dir-iterator: prevent top-level symlinks without FOLLOW_SYMLINKS

When using the dir_iterator API, we first stat(2) the base path, and
then use that as a starting point to enumerate the directory's contents.

If the directory contains symbolic links, we will immediately die() upon
encountering them without the `FOLLOW_SYMLINKS` flag. The same is not
true when resolving the top-level directory, though.

As explained in a previous commit, this oversight in 6f054f9fb3
(builtin/clone.c: disallow `--local` clones with symlinks, 2022-07-28)
can be used as an attack vector to include arbitrary files on a victim's
filesystem from outside of the repository.

Prevent resolving top-level symlinks unless the FOLLOW_SYMLINKS flag is
given, which will cause clones of a repository with a symlink'd
"$GIT_DIR/objects" directory to fail.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 9 insertions, 4 deletions

diff --git a/dir-iterator.c b/dir-iterator.c
index b17e9f970a..3764dd81a1 100644
--- a/dir-iterator.c
+++ b/dir-iterator.c
@@ -203,7 +203,7 @@ struct dir_iterator *dir_iterator_begin(const char *path, unsigned int flags)
 {
 	struct dir_iterator_int *iter = xcalloc(1, sizeof(*iter));
 	struct dir_iterator *dir_iterator = &iter->base;
-	int saved_errno;
+	int saved_errno, err;
 
 	strbuf_init(&iter->base.path, PATH_MAX);
 	strbuf_addstr(&iter->base.path, path);
@@ -213,10 +213,15 @@ struct dir_iterator *dir_iterator_begin(const char *path, unsigned int flags)
 	iter->flags = flags;
 
 	/*
-	 * Note: stat already checks for NULL or empty strings and
-	 * inexistent paths.
+	 * Note: stat/lstat already checks for NULL or empty strings and
+	 * nonexistent paths.
 	 */
-	if (stat(iter->base.path.buf, &iter->base.st) < 0) {
+	if (iter->flags & DIR_ITERATOR_FOLLOW_SYMLINKS)
+		err = stat(iter->base.path.buf, &iter->base.st);
+	else
+		err = lstat(iter->base.path.buf, &iter->base.st);
+
+	if (err < 0) {
 		saved_errno = errno;
 		goto error_out;
 	}