linux-kernel - Re: [PATCH] udevadm-info: Don't access sysfs 'resource<N>' files

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Date:	Mon, 18 Mar 2013 10:24:40 -0600
From:	Alex Williamson <alex.williamson@...hat.com>
To:	Kay Sievers <kay@...y.org>
Cc:	Greg KH <gregkh@...uxfoundation.org>,
	Myron Stowe <mstowe@...hat.com>,
	Myron Stowe <myron.stowe@...hat.com>,
	linux-hotplug@...r.kernel.org, linux-pci@...r.kernel.org,
	yuxiangl@...vell.com, yxlraid@...il.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] udevadm-info: Don't access sysfs 'resource<N>' files

On Sun, 2013-03-17 at 15:00 +0100, Kay Sievers wrote:
> On Sun, Mar 17, 2013 at 2:38 PM, Alex Williamson
> <alex.williamson@...hat.com> wrote:
> > I'm assuming that the device only breaks because udevadm is dumping the
> > full I/O port register space of the device and that if an actual driver
> > was interacting with it through this interface that it would work.  Who
> > knows how many devices will have read side-effects by udevadm blindly
> > dumping these files.  Thanks,
> 
> Sysfs is a too public interface to export things there which make
> devices/driver choke on a simple read() of an attribute.

That's why the default permissions for the file do not allow users to
read it.  I wish we could do something as clever as the MMIO resource
files, but I/O port spaces don't allow mmap for the predominant
architecture.  Eventually VFIO is meant to replace this access and does
move device register access behind ioctls, but for now legacy KVM device
assignment relies on these files and so might some UIO drivers.

> This is nothing specific to udevadm, any tool can do that. Udevadm
> will never read any of the files during normal operation. The admin
> explicitly asked udevadm with a specific command to dump all the stuff
> the device offers.

Isn't it possible udevadm could drop privileges or filter out non-world
readable files? 

> The kernel driver needs to be fixed to allow that, in the worst case,
> the attributes not exported at all. People should take more care what
> they export in /sys, it's not a hidden and private ioctl what's
> exported there, stuff is very visible and will be looked at.

File permissions...

> Telling userspace not to use specific stuff in /sys I would not expect
> to work as a strategy; there is too much weird stuff out there that
> will always try to do that ...

I agree, the kernel needs to protect itself from malicious apps, but if
you run a malicious app with admin access, how much can/should we do?
If we're going to ignore file permissions, why limit ourselves to
read(), should we make everything safe against write() as well?  Thanks,

Alex

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