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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>
CC: <stable@vger.kernel.org>
Signed-off-by: Steve French <smfrench@gmail.com>
e1e9bda22d