Date:	Fri, 8 Jul 2011 09:29:16 +0300
From:	Amir Goldstein <amir73il@...il.com>
To:	Allison Henderson <achender@...ux.vnet.ibm.com>
Cc:	Andreas Dilger <adilger@...ger.ca>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH 1/2 v3] EXT4: Secure Delete: Zero out file data

On Fri, Jul 8, 2011 at 4:55 AM, Allison Henderson
<achender@...ux.vnet.ibm.com> wrote:
> On 07/07/2011 05:09 PM, Amir Goldstein wrote:
>>
>> On Thu, Jul 7, 2011 at 11:19 PM, Allison Henderson
>> <achender@...ux.vnet.ibm.com>  wrote:
>>>
>>> On 07/07/2011 12:52 PM, Andreas Dilger wrote:
>>>>
>>>> On 2011-07-07, at 1:05 AM, Amir Goldstein wrote:
>>>>>
>>>>> On Thu, Jul 7, 2011 at 12:05 AM, Allison Henderson
>>>>> <achender@...ux.vnet.ibm.com>    wrote:
>>>>>>
>>>>>> On 07/02/2011 02:33 AM, Amir Goldstein wrote:
>>>>>>>
>>>>>>> On Fri, Jul 1, 2011 at 12:22 AM, Allison Henderson
>>>>>>> <achender@...ux.vnet.ibm.com>     wrote:
>>>>>>>>
>>>>>>>> @@ -4485,6 +4485,14 @@ void ext4_free_blocks(handle_t *handle,
>>>>>>>> struct
>>>>>>>> inode *inode,
>>>>>>>>        ext4_debug("freeing block %llu\n", block);
>>>>>>>>        trace_ext4_free_blocks(inode, block, count, flags);
>>>>>>>>
>>>>>>>> +       if (flags&     EXT4_FREE_BLOCKS_ZERO) {
>>>>>>>> +               err = sb_issue_zeroout(inode->i_sb, block, count,
>>>>>>>> GFP_NOFS);
>>>>>>>
>>>>>>> But the delete of these blocks in not yet committed,
>>>>>>> so after reboot, you can end up with a non-deleted but zeroed file
>>>>>>> data.
>>>>>>> Is that acceptable? I should think not.
>>>>>>>
>>>>>>> One way around this is a 2-phase unlink/truncate.
>>>>>>> Phase 1: add to orphan list and register a callback on commit
>>>>>>> Phase 2: issue zeroout and free the blocks
>>>>>>>
>>>>>>> This won't work for punch hole, but then again, for punch hole
>>>>>>> it's probably OK to end up with zeroed data, but non-deleted blocks.
>>>>>>> Right?
>>>>>>
>>>>>> Hi, I had a quick question about the orphan list.  I notice that
>>>>>> ext4_ext_truncate and also ext4_ext_punch_hole already have a call to
>>>>>> ext4_orphan_add that happens really early before any calls to free
>>>>>> blocks.
>>>>>>  Does this address your earlier concerns, or is there another reason I
>>>>>> missed?  Thx!
>>>>>
>>>>> It doesn't address the concerns of getting a non-deleted file with
>>>>> zeroed
>>>>> data
>>>>> after crash, because the existence of the inode on the orphan list
>>>>> after
>>>>> crash
>>>>> depends on the transaction that added it to the list being committed.
>>>>> And your patch zeroes the blocks before that transaction is committed.
>>>>>
>>>>> However, the orphan list gives you a very good framework to implement
>>>>> deferred delete (by a kernel thread) as Andreas suggested.
>>>>> Unlink should be simple, because freeing unlinked inode blocks it is
>>>>> anyway
>>>>> deferred till the inode refcount drops to zero.
>>>>
>>>> Right.  The patch that I referenced moved all of the blocks from unlink
>>>> and truncate-to-zero from the current inode to a new temporary inode on
>>>> the
>>>> orphan list (simply copying the i_blocks field + i_block and i_size,
>>>> IIRC,
>>>> and zeroing them on the original inode).
>>>>
>>>>> Truncate is more tricky, because of the truncate shrink/extend
>>>>> requirement
>>>>> (that all data is zeroes after extending the inode's size via truncate
>>>>> system call), so a shrinking-deferred truncate would have to mark all
>>>>> the
>>>>> to-be-deleted extents uninitialized.
>>>>
>>>> It would be possible to do this for partial truncate/punch as well, to
>>>> move whole blocks over to a new inode on the orphan list and zeroing
>>>> only
>>>> the 1 or 2 partial blocks inline.
>>>>
>>>> It should even be possible to leverage the "block migrate" facility used
>>>> by defrag, so that we don't duplicate this code.  That would mean just
>>>> allocating a temp "unlink" inode in the kernel and putting it on the
>>>> orphan
>>>> list (like an open-unlinked file), migrate the selected range of blocks,
>>>> and then zeroing the blocks in the background before unlinking the
>>>> inode.
>>>>
>>>> I don't think that just marking the deleted extents as uninitialized is
>>>> enough, since it would still leave "private" data on disk that could be
>>>> read afterward.  This would also only work for extent-mapped
>>>> filesystems.
>>>>
>>>> There may need to be some work to enable the migrate code on
>>>> block-mapped
>>>> files, if you want to allow secure-delete on those files, but that is
>>>> good
>>>> IMHO since it also means that we could defrag block-mapped files.
>>>>
>>>> Cheers, Andreas
>>>>
>>>
>>> Ah, ok then.  Yes, part of the requirements was to make secure delete
>>> work
>>> for partial truncates, punch hole, and also indexed files.  So that will
>>> save me some time if I can get the migrate routines work.  Thx for the
>>> pointers all!
>>>
>>
>> I realized that there is a basic flaw in the concept of
>> deferred-secure-delete.
>>  From a security point of view, after a crash during a secure-delete,
>> if the file is not there, all its data should have been wiped.
>> Orphan cleanup on the next mount may be done on a system that
>> doesn't respect secure delete.
>> So for real security, the unlink/truncate command cannot return before
>> all data is wiped.
>> The unlink/truncate metadata changes must not even be committed
>> before all data is wiped (or at least part of the data with partial
>> truncate).
>>
>> Amir.
>
>
> I see, so then it sounds like using a background thread to do the zeroing
> would not help us if we have to wait for it complete anyway. Going back to
> the 2 phase approach, this means that we need to do the zero out and then
> the free before we do the orphan list and commit? Just trying to make sure I
> understand things correctly :)
>

Well, that really depends of the precise definition of "secure delete".
If you agree with the "100% secure" interpretation, then your current patch
is "almost" correct.
I can see 2 things that are missing:
1. ext4_unlink() will have to invoke ext4_truncate(0) directly just
like truncate system call does.
This is to prevent an attacker from keeping the protected file open
and preventing freeing
of it's data when the file is removed from the name space.
2. ext4_truncate() currently changes i_disksize first (and adds inode
to orphan list)
and then frees the blocks. for 100% secure delete, you cannot change i_disksize
before zeroing the blocks, so it has to be:
- zeroout range
- change i_disksize and add to orphan list
- free blocks

I don't see how it could be done any other way, but maybe someone else can...

Amir.
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