attr: fix out-of-bounds read with unreasonable amount of patterns

The `struct attr_stack` tracks the stack of all patterns together with
their attributes. When parsing a gitattributes file that has more than
2^31 such patterns though we may trigger multiple out-of-bounds reads on
64 bit platforms. This is because while the `num_matches` variable is an
unsigned integer, we always use a signed integer to iterate over them.

I have not been able to reproduce this issue due to memory constraints
on my systems. But despite the out-of-bounds reads, the worst thing that
can seemingly happen is to call free(3P) with a garbage pointer when
calling `attr_stack_free()`.

Fix this bug by using unsigned integers to iterate over the array. While
this makes the iteration somewhat awkward when iterating in reverse, it
is at least better than knowingly running into an out-of-bounds read.
While at it, convert the call to `ALLOC_GROW` to use `ALLOC_GROW_BY`
instead.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

attr.c

@@ -446,7 +446,7 @@ struct attr_stack {
 
 static void attr_stack_free(struct attr_stack *e)
 {
-	int i;
+	unsigned i;
 	free(e->origin);
 	for (i = 0; i < e->num_matches; i++) {
 		struct match_attr *a = e->attrs[i];
@@ -660,8 +660,8 @@ static void handle_attr_line(struct attr_stack *res,
 	a = parse_attr_line(line, src, lineno, macro_ok);
 	if (!a)
 		return;
-	ALLOC_GROW(res->attrs, res->num_matches + 1, res->alloc);
-	res->attrs[res->num_matches++] = a;
+	ALLOC_GROW_BY(res->attrs, res->num_matches, 1, res->alloc);
+	res->attrs[res->num_matches - 1] = a;
 }
 
 static struct attr_stack *read_attr_from_array(const char **list)
@@ -1025,11 +1025,11 @@ static int fill(const char *path, int pathlen, int basename_offset,
 		struct all_attrs_item *all_attrs, int rem)
 {
 	for (; rem > 0 && stack; stack = stack->prev) {
-		int i;
+		unsigned i;
 		const char *base = stack->origin ? stack->origin : "";
 
-		for (i = stack->num_matches - 1; 0 < rem && 0 <= i; i--) {
-			const struct match_attr *a = stack->attrs[i];
+		for (i = stack->num_matches; 0 < rem && 0 < i; i--) {
+			const struct match_attr *a = stack->attrs[i - 1];
 			if (a->is_macro)
 				continue;
 			if (path_matches(path, pathlen, basename_offset,
@@ -1060,9 +1060,9 @@ static void determine_macros(struct all_attrs_item *all_attrs,
 			     const struct attr_stack *stack)
 {
 	for (; stack; stack = stack->prev) {
-		int i;
-		for (i = stack->num_matches - 1; i >= 0; i--) {
-			const struct match_attr *ma = stack->attrs[i];
+		unsigned i;
+		for (i = stack->num_matches; i > 0; i--) {
+			const struct match_attr *ma = stack->attrs[i - 1];
 			if (ma->is_macro) {
 				int n = ma->u.attr->attr_nr;
 				if (!all_attrs[n].macro) {