linux-kernel - Re: [PATCH 2/2] blk-zoned: allow BLKREPORTZONE without CAP_SYS

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Date:   Thu, 3 Jun 2021 16:44:04 +0200
From:   David Sterba <dsterba@...e.cz>
To:     Damien Le Moal <Damien.LeMoal@....com>
Cc:     Niklas Cassel <Niklas.Cassel@....com>,
        Jens Axboe <axboe@...nel.dk>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] blk-zoned: allow BLKREPORTZONE without CAP_SYS_ADMIN

On Thu, Jun 03, 2021 at 11:20:33AM +0000, Damien Le Moal wrote:
> On 2021/06/03 19:07, David Sterba wrote:
> > On Thu, Jun 03, 2021 at 10:00:08AM +0000, Damien Le Moal wrote:
> >> On 2021/06/03 18:54, David Sterba wrote:
> >>> On Mon, May 31, 2021 at 01:54:53PM +0000, Niklas Cassel wrote:
> >>>> From: Niklas Cassel <niklas.cassel@....com>
> >>>>
> >>>> Performing a BLKREPORTZONE operation should be allowed under the same
> >>>> permissions as read(). (read() does not require CAP_SYS_ADMIN).
> >>>>
> >>>> Remove the CAP_SYS_ADMIN requirement, and instead check that the fd was
> >>>> successfully opened with FMODE_READ. This way BLKREPORTZONE will match
> >>>> the access control requirement of read().
> >>>
> >>> Does this mean that a process that does not have read nor write access
> >>> to the device itself (blocks) is capable of reading the zone
> >>> information? Eg. some monitoring tool.
> >>
> >> With this change, to do a report zones, the process will only need to have read
> >> access to the device. And if it has read access, it also means that it can read
> >> the zones content.
> > 
> > Ok, so this is a bit restricting. The zone information is like block
> > device metadata, comparing it to a file that has permissionx 0600 I can
> > see the all the stat info (name, tiemstamps) but can't read the data.
> > 
> > But as the ioctl work, it needs a file descriptor and there's probably
> > no way to separate the permissions to read blocks and just the metadata.
> > For a monitoring/reporting tool this would be useful. Eg. for btrfs it
> > could be part of filesystem status overview regarding full or near-full
> > zones and emitting an early warning or poking some service to start the
> > reclaim.
> 
> You lost me... the change is less restrictive than before because the process
> does not need SYS_CAP_ADMIN anymore. The block device file open is untouched, no
> change. So whatever process could open it before, will still be able to do so as
> is. More processes will be able to do report zones with the change. That is all
> really that changes, so I do not see what potentially breaks, nor how this may
> prevent writing some monitoring tool. Whoever can open the block device file has
> FMODE_READ rights, no ? Am I missing something here ?

I'm not saying the patch is wrong or not doing what it says. What caught
my attention was the fact that the admin capabilities is not that
different from requiring the read permissions. Which for a block device
is not easy to get. Normally block devices have 0660 and group is
'disk', so yeah I can add a monitoring daemon to the group. But that
still would allow it to read all block devices.

So yeah, the patch is making it easier but from my POV it's not that
practical. A more fine grained access control would be needed, or
another way how to read just the zone info. Reading the information from
eg. sysfs has it's own issues.