Date:   Fri, 26 Apr 2019 15:55:32 +0000
From:   Parav Pandit <parav@...lanox.com>
To:     Alex Williamson <alex.williamson@...hat.com>
CC:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kwankhede@...dia.com" <kwankhede@...dia.com>,
        "cjia@...dia.com" <cjia@...dia.com>
Subject: RE: [PATCHv1 7/7] vfio/mdev: Fix race conditions with mdev device
 life cycle APIs



> -----Original Message-----
> From: Alex Williamson <alex.williamson@...hat.com>
> Sent: Friday, April 26, 2019 10:34 AM
> To: Parav Pandit <parav@...lanox.com>
> Cc: kvm@...r.kernel.org; linux-kernel@...r.kernel.org;
> kwankhede@...dia.com; cjia@...dia.com
> Subject: Re: [PATCHv1 7/7] vfio/mdev: Fix race conditions with mdev device
> life cycle APIs
> 
> On Thu, 25 Apr 2019 23:29:26 +0000
> Parav Pandit <parav@...lanox.com> wrote:
> 
> > Hi Alex,
> >
> > First, sorry for my late reply.
> >
> > > -----Original Message-----
> > > From: Alex Williamson <alex.williamson@...hat.com>
> > > Sent: Tuesday, April 23, 2019 2:22 PM
> > > To: Parav Pandit <parav@...lanox.com>
> > > Cc: kvm@...r.kernel.org; linux-kernel@...r.kernel.org;
> > > kwankhede@...dia.com; cjia@...dia.com
> > > Subject: Re: [PATCHv1 7/7] vfio/mdev: Fix race conditions with mdev
> > > device life cycle APIs
> > >
> > > On Thu, 4 Apr 2019 23:05:43 +0000
> > > Parav Pandit <parav@...lanox.com> wrote:
> > >
> > > > > -----Original Message-----
> > > > > From: Alex Williamson <alex.williamson@...hat.com>
> > > > > Sent: Thursday, April 4, 2019 3:44 PM
> > > > > To: Parav Pandit <parav@...lanox.com>
> > > > > Cc: kvm@...r.kernel.org; linux-kernel@...r.kernel.org;
> > > > > kwankhede@...dia.com; cjia@...dia.com
> > > > > Subject: Re: [PATCHv1 7/7] vfio/mdev: Fix race conditions with
> > > > > mdev device life cycle APIs
> > > > >
> > > > > On Thu, 4 Apr 2019 00:02:22 +0000 Parav Pandit
> > > > > <parav@...lanox.com> wrote:
> > > > >
> > > > > > > -----Original Message-----
> > > > > > > From: Alex Williamson <alex.williamson@...hat.com>
> > > > > > > Sent: Wednesday, April 3, 2019 4:27 PM
> > > > > > > To: Parav Pandit <parav@...lanox.com>
> > > > > > > Cc: kvm@...r.kernel.org; linux-kernel@...r.kernel.org;
> > > > > > > kwankhede@...dia.com; cjia@...dia.com
> > > > > > > Subject: Re: [PATCHv1 7/7] vfio/mdev: Fix race conditions
> > > > > > > with mdev device life cycle APIs
> > > > > > >
> > > > > > > On Tue, 26 Mar 2019 22:45:45 -0500 Parav Pandit
> > > > > > > <parav@...lanox.com> wrote:
> > > > > > >
> > > > > > > > Below race condition and call trace exist with current
> > > > > > > > device life cycle sequence.
> > > > > > > >
> > > > > > > > 1. In following sequence, child devices created while
> > > > > > > > removing mdev parent device can be left out, or it may
> > > > > > > > lead to race of removing half initialized child mdev devices.
> > > > > > > >
> > > > > > > > issue-1:
> > > > > > > > --------
> > > > > > > >        cpu-0                         cpu-1
> > > > > > > >        -----                         -----
> > > > > > > >                                   mdev_unregister_device()
> > > > > > > >                                      device_for_each_child()
> > > > > > > >
> > > > > > > > mdev_device_remove_cb()
> > > > > > > >
> > > > > > > > mdev_device_remove()
> > > > > > > > create_store()
> > > > > > > >   mdev_device_create()                   [...]
