linux-kernel - [PATCH 3.12 40/83] cifs: fix use-after-free bug in find_writable

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Date:	Mon, 27 Apr 2015 23:32:11 +0200
From:	Jiri Slaby <jslaby@...e.cz>
To:	stable@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, David Disseldorp <ddiss@...e.de>,
	Steve French <smfrench@...il.com>, Jiri Slaby <jslaby@...e.cz>
Subject: [PATCH 3.12 40/83] cifs: fix use-after-free bug in find_writable_file

From: David Disseldorp <ddiss@...e.de>

3.12-stable review patch.  If anyone has any objections, please let me know.

===============

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@...e.de>
Reviewed-by: Jeff Layton <jlayton@...ba.org>
Signed-off-by: Steve French <smfrench@...il.com>
Signed-off-by: Jiri Slaby <jslaby@...e.cz>
---
 fs/cifs/file.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/cifs/file.c b/fs/cifs/file.c
index ead2473f6839..381e60e6ef92 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;
 		}
 	}
-- 
2.3.5

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