linux-kernel - Re: [PATCH 0/3] SG_IO command filtering via sysfs

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Date:   Fri, 16 Nov 2018 10:45:11 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     "Theodore Y. Ts'o" <tytso@....edu>, linux-kernel@...r.kernel.org,
        linux-scsi@...r.kernel.org, Hannes Reinecke <hare@...e.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        James Bottomley <James.Bottomley@...senpartnership.com>
Subject: Re: [PATCH 0/3] SG_IO command filtering via sysfs

On 16/11/18 10:32, Christoph Hellwig wrote:
> On Mon, Nov 12, 2018 at 11:17:29AM +0100, Paolo Bonzini wrote:
>>> Well, that's what we have the security_file_ioctl() LSM hook for so that
>>> your security model can arbitrate access to ioctls.
>>
>> Doesn't that have TOC-TOU races by design?
> 
> If you want to look at the command - yes.  If you just want to filter
> read vs write vs ioctl, no.

Yeah, but looking at the command is what Ted wants.  The thing that we
did in RHEL was a single sysfs bool that allows unfiltered access,
because it was sort of enough and made the delta very small.  But for
upstream I want to do it right, even if that means learning all that
new-fangled BPF stuff. :)

>> Also, what about SG_IO giving write access to files that are only opened
>> read-only (and only have read permissions)?
> 
> Allowing SG_IO on read-only permissions sounds like a reall bad idea,
> filtering or not.

I would even agree, however it's allowed right now and I would be
surprised if no one was relying on it in good faith ("I'm just doing an
INQUIRY, why do I need to open O_RDWR").  And indeed:

$ sudo chmod a+r /dev/sda
$ strace -e openat sg_inq /dev/sda
openat(AT_FDCWD, "/dev/sda", O_RDONLY|O_NONBLOCK) = 3
                             ^^^^^^^^

So it would be a regression.

Paolo