configfs: Fix deadlock with racing rmdir() and rename()

This patch fixes the deadlock between racing sys_rename() and configfs_rmdir().

The idea is to avoid locking i_mutexes of default groups in
configfs_detach_prep(), and rely instead on the new configfs_dirent_lock to
protect against configfs_dirent's linkage mutations. To ensure that an mkdir()
racing with rmdir() will not create new items in a to-be-removed default group,
we make configfs_new_dirent() check for the CONFIGFS_USET_DROPPING flag right
before linking the new dirent, and return error if the flag is set. This makes
racing mkdir()/symlink()/dir_open() fail in places where errors could already
happen, resp. in (attach_item()|attach_group())/create_link()/new_dirent().

configfs_depend() remains safe since it locks all the path from configfs root,
and is thus mutually exclusive with rmdir().

An advantage of this is that now detach_groups() unconditionnaly takes the
default groups i_mutex, which makes it more consistent with populate_groups().

Signed-off-by: Louis Rilling <Louis.Rilling@kerlabs.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>

1 files changed, 25 insertions, 20 deletions

diff --git a/fs/configfs/dir.c b/fs/configfs/dir.c
index 918a332babfe..d5b5985716ba 100644
--- a/fs/configfs/dir.c
+++ b/fs/configfs/dir.c
@@ -43,6 +43,10 @@ DECLARE_RWSEM(configfs_rename_sem);
  * and configfs_dirent_lock locked, in that order.
  * This allows one to safely traverse configfs_dirent trees and symlinks without
  * having to lock inodes.
+ *
+ * Protects setting of CONFIGFS_USET_DROPPING: checking the flag
+ * unlocked is not reliable unless in detach_groups() called from
+ * rmdir()/unregister() and from configfs_attach_group()
  */
 DEFINE_SPINLOCK(configfs_dirent_lock);
 
@@ -91,6 +95,11 @@ static struct configfs_dirent *configfs_new_dirent(struct configfs_dirent * pare
         INIT_LIST_HEAD(&sd->s_children);
         sd->s_element = element;
         spin_lock(&configfs_dirent_lock);
+        if (parent_sd->s_type & CONFIGFS_USET_DROPPING) {
+                spin_unlock(&configfs_dirent_lock);
+                kmem_cache_free(configfs_dir_cachep, sd);
+                return ERR_PTR(-ENOENT);
+        }
         list_add(&sd->s_sibling, &parent_sd->s_children);
         spin_unlock(&configfs_dirent_lock);
 
@@ -349,11 +358,11 @@ static struct dentry * configfs_lookup(struct inode *dir,
 
 /*
  * Only subdirectories count here.  Files (CONFIGFS_NOT_PINNED) are
- * attributes and are removed by rmdir().  We recurse, taking i_mutex
- * on all children that are candidates for default detach.  If the
- * result is clean, then configfs_detach_group() will handle dropping
- * i_mutex.  If there is an error, the caller will clean up the i_mutex
- * holders via configfs_detach_rollback().
+ * attributes and are removed by rmdir().  We recurse, setting
+ * CONFIGFS_USET_DROPPING on all children that are candidates for
+ * default detach.
+ * If there is an error, the caller will reset the flags via
+ * configfs_detach_rollback().
  */
 static int configfs_detach_prep(struct dentry *dentry)
 {
@@ -370,8 +379,7 @@ static int configfs_detach_prep(struct dentry *dentry)
                 if (sd->s_type & CONFIGFS_NOT_PINNED)
                         continue;
                 if (sd->s_type & CONFIGFS_USET_DEFAULT) {
-                        mutex_lock(&sd->s_dentry->d_inode->i_mutex);
-                        /* Mark that we've taken i_mutex */
+                        /* Mark that we're trying to drop the group */
                         sd->s_type |= CONFIGFS_USET_DROPPING;
 
                         /*
@@ -392,7 +400,7 @@ out:
 }
 
 /*
- * Walk the tree, dropping i_mutex wherever CONFIGFS_USET_DROPPING is
+ * Walk the tree, resetting CONFIGFS_USET_DROPPING wherever it was
  * set.
  */
 static void configfs_detach_rollback(struct dentry *dentry)
@@ -403,11 +411,7 @@ static void configfs_detach_rollback(struct dentry *dentry)
         list_for_each_entry(sd, &parent_sd->s_children, s_sibling) {
                 if (sd->s_type & CONFIGFS_USET_DEFAULT) {
                         configfs_detach_rollback(sd->s_dentry);
-
-                        if (sd->s_type & CONFIGFS_USET_DROPPING) {
-                                sd->s_type &= ~CONFIGFS_USET_DROPPING;
-                                mutex_unlock(&sd->s_dentry->d_inode->i_mutex);
-                        }
+                        sd->s_type &= ~CONFIGFS_USET_DROPPING;
                 }
         }
 }
@@ -486,16 +490,12 @@ static void detach_groups(struct config_group *group)
 
                 child = sd->s_dentry;
 
+                mutex_lock(&child->d_inode->i_mutex);
+
                 configfs_detach_group(sd->s_element);
                 child->d_inode->i_flags |= S_DEAD;
 
-                /*
-                 * From rmdir/unregister, a configfs_detach_prep() pass
-                 * has taken our i_mutex for us.  Drop it.
-                 * From mkdir/register cleanup, there is no sem held.
-                 */
-                if (sd->s_type & CONFIGFS_USET_DROPPING)
-                        mutex_unlock(&child->d_inode->i_mutex);
+                mutex_unlock(&child->d_inode->i_mutex);
 
                 d_delete(child);
                 dput(child);
@@ -1181,12 +1181,15 @@ static int configfs_rmdir(struct inode *dir, struct dentry *dentry)
                 return -EINVAL;
         }
 
+        spin_lock(&configfs_dirent_lock);
         ret = configfs_detach_prep(dentry);
         if (ret) {
                 configfs_detach_rollback(dentry);
+                spin_unlock(&configfs_dirent_lock);
                 config_item_put(parent_item);
                 return ret;
         }
+        spin_unlock(&configfs_dirent_lock);
 
         /* Get a working ref for the duration of this function */
         item = configfs_get_config_item(dentry);
@@ -1476,9 +1479,11 @@ void configfs_unregister_subsystem(struct configfs_subsystem *subsys)
         mutex_lock_nested(&configfs_sb->s_root->d_inode->i_mutex,
                           I_MUTEX_PARENT);
         mutex_lock_nested(&dentry->d_inode->i_mutex, I_MUTEX_CHILD);
+        spin_lock(&configfs_dirent_lock);
         if (configfs_detach_prep(dentry)) {
                 printk(KERN_ERR "configfs: Tried to unregister non-empty subsystem!\n");
         }
+        spin_unlock(&configfs_dirent_lock);
         configfs_detach_group(&group->cg_item);
         dentry->d_inode->i_flags |= S_DEAD;
         mutex_unlock(&dentry->d_inode->i_mutex);