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 6f054f9
(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>

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;
 	}

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.

t/t0066-dir-iterator.sh

@@ -109,7 +109,9 @@ test_expect_success SYMLINKS 'setup dirs with symlinks' '
 	mkdir -p dir5/a/c &&
 	ln -s ../c dir5/a/b/d &&
 	ln -s ../ dir5/a/b/e &&
-	ln -s ../../ dir5/a/b/f
+	ln -s ../../ dir5/a/b/f &&
+
+	ln -s dir4 dir6
 '
 
 test_expect_success SYMLINKS 'dir-iterator should not follow symlinks by default' '
@@ -145,4 +147,27 @@ test_expect_success SYMLINKS 'dir-iterator should follow symlinks w/ follow flag
 	test_cmp expected-follow-sorted-output actual-follow-sorted-output
 '
 
+test_expect_success SYMLINKS 'dir-iterator does not resolve top-level symlinks' '
+	test_must_fail test-tool dir-iterator ./dir6 >out &&
+
+	grep "ENOTDIR" out
+'
+
+test_expect_success SYMLINKS 'dir-iterator resolves top-level symlinks w/ follow flag' '
+	cat >expected-follow-sorted-output <<-EOF &&
+	[d] (a) [a] ./dir6/a
+	[d] (a/f) [f] ./dir6/a/f
+	[d] (a/f/c) [c] ./dir6/a/f/c
+	[d] (b) [b] ./dir6/b
+	[d] (b/c) [c] ./dir6/b/c
+	[f] (a/d) [d] ./dir6/a/d
+	[f] (a/e) [e] ./dir6/a/e
+	EOF
+
+	test-tool dir-iterator --follow-symlinks ./dir6 >out &&
+	sort out >actual-follow-sorted-output &&
+
+	test_cmp expected-follow-sorted-output actual-follow-sorted-output
+'
+
 test_done

t/t5604-clone-reference.sh

@@ -341,4 +341,20 @@ test_expect_success SYMLINKS 'clone repo with symlinked or unknown files at obje
 	test_must_be_empty T--shared.objects-symlinks.raw
 '
 
+test_expect_success SYMLINKS 'clone repo with symlinked objects directory' '
+	test_when_finished "rm -fr sensitive malicious" &&
+
+	mkdir -p sensitive &&
+	echo "secret" >sensitive/file &&
+
+	git init malicious &&
+	rm -fr malicious/.git/objects &&
+	ln -s "$(pwd)/sensitive" ./malicious/.git/objects &&
+
+	test_must_fail git clone --local malicious clone 2>err &&
+
+	test_path_is_missing clone &&
+	grep "failed to start iterator over" err
+'
+
 test_done