filter.c
+++ b/kernel/trace/trace_events_filter.c
@@ -1704,18 +1704,16 @@ static int create_filter(struct trace_event_call *call,
                         struct event_filter **filterp)
 {
        struct filter_parse_error *pe = NULL;
-       struct event_filter *filter = NULL;
        int err;
 
-       err = create_filter_start(filter_string, set_str, &pe, &filter);
+       err = create_filter_start(filter_string, set_str, &pe, filterp);
        if (err)
                return err;
 
-       err = process_preds(call, filter_string, filter, pe);
+       err = process_preds(call, filter_string, *filterp, pe);
        if (err && set_str)
-               append_filter_err(pe, filter);
+               append_filter_err(pe, *filterp);
 
-       *filterp = filter;
        return err;
 }
 
@@ -1739,24 +1737,22 @@ static int create_system_filter(struct trace_subsystem_dir *dir,
                                struct trace_array *tr,
                                char *filter_str, struct event_filter **filterp)
 {
-       struct event_filter *filter = NULL;
        struct filter_parse_error *pe = NULL;
        int err;
 
-       err = create_filter_start(filter_str, true, &pe, &filter);
+       err = create_filter_start(filter_str, true, &pe, filterp);
        if (!err) {
                err = process_system_preds(dir, tr, pe, filter_str);
                if (!err) {
                        /* System filters just show a default message */
-                       kfree(filter->filter_string);
-                       filter->filter_string = NULL;
+                       kfree((*filterp)->filter_string);
+                       (*filterp)->filter_string = NULL;
                } else {
-                       append_filter_err(pe, filter);
+                       append_filter_err(pe, *filterp);
                }
        }
        create_filter_finish(pe);
 
-       *filterp = filter;
        return err;
 }
 
@@ -1764,7 +1760,7 @@ static int create_system_filter(struct trace_subsystem_dir *dir,
 int apply_event_filter(struct trace_event_file *file, char *filter_string)
 {
        struct trace_event_call *call = file->event_call;
-       struct event_filter *filter;
+       struct event_filter *filter = NULL;
        int err;
 
        if (!strcmp(strstrip(filter_string), "0")) {
@@ -1817,7 +1813,7 @@ int apply_subsystem_event_filter(struct trace_subsystem_dir *dir,
 {
        struct event_subsystem *system = dir->subsystem;
        struct trace_array *tr = dir->tr;
-       struct event_filter *filter;
+       struct event_filter *filter = NULL;
        int err = 0;
 
        mutex_lock(&event_mutex);
@@ -2024,7 +2020,7 @@ int ftrace_profile_set_filter(struct perf_event *event, int event_id,
                              char *filter_str)
 {
        int err;
-       struct event_filter *filter;
+       struct event_filter *filter = NULL;
        struct trace_event_call *call;
 
        mutex_lock(&event_mutex);