> > > > > > > >        device_register()
> > > > > > > >                                   parent_remove_sysfs_files()
> > > > > > > >                                   /* BUG: device added by cpu-0
> > > > > > > >                                    * whose parent is getting removed.
> > > > > > > >                                    */
> > > > > > > >
> > > > > > > > issue-2:
> > > > > > > > --------
> > > > > > > >        cpu-0                         cpu-1
> > > > > > > >        -----                         -----
> > > > > > > > create_store()
> > > > > > > >   mdev_device_create()                   [...]
> > > > > > > >        device_register()
> > > > > > > >
> > > > > > > >        [...]                      mdev_unregister_device()
> > > > > > > >                                      device_for_each_child()
> > > > > > > >
> > > > > > > > mdev_device_remove_cb()
> > > > > > > >
> > > > > > > > mdev_device_remove()
> > > > > > > >
> > > > > > > >        mdev_create_sysfs_files()
> > > > > > > >        /* BUG: create is adding
> > > > > > > >         * sysfs files for a device
> > > > > > > >         * which is undergoing removal.
> > > > > > > >         */
> > > > > > > >
> > > > > > > > parent_remove_sysfs_files()
> > > > > > > >
> > > > > > > > 2. Below crash is observed when user initiated remove is
> > > > > > > > in progress and mdev_unregister_driver() completes parent
> > > > > unregistration.
> > > > > > > >
> > > > > > > >        cpu-0                         cpu-1
> > > > > > > >        -----                         -----
> > > > > > > > remove_store()
> > > > > > > >    mdev_device_remove()
> > > > > > > >    active = false;
> > > > > > > >                                   mdev_unregister_device()
> > > > > > > >                                     remove type
> > > > > > > >    [...]
> > > > > > > >    mdev_remove_ops() crashes.
> > > > > > > >
> > > > > > > > This is similar race like create() racing with
> > > mdev_unregister_device().
> > > > > > > >
> > > > > > > > mtty mtty: MDEV: Registered
> > > > > > > > iommu: Adding device 83b8f4f2-509f-382f-3c1e-e6bfe0fa1001
> > > > > > > > to group
> > > > > > > > 57 vfio_mdev 83b8f4f2-509f-382f-3c1e-e6bfe0fa1001: MDEV:
> > > > > > > > group_id = 57 mtty mtty: MDEV: Unregistering
> > > > > > > > mtty_dev: Unloaded!
> > > > > > > > BUG: unable to handle kernel paging request at
> > > > > > > > ffffffffc027d668 PGD
> > > > > > > > af9818067 P4D af9818067 PUD af981a067 PMD 8583c3067 PTE 0
> > > > > > > > Oops: 0000 [#1] SMP PTI
> > > > > > > > CPU: 15 PID: 3517 Comm: bash Kdump: loaded Not tainted
> > > > > > > > 5.0.0-rc7-vdevbus+ #2 Hardware name: Supermicro
> > > > > > > > SYS-6028U-TR4+/X10DRU-i+, BIOS 2.0b 08/09/2016
> > > > > > > > RIP: 0010:mdev_device_remove_ops+0x1a/0x50 [mdev] Call
> Trace:
> > > > > > > >  mdev_device_remove+0xef/0x130 [mdev]
> > > > > > > >  remove_store+0x77/0xa0 [mdev]
> > > > > > > >  kernfs_fop_write+0x113/0x1a0
> > > > > > > >  __vfs_write+0x33/0x1b0
> > > > > > > >  ? rcu_read_lock_sched_held+0x64/0x70
> > > > > > > >  ? rcu_sync_lockdep_assert+0x2a/0x50  ?
> > > > > > > > __sb_start_write+0x121/0x1b0  ? vfs_write+0x17c/0x1b0
> > > > > > > >  vfs_write+0xad/0x1b0
> > > > > > > >  ? trace_hardirqs_on_thunk+0x1a/0x1c
> > > > > > > >  ksys_write+0x55/0xc0
> > > > > > > >  do_syscall_64+0x5a/0x210
> > > > > > > >
> > > > > > > > Therefore, mdev core is improved to overcome above issues.
