linux-kernel - Re: [RFC] PCI: allow sysfs file owner to read the config space with CAP_SYS

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Date:   Mon, 19 Oct 2020 15:47:29 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Allen <allen.lkml@...il.com>
Cc:     linux-pci@...r.kernel.org, Bjorn Helgaas <bhelgaas@...gle.com>,
        ast@...nel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Allen Pais <apais@...ux.microsoft.com>,
        Allen Pais <allen.pais@...l.com>
Subject: Re: [RFC] PCI: allow sysfs file owner to read the config space with
 CAP_SYS_RAWIO

On Mon, Oct 19, 2020 at 06:51:39PM +0530, Allen wrote:
> > > > >
> > > > >  Access to pci config space is explictly checked with CAP_SYS_ADMIN
> > > > > in order to read configuration space past the frist 64B.
> > > > >
> > > > >  Since the path is only for reading, could we use CAP_SYS_RAWIO?
> > > >
> > > > Why?  What needs this reduced capability?
> > >
> > > Thanks for the review.
> > >
> > > We need read access to /sys/bus/pci/devices/,  We need write access to config,
> > > remove, rescan & enable files under the device directory for each PCIe
> > > functions & the downstream PCIe port.
> > >
> > > We need r/w access to sysfs to unbind and rebind the root complex.
> >
> > That didn't answer my question at all.
> 
> Sorry about that, breaking it down:
> 
> When the machine first boots, the VFIO device bindings under /dev/vfio
> are not present.
> 
> root@...alhost:/tmp# ls -l /dev/vfio/
> total 0
> crw-rw-rw-. 1 root root 10, 196 Jan  5 01:47 vfio
> 
> We have an agent which needs to run the following commands (We get
> access denied here and need permissions to do this).
> echo -n xxxx yyyy > /sys/module/vfio_pci/drivers/pci:vfio-pci/new_id
> echo -n xxxx yyyy > /sys/module/vfio_pci/drivers/pci:vfio-pci/new_id
> 
> And we want to avoid handing CAP_SYS_ADMIN here. Which is why the
> thought about CAP_SYS_RAWIO.

But that is not what you were asking this patch to do at all.  So why
bring it up?

new_id is NOT for "raw io" control, that should be only for admin
priviliges.

And just because the vfio driver "abuses" this
traditionally-debug-functionality doesn't mean you get to abuse the
permission levels either.

> > Why can't you have the process that wants to do all of the above, have
> > admin rights as well?  Doing all of that is _very_ low-level and can
> > cause all sorts of horrible things to happen to your machine, and is not
> > really "raw io" in the traditional sense at all, right?
> 
> 
> If the above approach is going to cause the system to do horrible things,
> then I'll drop the idea.

Of course it can cause the system to do horrible things, try it yourself
and see!

greg k-h