[PATCH vfs/for-next] vfs: fix vfs_get_single_reconf_super error handling

[Eric Biggers <ebiggers@kernel.org>]

From: Eric Biggers <ebiggers@google.com>

syzbot reported an invalid free in debugfs_release_dentry().  The
reproducer tries to mount debugfs with the 'dirsync' option, which is
not allowed.  The bug is that if reconfigure_super() fails in
vfs_get_super(), deactivate_locked_super() is called, but also
fs_context::root is left non-NULL which causes deactivate_super() to be
called again later.

Fix it by releasing fs_context::root in the error path.

Reported-by: syzbot+5aca688dac0796c56129@syzkaller.appspotmail.com
Fixes: e478b48498a7 ("vfs: Add a single-or-reconfig keying to vfs_get_super()")
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
---
 fs/super.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/super.c b/fs/super.c
index 0f913376fc4c..99195e15be05 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -1194,8 +1194,11 @@ int vfs_get_super(struct fs_context *fc,
 		fc->root = dget(sb->s_root);
 		if (keying == vfs_get_single_reconf_super) {
 			err = reconfigure_super(fc);
-			if (err < 0)
+			if (err < 0) {
+				dput(fc->root);
+				fc->root = NULL;
 				goto error;
+			}
 		}
 	}
 
-- 
2.23.0