> > > > > > > >
> > > > > > > > Wait for any ongoing mdev create() and remove() to finish
> > > > > > > > before unregistering parent device using srcu. This
> > > > > > > > continues to allow multiple create and remove to progress in
> parallel.
> > > > > > > > At the same time guard parent removal while parent is
> > > > > > > > being access by create() and remove
> > > > > > > callbacks.
> > > > > > > >
> > > > > > > > mdev_device_remove() is refactored to not block on srcu
> > > > > > > > when device is removed as part of parent removal.
> > > > > > > >
> > > > > > > > Fixes: 7b96953bc640 ("vfio: Mediated device Core driver")
> > > > > > > > Signed-off-by: Parav Pandit <parav@...lanox.com>
> > > > > > > > ---
> > > > > > > >  drivers/vfio/mdev/mdev_core.c    | 83
> > > > > > > ++++++++++++++++++++++++++++++++++------
> > > > > > > >  drivers/vfio/mdev/mdev_private.h |  6 +++
> > > > > > > >  2 files changed, 77 insertions(+), 12 deletions(-)
> > > > > > > >
> > > > > > > > diff --git a/drivers/vfio/mdev/mdev_core.c
> > > > > > > > b/drivers/vfio/mdev/mdev_core.c index aefcf34..fa233c8
> > > > > > > > 100644
> > > > > > > > --- a/drivers/vfio/mdev/mdev_core.c
> > > > > > > > +++ b/drivers/vfio/mdev/mdev_core.c
> > > > > > > > @@ -84,6 +84,7 @@ static void mdev_release_parent(struct
> > > > > > > > kref
> > > *kref)
> > > > > > > >  						  ref);
> > > > > > > >  	struct device *dev = parent->dev;
> > > > > > > >
> > > > > > > > +	cleanup_srcu_struct(&parent->unreg_srcu);
> > > > > > > >  	kfree(parent);
> > > > > > > >  	put_device(dev);
> > > > > > > >  }
> > > > > > > > @@ -147,10 +148,30 @@ static int
> > > mdev_device_remove_ops(struct
> > > > > > > mdev_device *mdev, bool force_remove)
> > > > > > > >  	return 0;
> > > > > > > >  }
> > > > > > > >
> > > > > > > > +static int mdev_device_remove_common(struct mdev_device
> > > *mdev,
> > > > > > > > +				     bool force_remove) {
> > > > > > > > +	struct mdev_type *type;
> > > > > > > > +	int ret;
> > > > > > > > +
> > > > > > > > +	type = to_mdev_type(mdev->type_kobj);
> > > > > > >
> > > > > > > I know you're just moving this into the common function, but
> > > > > > > I think we're just caching this for aesthetics, the mdev
> > > > > > > object is still valid after the remove ops and I don't see
> > > > > > > anything touching this field.  If so, maybe we should remove
> > > > > > > 'type' or at least set it right before it's used so it
> > > > > > > doesn't appear that we're preserving it before
> > > > > the remove op.
> > > > > > >
> > > > > > Sure, yes.
> > > > > > Type assignment should be done just before calling
> > > > > mdev_remove_sysfs_files().
> > > > > > Will send v2.
> > > > > >
> > > > > > > > +
> > > > > > > > +	ret = mdev_device_remove_ops(mdev, force_remove);
> > > > > > > > +	if (ret && !force_remove) {
> > > > > > > > +		mutex_lock(&mdev_list_lock);
> > > > > > > > +		mdev->active = true;
> > > > > > > > +		mutex_unlock(&mdev_list_lock);
> > > > > > >
> > > > > > > The mutex around this is a change from the previous code and
> > > > > > > I'm not sure it adds anything.  If there's a thread testing
> > > > > > > for active racing with this thread setting active to true,
> > > > > > > there's no meaningful difference in the result by acquiring the
> mutex.
> > > > > > > 'active' may change from false->true during the critical
> > > > > > > section of the other thread, but I don't think there are any
> > > > > > > strange out of order things that give the wrong result, the
> > > > > > > other thread either sees true
> > > > > or false and continues or exits, regardless of this mutex.
> > > > > > >
> > > > > > Yes, I can drop the mutex.
> > > > > > In future remove sequence fix, this will anyway vanish.
