[3.16.y-ckt stable] Patch "cifs: fix use-after-free bug in find_writable_file" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    cifs: fix use-after-free bug in find_writable_file

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.16.y-queue

This patch is scheduled to be released in version 3.16.7-ckt10.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.16.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From dbd777a1af27ec690f4e34d80a6e32d5f27eea62 Mon Sep 17 00:00:00 2001
From: David Disseldorp <ddiss at suse.de>
Date: Fri, 13 Mar 2015 14:20:29 +0100
Subject: cifs: fix use-after-free bug in find_writable_file

commit e1e9bda22d7ddf88515e8fe401887e313922823e upstream.

Under intermittent network outages, find_writable_file() is susceptible
to the following race condition, which results in a user-after-free in
the cifs_writepages code-path:

Thread 1                                        Thread 2
========                                        ========

inv_file = NULL
refind = 0
spin_lock(&cifs_file_list_lock)

// invalidHandle found on openFileList

inv_file = open_file
// inv_file->count currently 1

cifsFileInfo_get(inv_file)
// inv_file->count = 2

spin_unlock(&cifs_file_list_lock);

cifs_reopen_file()                            cifs_close()
// fails (rc != 0)                            ->cifsFileInfo_put()
                                       spin_lock(&cifs_file_list_lock)
                                       // inv_file->count = 1
                                       spin_unlock(&cifs_file_list_lock)

spin_lock(&cifs_file_list_lock);
list_move_tail(&inv_file->flist,
      &cifs_inode->openFileList);
spin_unlock(&cifs_file_list_lock);

cifsFileInfo_put(inv_file);
->spin_lock(&cifs_file_list_lock)

  // inv_file->count = 0
  list_del(&cifs_file->flist);
  // cleanup!!
  kfree(cifs_file);

  spin_unlock(&cifs_file_list_lock);

spin_lock(&cifs_file_list_lock);
++refind;
// refind = 1
goto refind_writable;

At this point we loop back through with an invalid inv_file pointer
and a refind value of 1. On second pass, inv_file is not overwritten on
openFileList traversal, and is subsequently dereferenced.

Signed-off-by: David Disseldorp <ddiss at suse.de>
Reviewed-by: Jeff Layton <jlayton at samba.org>
Signed-off-by: Steve French <smfrench at gmail.com>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 fs/cifs/file.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/cifs/file.c b/fs/cifs/file.c
index 58dff97aafef..e645b9f4f6a3 100644
--- a/fs/cifs/file.c
+++ b/fs/cifs/file.c
@@ -1821,6 +1821,7 @@ refind_writable:
 			cifsFileInfo_put(inv_file);
 			spin_lock(&cifs_file_list_lock);
 			++refind;
+			inv_file = NULL;
 			goto refind_writable;
 		}
 	}