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 08:31:33 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Ric Wheeler <ricwheeler@...il.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

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.

> 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
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