[PATCH 3.2 147/221] cifs: fix use-after-free bug in find_writable_file

Ben Hutchings
2015-05-05T02:11:42Z

From: David Disseldorp <ddiss@suse.de>

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@suse.de>
Reviewed-by: Jeff Layton <jlayton@samba.org>
Signed-off-by: Steve French <smfrench@gmail.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
fs/cifs/file.c | 1 +
1 file changed, 1 insertion(+)

--- a/fs/cifs/file.c
+++ b/fs/cifs/file.c
@@ -1608,6 +1608,7 @@ refind_writable:
cifsFileInfo_put(inv_file);
spin_lock(&cifs_file_list_lock);
++refind;
+ inv_file = NULL;
goto refind_writable;
}
}