Re: ubifs, race between link and unlink/rename?

[Artem Bityutskiy <dedekind@infradead.org>]

On Thu, 2009-05-14 at 01:31 +0900, hooanon05@yahoo.co.jp wrote:
> Hello,
> 
> Is there a race condition in ubifs?
> Here is a scenario.
> 
> Process A		Process B
> ----------------------+---------------------------
> create("dirA/fileA"); |
> unlink("dirA/fileA"); |	link("dirA/fileA", "dirB/fileB");
> 		      | unlink("dirB/fileB");
> ----------------------+---------------------------

From: Hunter Adrian <adrian.hunter@nokia.com>
Date: Thu, 14 May 2009 06:32:30 +0200
Subject: [PATCH] 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>
---
 fs/ubifs/dir.c |   19 +++++++++++++++++++
 1 files changed, 19 insertions(+), 0 deletions(-)

diff --git a/fs/ubifs/dir.c b/fs/ubifs/dir.c
index f55d523..552fb01 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;
-- 
1.6.0.6

-- 
Best regards,
Artem Bityutskiy (Битюцкий Артём)

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