checkpolicy/fuzz: override YY_FATAL_ERROR

The default action of the lexer macro YY_FATAL_ERROR(msg) is to print
the message and call exit().  This might happen on an overlong token
(8192 bytes) that does not fit into the token buffer.
Fuzz targets must not call exit() though, since an exit is treated as an
abnormal behavior, see https://llvm.org/docs/LibFuzzer.html#fuzz-target.

Since YY_FATAL_ERROR is used in functions with different return value
types and is expected to not return, jump to a location in the fuzzer
right before yyparse() instead.

Reported-by: oss-fuzz (issue 67728)
Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
Acked-by: James Carter <jwcart2@gmail.com>

checkpolicy/fuzz/checkpolicy-fuzzer.c

@@ -1,4 +1,5 @@
 #include <assert.h>
+#include <setjmp.h>
 #include <unistd.h>
 #include <sys/mman.h>
 
@@ -30,6 +31,7 @@ extern void yyrestart(FILE *);
 extern int yylex_destroy(void);
 extern void set_source_file(const char *name);
 
+jmp_buf fuzzing_pre_parse_stack_state;
 
 // Set to 1 for verbose libsepol logging
 #define VERBOSE 0
@@ -99,6 +101,13 @@ static int read_source_policy(policydb_t *p, const uint8_t *data, size_t size)
 
 	init_parser(1);
 
+	if (!setjmp(fuzzing_pre_parse_stack_state)) {
+		queue_destroy(id_queue);
+		fclose(yyin);
+		yylex_destroy();
+		return -1;
+	}
+
 	rc = yyparse();
 	// TODO: drop global variable policydb_errors if proven to be redundant
 	assert(rc || !policydb_errors);

checkpolicy/policy_scan.l

@@ -41,6 +41,21 @@ int werror = 0;
 int yyerror(const char *msg);
 int yywarn(const char *msg);
 
+#ifdef FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION
+/*
+ * Version that does not exit, like yy_fatal_error(),
+ * since fuzz targets must not call exit().
+ */
+#include <setjmp.h>
+extern jmp_buf fuzzing_pre_parse_stack_state;
+void yyfatal(const char *msg)
+{
+	yyerror(msg);
+	longjmp(fuzzing_pre_parse_stack_state, 1);
+}
+#define YY_FATAL_ERROR(msg) yyfatal(msg)
+#endif
+
 void set_source_file(const char *name);
 
 char source_file[PATH_MAX];