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

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Date: Mon, 1 Jul 2024 13:48:07 +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 Mon, Jul 01, 2024 at 01:44:01PM +0800, Yi Liu wrote:
> On 2024/7/1 09:47, Yan Zhao wrote:
> > On Sun, Jun 30, 2024 at 03:06:05PM +0800, Yi Liu wrote:
> > > On 2024/6/28 23:28, Yan Zhao wrote:
> > > > 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?
> > > 
> > > this should happen within the process context that has opened the group. it
> > > should be safe, and that would be tracked by the open_count.
> > Thanks for explanation.
> > 
> > Even within a single process, for the group path, it appears that accesses to
> > the multiple opened device fds still require proper synchronization.
> 
> this is for sure as they are accessing the same device.
> 
> > With proper synchronizations, for cdev path, accesses from different processes
> > can still function correctly.
> > Additionally, the group fd can also be passed to another process, allowing
> > device fds to be acquired and accessed from a different process.
> 
> I think the secure boundary is within a process. If there are multiple
> processes accessing a single device, then the boundary is broken.
> 
> > On the other hand, cdev path might also support multiple opened fds from a
> > single process by checking task gid.
> > 
> > The device cdev path simply opts not to do that because it is unnecessary, right?
> 
> This is part of the reason. The major reason is that the vfio group can be
> compiled out. Without the vfio group, it's a bit complicated to ensure all
> the devices within the same iommu group been opened by one user. As no
> known usage of it, so we didn't explore it very much. Actually, if multiple
> FDs are needed, may be dup() is a choice. Do you have such a need?
No, I don't have such a need.
I just find it's confusing to say "Only the group path allows the device to be
opened multiple times. The device cdev path doesn't have a secure way for it",
since it's still doable to achieve the same "secure" level in cdev path and the
group path is not that "secure" :)