locks: Fix potential OOPS in generic_setlease()

This code is run under lock_kernel(), which is dropped during
sleeping operations, so the following race is possible:

CPU1:                                CPU2:
  vfs_setlease();                    vfs_setlease();
  lock_kernel();
                                     lock_kernel(); /* spin */
  generic_setlease():
    ...
    for (before = ...)
    /* here we found some lease after
     * which we will insert the new one
     */
    fl = locks_alloc_lock();
    /* go to sleep in this allocation and
     * drop the BKL
     */
                                     generic_setlease():
                                       ...
                                       for (before = ...)
                                       /* here we find the "before" pointing
                                        * at the one we found on CPU1
                                        */
                                      ->fl_change(my_before, arg);
                                              lease_modify();
                                                     locks_free_lock();
                                                     /* and we freed it */
                                     ...
                                     unlock_kernel();
   locks_insert_lock(before, fl);
   /* OOPS! We have just tried to add the lease
    * at the tail of already removed one
    */

The similar races are already handled in other code - all the
allocations are performed before any checks/updates.

Thanks to Kamalesh Babulal for testing and for a bug report on an
earlier version.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Cc: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>

1 files changed, 12 insertions, 9 deletions

diff --git a/fs/locks.c b/fs/locks.c
index 282b6c11670a..43dbc7f566fa 100644
--- a/fs/locks.c
+++ b/fs/locks.c
@@ -1343,6 +1343,7 @@ int fcntl_getlease(struct file *filp)
 int generic_setlease(struct file *filp, long arg, struct file_lock **flp)
 {
         struct file_lock *fl, **before, **my_before = NULL, *lease;
+        struct file_lock *new_fl = NULL;
         struct dentry *dentry = filp->f_path.dentry;
         struct inode *inode = dentry->d_inode;
         int error, rdlease_count = 0, wrlease_count = 0;
@@ -1369,6 +1370,11 @@ int generic_setlease(struct file *filp, long arg, struct file_lock **flp)
                 || (atomic_read(&inode->i_count) > 1)))
                 goto out;
 
+        error = -ENOMEM;
+        new_fl = locks_alloc_lock();
+        if (new_fl == NULL)
+                goto out;
+
         /*
          * At this point, we know that if there is an exclusive
          * lease on this file, then we hold it on this filp
@@ -1411,18 +1417,15 @@ int generic_setlease(struct file *filp, long arg, struct file_lock **flp)
         if (!leases_enable)
                 goto out;
 
-        error = -ENOMEM;
-        fl = locks_alloc_lock();
-        if (fl == NULL)
-                goto out;
-
-        locks_copy_lock(fl, lease);
+        locks_copy_lock(new_fl, lease);
+        locks_insert_lock(before, new_fl);
 
-        locks_insert_lock(before, fl);
+        *flp = new_fl;
+        return 0;
 
-        *flp = fl;
-        error = 0;
 out:
+        if (new_fl != NULL)
+                locks_free_lock(new_fl);
         return error;
 }
 EXPORT_SYMBOL(generic_setlease);