Re: [PATCH v6 02/16] chardev: introduce cdev_get_by_path() - Greg Kroah-Hartman

From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: Sagi Grimberg <sagi@grimberg.me>
Cc: Logan Gunthorpe <logang@deltatee.com>,
	linux-kernel@vger.kernel.org, linux-nvme@lists.infradead.org,
	linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	Christoph Hellwig <hch@lst.de>, Keith Busch <kbusch@kernel.org>,
	Jens Axboe <axboe@fb.com>,
	Chaitanya Kulkarni <Chaitanya.Kulkarni@wdc.com>,
	Max Gurtovoy <maxg@mellanox.com>,
	Stephen Bates <sbates@raithlin.com>,
	Alexander Viro <viro@zeniv.linux.org.uk>
Subject: Re: [PATCH v6 02/16] chardev: introduce cdev_get_by_path()
Date: Thu, 25 Jul 2019 21:43:12 +0200	[thread overview]
Message-ID: <20190725194312.GA13090@kroah.com> (raw)
In-Reply-To: <966fa988-de56-effe-dd52-3515ee83629c@grimberg.me>

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