> > > > > >
> > > > > > Shall we finish this series with these 7 patches?
> > > > > > Once you ack it will send v2 for these 7 patches and follow on
> > > > > > to that we
> > > > > cleanup the sequencing?
> > > > >
> > > > > Do you intend to move the removal of the mdev sanitization loop
> > > > > from
> > > > > 6/7 to this patch?  I don't think we can really claim that what
> > > > > it's trying to do is unnecessary until after we have the new
> > > > > code here that prevents the sysfs remove path from running
> > > > > concurrent to the parent remove path.  It's not really related
> > > > > to the changes in 6/7 anyway.  In fact, rather than moving that
> > > > > chunk here, it could be added as a follow-on patch with
> > > > > explanation of why it is now unnecessary.  Thanks,
> > > > >
> > > > Device type cannot change from mdev to something else when it was
> > > invoked by the remove() sysfs cb.
> > > > Neither it can be something else in parent removal is passes bus
> > > comparison check.
> > >
> > > Hi Parav,
> > >
> > > I tried to explain the concern in
> > > Message-ID: <20190402163309.414c45ad@...home> It hinges on the fact
> > > that
> > > remove_store() calls device_remove_file_self() before calling
> > > mdev_device_remove().  Therefore imagine this scenarios:
> > >
> > > Thread A			Thread B
> > >
> > > mdev_device_remove()
> > >  mdev_remove_sysfs_files()
> > > 				remove_store()
> > > 				 device_remove_file_self()
> > >   sysfs_remove_files...
> > > 				 mdev_device_remove()
> > > 				  return -EAGAIN
> > >                                  device_create_file()
> > >  device_unregister()
> > >  mdev_put_parent()
> > >
> > > So Thread B recreated a stale sysfs attribute.  If it prevents the
> > > mdev from being released via mdev_device_release() then it will
> > > forever be !active and calling remove store will continue to error
> > > and recreate it.  If the mdev does get freed, then remove_store() is
> > > working with a stale device, which the sanitizing loop removed in
> > > 6/7 is meant to catch.  Therefore, it makes sense to me to relocate
> > > that loop removal until after we clean up the mess around removal.
> > >
> > If device gets freed in mdev_device_release(), than remove_store()
> shouldn't find a valid entry.
> > Isn't it?
> 
> That's the question that I haven't tested or investigated further, if
> remove_store() re-adds the sysfs file after mdev_device_remove() would
> remove it, does the sysfs attribute still exist and can remove_store() still be
> called with a bogus pointer.
> 
> > > BTW, I exchanged email with Kirti offline and I think we're in
> > > agreement around your plans to fix this.  The confusion was around
> > > whether the vendor driver remove callback can be called while the
> > > device is still in use, but I believe vfio-core will prevent that
> > > with the correct bus removal logic in place.
> > Yes. vfio-core waits there. I think I shared the trace of it.
> > If this mdev is used by non vfio driver, such as mlx5_core, than
> > remove() of the mlx5_core driver will get called, and there it will
> > follow standard PCI bus style forced removal anyway. So we are good
> > there.
> >
> > > So where do we stand on this series?  I think patches 1-5 look good.
> > Yes. There were more review-by tags that I guess you need to include.
> 
> Yep, got 'em.
> 
> > > Should I incorporate them for v5.2?
> > Yes. that will be good. So next series can be shorter. :-)
> >
> > > Patch 6 looks ok, except I'd rather see the sanitizing loop stay
> > > until we can otherwise fix the race
> > > above.
> > I can put back the sanitizing look, once it looks valid. Will wait for
> > your response.
> 
> Yep, I think patch 6 is good w/o the removal of the sanitizing loop.
> Will you repost it?
> 
Just the patch-6 or 1 to 6?

> > > Patch 7 needed more work, iirc.  Thanks,
> > For a moment if we assume sanitizing loop exists, than patch-7 looks
> > good?
> 
> Patch 7 is a bit less trivial, so I think as we're close to the merge window for
> v5.2, I'd rather push it out to be included with the later re-works.  Thanks,
> 
I agree it little less trivial, I tried to place as much comment as possible, but it is an important fix.
Let me repost 6-7 and decide if it can be included or not?

> Alex

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