linux-kernel - Re: [PATCH] vfio: Reuse file f

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Message-ID: <Zn7WofbKsjhlN41U@yzhao56-desk.sh.intel.com>
Date: Fri, 28 Jun 2024 23:28:33 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Yi Liu <yi.l.liu@...el.com>
CC: Jason Gunthorpe <jgg@...dia.com>, "Tian, Kevin" <kevin.tian@...el.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "alex.williamson@...hat.com"
	<alex.williamson@...hat.com>, "peterx@...hat.com" <peterx@...hat.com>,
	"ajones@...tanamicro.com" <ajones@...tanamicro.com>
Subject: Re: [PATCH] vfio: Reuse file f_inode as vfio device inode

On Fri, Jun 28, 2024 at 05:48:11PM +0800, Yi Liu wrote:
> On 2024/6/28 13:21, Yan Zhao wrote:
> > On Thu, Jun 27, 2024 at 09:42:09AM -0300, Jason Gunthorpe wrote:
> > > On Thu, Jun 27, 2024 at 05:51:01PM +0800, Yan Zhao wrote:
> > > 
> > > > > > > This doesn't seem right.. There is only one device but multiple file
> > > > > > > can be opened on that device.
> > > > Maybe we can put this assignment to vfio_df_ioctl_bind_iommufd() after
> > > > vfio_df_open() makes sure device->open_count is 1.
> > > 
> > > Yeah, that seems better.
> > > 
> > > Logically it would be best if all places set the inode once the
> > > inode/FD has been made to be the one and only way to access it.
> > For group path, I'm afraid there's no such a place ensuring only one active fd
> > in kernel.
> > I tried modifying QEMU to allow two openings and two assignments of the same
> > device. It works and appears to guest that there were 2 devices, though this
> > ultimately leads to device malfunctions in guest.
> > 
> > > > BTW, in group path, what's the benefit of allowing multiple open of device?
> > > 
> > > I don't know, the thing that opened the first FD can just dup it, no
> > > idea why two different FDs would be useful. It is something we removed
> > > in the cdev flow
> > > 
> > Thanks. However, from the code, it reads like a drawback of the cdev flow :)
> > I don't understand why the group path is secure though.
> > 
> >          /*
> >           * Only the group path allows the device to be opened multiple
> >           * times.  The device cdev path doesn't have a secure way for it.
> >           */
> >          if (device->open_count != 0 && !df->group)
> >                  return -EINVAL;
> > 
> > 
> 
> The group path only allow single group open, so the device FDs retrieved
> via the group is just within the opener of the group. This secure is built
> on top of single open of group.
What if the group is opened for only once but VFIO_GROUP_GET_DEVICE_FD
ioctl is called for multiple times?