[PATCH 3/3] ubifs: ensure only one in-memory xattr inode is created

[Hou Tao]

ubifs may create two in-memory inodes for one xattr if
there are concurrent ubifs_xattr_get() and ubifs_xattr_set(),
as show in the following case:

ubifs_xattr_get()              ubifs_xattr_set()
                                 // the first created inode A
                                 // fill inode A
                                 ubifs_new_inode()
                                 ubifs_jnl_update()
                                   // mapping xattr name to inum
                                   ubifs_tnc_add_nm()
                                   // add xattr inode node
                                   ubifs_tnc_add()

  // find inum through xattr name
  ubifs_tnc_lookup_nm()
  iget_xattr()
    ubifs_iget()
      // not found in hash table
      // so create a new inode B
      // and keep it in hash table
      iget_locked()
      // find xattr inode node
      // fill inode B
      ubifs_tnc_lookup
      unlock_new_inode
                                 // inode A is also inserted into
                                 // hash table

If we update the xattr value afterwards, only the values in inode A will
be updated. So when we ty to remove the xattr name, and in the same
time get the xattr name, ubifs_xattr_get() may return the stale value
in inode B, as show in the following case:

ubifs_xattr_get()              ubifs_xattr_remove()

// get xattr inum
ubifs_tnc_lookup_nm()
				// return inode A
				iget_xattr()
				clear_nlink()
				remove_xattr()
				iput()
				  evict()
				    ubifs_evict_inode()
				    remove_inode_hash()
// return inode B
// return a stale xattr value
iget_xattr()

Fix it by moving insert_inode_hash() before ubifs_jnl_update(),
but after the initialization of inode is completed, so only
one inode is created for xattr value.

Signed-off-by: Hou Tao <houtao1 at huawei.com>
---
 fs/ubifs/xattr.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/fs/ubifs/xattr.c b/fs/ubifs/xattr.c
index 82be2c2d2db5..10fcb454bb01 100644
--- a/fs/ubifs/xattr.c
+++ b/fs/ubifs/xattr.c
@@ -133,6 +133,15 @@ static int create_xattr(struct ubifs_info *c, struct inode *host,
 	inode->i_size = ui->ui_size = size;
 	ui->data_len = size;
 
+	/*
+	 * Ensure iget_xattr() in ubifs_xattr_get() will find the inode
+	 * instead of creating a new one.
+	 * The initialization of xattr inode is completed here, so using
+	 * insert_inode_hash() instead of insert_inode_locked(). The
+	 * latter can lead to iget_xattr() return -ESTALE.
+	 */
+	insert_inode_hash(inode);
+
 	mutex_lock(&host_ui->ui_mutex);
 	host->i_ctime = current_time(host);
 	host_ui->xattr_cnt += 1;
@@ -156,7 +165,6 @@ static int create_xattr(struct ubifs_info *c, struct inode *host,
 	mutex_unlock(&host_ui->ui_mutex);
 
 	ubifs_release_budget(c, &req);
-	insert_inode_hash(inode);
 	iput(inode);
 	return 0;
 
-- 
2.25.0.4.g0ad7144999