linux-kernel - Re: [PATCH v6 02/16] chardev: introduce cdev_get_by

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Message-ID: <20190725194312.GA13090@kroah.com>
Date:   Thu, 25 Jul 2019 21:43:12 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Sagi Grimberg <sagi@...mberg.me>
Cc:     Logan Gunthorpe <logang@...tatee.com>,
        linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Christoph Hellwig <hch@....de>,
        Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...com>,
        Chaitanya Kulkarni <Chaitanya.Kulkarni@....com>,
        Max Gurtovoy <maxg@...lanox.com>,
        Stephen Bates <sbates@...thlin.com>,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v6 02/16] chardev: introduce cdev_get_by_path()

On Thu, Jul 25, 2019 at 12:37:11PM -0700, Sagi Grimberg wrote:
> 
> > > > > > > Why do you have a "string" within the kernel and are not using the
> > > > > > > normal open() call from userspace on the character device node on the
> > > > > > > filesystem in your namespace/mount/whatever?
> > > > > > 
> > > > > > NVMe-OF is configured using configfs. The target is specified by the
> > > > > > user writing a path to a configfs attribute. This is the way it works
> > > > > > today but with blkdev_get_by_path()[1]. For the passthru code, we need
> > > > > > to get a nvme_ctrl instead of a block_device, but the principal is the same.
> > > > > 
> > > > > Why isn't a fd being passed in there instead of a random string?
> > > > 
> > > > I wouldn't know the answer to this but I assume because once we decided
> > > > to use configfs, there was no way for the user to pass the kernel an fd.
> > > 
> > > That's definitely not changing. But this is not different than how we
> > > use the block device or file configuration, this just happen to need the
> > > nvme controller chardev now to issue I/O.
> > 
> > So, as was kind of alluded to in another part of the thread, what are
> > you doing about permissions?  It seems that any user/group permissions
> > are out the window when you have the kernel itself do the opening of the
> > char device, right?  Why is that ok?  You can pass it _any_ character
> > device node and away it goes?  What if you give it a "wrong" one?  Char
> > devices are very different from block devices this way.
> 
> We could condition any configfs operation on capable(CAP_NET_ADMIN) to
> close that hole for now..

Why that specific permission?

And what about the "pass any random char device name" issue?  What
happens if you pass /dev/random/ as the string?

thanks,

greg k-h