list-objects: respect max_allowed_tree_depth

The tree traversal in list-objects.c, which is used by "rev-list
--objects", etc, uses recursion and may run out of stack space. Let's
teach it about the new core.maxTreeDepth config option.

We unfortunately can't return an error here, as this code doesn't
produce an error return at all. We'll die() instead, which matches the
behavior when we see an otherwise broken tree.

Note that this will also generally reject such deep trees from entering
the repository from a fetch or push, due to the use of rev-list in the
connectivity check. But it's not foolproof! We stop traversing when we
see an UNINTERESTING object, and the connectivity check marks existing
ref tips as UNINTERESTING. So imagine commit X has a tree
with maximum depth N. If you then create a new commit Y with a tree
entry "Y:subdir" that points to "X^{tree}", then the depth of Y will be
N+1. But a connectivity check running "git rev-list --objects Y --not X"
won't realize that; it will stop traversing at X^{tree}, since that was
already reachable.

So this will stop naive pushes of too-deep trees, but not carefully
crafted malicious ones. Doing it robustly and efficiently would require
caching the maximum depth of each tree (i.e., the longest path to any
leaf entry). That's much more complex and not strictly needed. If each
recursive algorithm limits itself already, then that's sufficient.
Blocking the objects from entering the repo would be a nice
belt-and-suspenders addition, but it's not worth the extra cost.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

list-objects.c

@@ -14,6 +14,7 @@
 #include "packfile.h"
 #include "object-store-ll.h"
 #include "trace.h"
+#include "environment.h"
 
 struct traversal_context {
 	struct rev_info *revs;
@@ -21,6 +22,7 @@ struct traversal_context {
 	show_commit_fn show_commit;
 	void *show_data;
 	struct filter *filter;
+	int depth;
 };
 
 static void show_commit(struct traversal_context *ctx,
@@ -118,7 +120,9 @@ static void process_tree_contents(struct traversal_context *ctx,
 				    entry.path, oid_to_hex(&tree->object.oid));
 			}
 			t->object.flags |= NOT_USER_GIVEN;
+			ctx->depth++;
 			process_tree(ctx, t, base, entry.path);
+			ctx->depth--;
 		}
 		else if (S_ISGITLINK(entry.mode))
 			; /* ignore gitlink */
@@ -156,6 +160,9 @@ static void process_tree(struct traversal_context *ctx,
 	    !revs->include_check_obj(&tree->object, revs->include_check_data))
 		return;
 
+	if (ctx->depth > max_allowed_tree_depth)
+		die("exceeded maximum allowed tree depth");
+
 	failed_parse = parse_tree_gently(tree, 1);
 	if (failed_parse) {
 		if (revs->ignore_missing_links)
@@ -349,6 +356,7 @@ static void traverse_non_commits(struct traversal_context *ctx,
 		if (!path)
 			path = "";
 		if (obj->type == OBJ_TREE) {
+			ctx->depth = 0;
 			process_tree(ctx, (struct tree *)obj, base, path);
 			continue;
 		}

t/t6700-tree-depth.sh

@@ -72,4 +72,13 @@ test_expect_success 'default limit for ls-tree fails gracefully' '
 	test_must_fail git ls-tree -r big >/dev/null
 '
 
+test_expect_success 'limit recursion of rev-list --objects' '
+	git $small_ok rev-list --objects small >/dev/null &&
+	test_must_fail git $small_no rev-list --objects small >/dev/null
+'
+
+test_expect_success 'default limit for rev-list fails gracefully' '
+	test_must_fail git rev-list --objects big >/dev/null
+'
+
 test_done