tracing: Check for no filter when processing event filters

The syzkaller detected a out-of-bounds issue with the events filter code,
specifically here:

	prog[N].pred = NULL;					/* #13 */
	prog[N].target = 1;		/* TRUE */
	prog[N+1].pred = NULL;
	prog[N+1].target = 0;		/* FALSE */
->	prog[N-1].target = N;
	prog[N-1].when_to_branch = false;

As that's the first reference to a "N-1" index, it appears that the code got
here with N = 0, which means the filter parser found no filter to parse
(which shouldn't ever happen, but apparently it did).

Add a new error to the parsing code that will check to make sure that N is
not zero before going into this part of the code. If N = 0, then -EINVAL is
returned, and a error message is added to the filter.

Cc: stable@vger.kernel.org
Fixes: 80765597bc ("tracing: Rewrite filter logic to be simpler and faster")
Reported-by: air icy <icytxw@gmail.com>
bugzilla url: https://bugzilla.kernel.org/show_bug.cgi?id=200019
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/trace_events_filter.c

@@ -78,7 +78,8 @@ static const char * ops[] = { OPS };
 	C(TOO_MANY_PREDS,	"Too many terms in predicate expression"), \
 	C(INVALID_FILTER,	"Meaningless filter expression"),	\
 	C(IP_FIELD_ONLY,	"Only 'ip' field is supported for function trace"), \
-	C(INVALID_VALUE,	"Invalid value (did you forget quotes)?"),
+	C(INVALID_VALUE,	"Invalid value (did you forget quotes)?"), \
+	C(NO_FILTER,		"No filter found"),
 
 #undef C
 #define C(a, b)		FILT_ERR_##a
@@ -550,6 +551,13 @@ predicate_parse(const char *str, int nr_parens, int nr_preds,
 		goto out_free;
 	}
 
+	if (!N) {
+		/* No program? */
+		ret = -EINVAL;
+		parse_error(pe, FILT_ERR_NO_FILTER, ptr - str);
+		goto out_free;
+	}
+
 	prog[N].pred = NULL;					/* #13 */
 	prog[N].target = 1;		/* TRUE */
 	prog[N+1].pred = NULL;