UBIFS: return error if link and unlink race

Consider a scenario when 'vfs_link(dirA/fileA)' and
'vfs_unlink(dirA/fileA, dirB/fileB)' race. 'vfs_link()' does not
lock 'dirA->i_mutex', so this is possible. Both of the functions
lock 'fileA->i_mutex' though. Suppose 'vfs_unlink()' wins, and takes
'fileA->i_mutex' mutex first. Suppose 'fileA->i_nlink' is 1. In this
case 'ubifs_unlink()' will drop the last reference, and put 'inodeA'
to the list of orphans. After this, 'vfs_link()' will link
'dirB/fileB' to 'inodeA'. Thir is a problem because, for example,
the subsequent 'vfs_unlink(dirB/fileB)' will add the same inode
to the list of orphans.

This problem was reported by J. R. Okajima <hooanon05@yahoo.co.jp>

[Artem: add more comments, amended commit message]

Signed-off-by: Adrian Hunter <adrian.hunter@nokia.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

1 files changed, 19 insertions, 0 deletions

diff --git a/fs/ubifs/dir.c b/fs/ubifs/dir.c
index f55d523c52bb..552fb0111fff 100644
--- a/fs/ubifs/dir.c
+++ b/fs/ubifs/dir.c
@@ -528,6 +528,25 @@ static int ubifs_link(struct dentry *old_dentry, struct inode *dir,
                 inode->i_nlink, dir->i_ino);
         ubifs_assert(mutex_is_locked(&dir->i_mutex));
         ubifs_assert(mutex_is_locked(&inode->i_mutex));
+
+        /*
+         * Return -ENOENT if we've raced with unlink and i_nlink is 0.  Doing
+         * otherwise has the potential to corrupt the orphan inode list.
+         *
+         * Indeed, consider a scenario when 'vfs_link(dirA/fileA)' and
+         * 'vfs_unlink(dirA/fileA, dirB/fileB)' race. 'vfs_link()' does not
+         * lock 'dirA->i_mutex', so this is possible. Both of the functions
+         * lock 'fileA->i_mutex' though. Suppose 'vfs_unlink()' wins, and takes
+         * 'fileA->i_mutex' mutex first. Suppose 'fileA->i_nlink' is 1. In this
+         * case 'ubifs_unlink()' will drop the last reference, and put 'inodeA'
+         * to the list of orphans. After this, 'vfs_link()' will link
+         * 'dirB/fileB' to 'inodeA'. This is a problem because, for example,
+         * the subsequent 'vfs_unlink(dirB/fileB)' will add the same inode
+         * to the list of orphans.
+         */
+         if (inode->i_nlink == 0)
+                 return -ENOENT;
+
         err = dbg_check_synced_i_size(inode);
         if (err)
                 return err;