linux-ext4 - Re: [PATCH 1/2 v3] EXT4: Secure Delete: Zero out file data

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Message-ID: <4E1699BD.3070208@redhat.com>
Date:	Fri, 08 Jul 2011 06:46:37 +0100
From:	Ric Wheeler <rwheeler@...hat.com>
To:	Andreas Dilger <adilger@...ger.ca>
CC:	Amir Goldstein <amir73il@...il.com>,
	Allison Henderson <achender@...ux.vnet.ibm.com>,
	linux-ext4@...r.kernel.org
Subject: Re: [PATCH 1/2 v3] EXT4: Secure Delete: Zero out file data

On 07/08/2011 03:46 AM, Andreas Dilger wrote:
> On 2011-07-07, at 6:09 PM, Amir Goldstein wrote:
>> On Thu, Jul 7, 2011 at 11:19 PM, Allison Henderson
>> <achender@...ux.vnet.ibm.com>  wrote:
>>> 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.
> I don't necessarily agree.  People who are using secure delete don't
> necessarily want their performance to go into the toilet, which is what
> would happen if each unlink/zero happened sequentially.  It is far more
> efficient to submit a large batch of unlink/zero operations and then do
> an sync() or fsync() at the end.  This allows multiple journal operations
> to be coalesced (e.g. unlinks from directory) and block zero requests to
> be merged.
>

I think that secure delete should be synchronous, slow and 100% crash proof. 
When you unlink a "secure delete" file, you should not be able to access the raw 
disk and find the data anywhere that used to be there.

This is an unusual operation and *most* users do not need it or care. Definitely 
not something that should happen by default.

In my experience having helped provide this feature in the past, people usually 
ended up doing something in user space (i.e., move the file to a directory where 
it will be shredded and then unlinked). This is similar to the scheme you 
outlined for a kernel implementation, but has the advantage that you could still 
see the original file name somewhere if you wanted to check to see if it was 
gone....

Ric



>> 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).
> That depends on the user, I think.  If someone does "rm -r {dir}" it may
> be better that the files are removed from the namespace more quickly, and
> secure deleted (in a batch) more quickly, than having "rm" wait for each
> file to be unlinked and maybe leave files in the namespace that didn't
> even get a chance to be unlinked.
>
> Cheers, Andreas
>
>
>
>
>
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