linux-kernel - Re: [Ping^3] Re: [PATCH] sg_io: allow UNMAP and WRITE SAME without CAP_SYS

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Date:	Thu, 06 Sep 2012 07:31:45 -0400
From:	Ric Wheeler <ricwheeler@...il.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
CC:	axboe@...nel.dk, Mike Snitzer <snitzer@...hat.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org
Subject: Re: [Ping^3] Re: [PATCH] sg_io: allow UNMAP and WRITE SAME without
 CAP_SYS_RAWIO

On 09/06/2012 02:31 AM, Paolo Bonzini wrote:
> Il 05/09/2012 22:18, Ric Wheeler ha scritto:
>> Hi Paolo,
>>
>> Both of these commands are destructive. WRITE_SAME (if done without the
>> discard bits set) can also take a very long time to be destructive and
>> tie up the storage.
> FORMAT_UNIT has the same characteristics and yet it is allowed (btw, I
> don't think WRITE SAME slowness is limited to the case where a real
> write is requested; discarding can be just as slow).
>
> Also, the two new commands are anyway restricted to programs that have
> write access to the disk.  If you have read-only access, you won't be
> able to issue any destructive command (there is one exception, START
> STOP UNIT is allowed even with read-only capability and is somewhat
> destructive).
>
> Honestly, the only reason why these two commands weren't included, is
> that the current whitelist is heavily tailored towards CD/DVD burning.

Hi Paolo,

I assume that FORMAT_UNIT is for CD/DVD needs - not sure what a S-ATA disk would 
do with that. If it is destructive, we should probably think about how to make 
it more secure and see how many applications we would break.

>
>> I think that restricting them to CAP_SYS_RAWIO seems reasonable - better
>> to vet and give the appropriate apps the needed capability than to
>> widely open up the safety check?
> CAP_SYS_RAWIO is so wide in its scope, that anything that requires it is
> insecure.
>
> Paolo

I don't see allowing anyone who can open the device to zero the data as better 
though :)

Ric



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