protexec: fix out-of-bounds stack write

In create_tempfile() we look for a suitable place to put the temporary
file into and, among others, look at $TMPDIR. If the value of this
environment variable exceeds the bounds of the local tmp_name[] buffer,
we ignore it. However, we still change the value of 'tmp_name_len' which
leads to follow-up errors.

On debug builds this can lead to hitting the assertion as can be seen
below:

$ TMPDIR=$(perl -e 'print "A"x1024') ./bin/array_access
Assertion failed at sljit_src/sljitProtExecAllocator.c:147
Aborted

For non-debug builds, however, this can lead to a memory corruption, by
abusing the fact that we change a trailing '/' to '\0' later on. With a
sufficiently high enough value for 'tmp_name_len' this can corrupt stack
frames up in the call chain.

Fix this by setting 'tmp_name_len' only if value it is based on is found
to be valid -- just like it was prior to commit 98323bd.

Fixes: 98323bd ("protexec: refactor create_tempfile() (#37)")
Signed-off-by: Mathias Krause <[email protected]>

sljit_src/sljitProtExecAllocator.c

@@ -107,6 +107,7 @@ static SLJIT_INLINE int create_tempfile(void)
 	int fd;
 	char tmp_name[256];
 	size_t tmp_name_len = 0;
+	size_t tmp_len;
 	char *dir;
 	struct stat st;
 #if defined(SLJIT_SINGLE_THREADED) && SLJIT_SINGLE_THREADED
@@ -125,18 +126,22 @@ static SLJIT_INLINE int create_tempfile(void)
 	dir = secure_getenv("TMPDIR");
 
 	if (dir) {
-		tmp_name_len = strlen(dir);
-		if (tmp_name_len > 0 && tmp_name_len < sizeof(tmp_name)) {
-			if ((stat(dir, &st) == 0) && S_ISDIR(st.st_mode))
+		tmp_len = strlen(dir);
+		if (tmp_len > 0 && tmp_len < sizeof(tmp_name)) {
+			if ((stat(dir, &st) == 0) && S_ISDIR(st.st_mode)) {
 				strcpy(tmp_name, dir);
+				tmp_name_len = tmp_len;
+			}
 		}
 	}
 
 #ifdef P_tmpdir
 	if (!tmp_name_len) {
-		tmp_name_len = strlen(P_tmpdir);
-		if (tmp_name_len > 0 && tmp_name_len < sizeof(tmp_name))
+		tmp_len = strlen(P_tmpdir);
+		if (tmp_len > 0 && tmp_len < sizeof(tmp_name)) {
 			strcpy(tmp_name, P_tmpdir);
+			tmp_name_len = tmp_len;
+		}
 	}
 #endif
 	if (!tmp_name_len) {