linux-kernel - Re: [PATCH v3 2/5] iommufd: Destroy vdevice on idevice destroy

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <aGSYLVmV17g832Ta@yilunxu-OptiPlex-7050>
Date: Wed, 2 Jul 2025 10:23:41 +0800
From: Xu Yilun <yilun.xu@...ux.intel.com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: "Tian, Kevin" <kevin.tian@...el.com>,
	"will@...nel.org" <will@...nel.org>,
	"aneesh.kumar@...nel.org" <aneesh.kumar@...nel.org>,
	"iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"joro@...tes.org" <joro@...tes.org>,
	"robin.murphy@....com" <robin.murphy@....com>,
	"shuah@...nel.org" <shuah@...nel.org>,
	"nicolinc@...dia.com" <nicolinc@...dia.com>,
	"aik@....com" <aik@....com>,
	"Williams, Dan J" <dan.j.williams@...el.com>,
	"baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.com>,
	"Xu, Yilun" <yilun.xu@...el.com>
Subject: Re: [PATCH v3 2/5] iommufd: Destroy vdevice on idevice destroy

On Tue, Jul 01, 2025 at 09:13:15AM -0300, Jason Gunthorpe wrote:
> On Tue, Jul 01, 2025 at 05:19:05PM +0800, Xu Yilun wrote:
> > On Mon, Jun 30, 2025 at 11:50:51AM -0300, Jason Gunthorpe wrote:
> > > On Mon, Jun 30, 2025 at 06:18:50PM +0800, Xu Yilun wrote:
> > > 
> > > > I need to reconsider this, seems we need a dedicated vdev lock to
> > > > synchronize concurrent vdev abort/destroy.
> > > 
> > > It is not possible to be concurrent
> > > 
> > > destroy is only called once after it is no longer possible to call
> > > abort.
> > 
> > I'm almost about to drop the "abort twice" idea. [1]
> > 
> > [1]: https://lore.kernel.org/linux-iommu/20250625123832.GF167785@nvidia.com/
> > 
> > See from the flow below,
> > 
> >   T1. iommufd_device_unbind(idev)
> > 	iommufd_device_destroy(obj)
> > 		mutex_lock(&idev->igroup->lock)
> >  		iommufd_vdevice_abort(idev->vdev.obj)
> > 		mutex_unlock(&idev->igroup->lock)
> > 	kfree(obj)
> > 
> >   T2. iommufd_destroy(vdev_id)
> > 	iommufd_vdevice_destroy(obj)
> > 		mutex_lock(&vdev->idev->igroup->lock)
> > 		iommufd_vdevice_abort(obj);
> > 		mutex_unlock(&vdev->idev->igroup->lock)
> > 	kfree(obj)
> > 
> > iommufd_vdevice_destroy() will access idev->igroup->lock, but it is
> > possible the idev is already freed at that time:
> > 
> >                                             iommufd_destroy(vdev_id)
> >                                             iommufd_vdevice_destroy(obj)
> >   iommufd_device_unbind(idev)
> >   iommufd_device_destroy(obj)
> >   mutex_lock(&idev->igroup->lock)
> >                                             mutex_lock(&vdev->idev->igroup->lock) (wait)
> >   iommufd_vdevice_abort(idev->vdev.obj)
> >   mutex_unlock(&idev->igroup->lock)
> >   kfree(obj)
> >                                             mutex_lock(&vdev->idev->igroup->lock) (PANIC)
> >                                             iommufd_vdevice_abort(obj)
> >                                             ...
> 
> Yes, you can't touch idev inside the destroy function at all, under
> any version. idev is only valid if you have a refcount on vdev.
> 
> But why are you touching this lock? Arrange things so abort doesn't
> touch the idev??

idev has a pointer idev->vdev to track the vdev's lifecycle.
idev->igroup->lock protects the pointer. At the end of
iommufd_vdevice_destroy() this pointer should be NULLed so that idev
knows vdev is really destroyed.

I haven't found a safer way for vdev to sync up its validness with idev
w/o touching idev.

I was thinking of using vdev->idev and some vdev lock for tracking
instead. Then iommufd_vdevice_abort() doesn't touch idev. But it is
still the same, just switch to put idev in risk:


                                               iommufd_destroy(vdev_id)
                                               iommufd_vdevice_destroy(obj)
  iommufd_device_unbind(idev)
  iommufd_device_destroy(obj)
                                               mutex_lock(&vdev->some_lock)
  mutex_lock(&idev->vdev->some_lock) (wait)
                                               iommufd_vdevice_abort(obj)
                                               mutex_unlock(&vdev->some_lock)
                                               kfree(obj)
  mutex_lock(&idev->vdev->some_lock) (PANIC)
  iommufd_vdevice_abort(idev->vdev.obj)
  ...

Thanks,
Yilun