Merge pull request #439: sparse index: fix use-after-free bug in cache_tree_verify()

This is a copy of [the v2 patch sent by Philip Wood](https://lore.kernel.org/git/pull.1053.v2.git.1633600244854.gitgitgadget@gmail.com/T/#u). This fixes a possible segfault when a `git rebase --apply` creates a full cache tree in a sparse index.

There is a slight change from Philip's patch that makes the test more robust to verbose output.

Author: derrickstolee

cache-tree.c

@@ -923,32 +923,48 @@ static void verify_one_sparse(struct repository *r,
 		    path->buf);
 }
 
-static void verify_one(struct repository *r,
-		       struct index_state *istate,
-		       struct cache_tree *it,
-		       struct strbuf *path)
+/*
+ * Returns:
+ *  0 - Verification completed.
+ *  1 - Restart verification - a call to ensure_full_index() freed the cache
+ *      tree that is being verified and verification needs to be restarted from
+ *      the new toplevel cache tree.
+ */
+static int verify_one(struct repository *r,
+		      struct index_state *istate,
+		      struct cache_tree *it,
+		      struct strbuf *path)
 {
 	int i, pos, len = path->len;
 	struct strbuf tree_buf = STRBUF_INIT;
 	struct object_id new_oid;
 
 	for (i = 0; i < it->subtree_nr; i++) {
 		strbuf_addf(path, "%s/", it->down[i]->name);
-		verify_one(r, istate, it->down[i]->cache_tree, path);
+		if (verify_one(r, istate, it->down[i]->cache_tree, path))
+			return 1;
 		strbuf_setlen(path, len);
 	}
 
 	if (it->entry_count < 0 ||
 	    /* no verification on tests (t7003) that replace trees */
 	    lookup_replace_object(r, &it->oid) != &it->oid)
-		return;
+		return 0;
 
 	if (path->len) {
+		/*
+		 * If the index is sparse and the cache tree is not
+		 * index_name_pos() may trigger ensure_full_index() which will
+		 * free the tree that is being verified.
+		 */
+		int is_sparse = istate->sparse_index;
 		pos = index_name_pos(istate, path->buf, path->len);
+		if (is_sparse && !istate->sparse_index)
+			return 1;
 
 		if (pos >= 0) {
 			verify_one_sparse(r, istate, it, path, pos);
-			return;
+			return 0;
 		}
 
 		pos = -pos - 1;
@@ -996,6 +1012,7 @@ static void verify_one(struct repository *r,
 		    oid_to_hex(&new_oid), oid_to_hex(&it->oid));
 	strbuf_setlen(path, len);
 	strbuf_release(&tree_buf);
+	return 0;
 }
 
 void cache_tree_verify(struct repository *r, struct index_state *istate)
@@ -1004,6 +1021,10 @@ void cache_tree_verify(struct repository *r, struct index_state *istate)
 
 	if (!istate->cache_tree)
 		return;
-	verify_one(r, istate, istate->cache_tree, &path);
+	if (verify_one(r, istate, istate->cache_tree, &path)) {
+		strbuf_reset(&path);
+		if (verify_one(r, istate, istate->cache_tree, &path))
+			BUG("ensure_full_index() called twice while verifying cache tree");
+	}
 	strbuf_release(&path);
 }

t/t1092-sparse-checkout-compatibility.sh

@@ -890,7 +890,7 @@ test_expect_success 'read-tree --merge with directory-file conflicts' '
 test_expect_success 'merge, cherry-pick, and rebase' '
 	init_repos &&
 
-	for OPERATION in "merge -m merge" cherry-pick rebase
+	for OPERATION in "merge -m merge" cherry-pick "rebase -q --apply" "rebase --merge"
 	do
 		test_all_match git checkout -B temp update-deep &&
 		test_all_match git $OPERATION update-folder1 &&