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, 5 insertions, 0 deletions

diff --git a/dir-iterator.h b/dir-iterator.h
index 08229157c6..e3b6ff2800 100644
--- a/dir-iterator.h
+++ b/dir-iterator.h
@@ -61,6 +61,11 @@
  *   not the symlinks themselves, which is the default behavior. Broken
  *   symlinks are ignored.
  *
+ *   Note: setting DIR_ITERATOR_FOLLOW_SYMLINKS affects resolving the
+ *   starting path as well (e.g., attempting to iterate starting at a
+ *   symbolic link pointing to a directory without FOLLOW_SYMLINKS will
+ *   result in an error).
+ *
  * Warning: circular symlinks are also followed when
  * DIR_ITERATOR_FOLLOW_SYMLINKS is set. The iteration may end up with
  * an ELOOP if they happen and DIR_ITERATOR_PEDANTIC is set.