Date:   Tue, 8 Mar 2022 13:17:03 +0000
From:   Lee Jones <lee.jones@...aro.org>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Greg KH <gregkh@...uxfoundation.org>, jasowang@...hat.com,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        stable@...r.kernel.org,
        syzbot+adc3cb32385586bec859@...kaller.appspotmail.com
Subject: Re: [PATCH 1/1] vhost: Protect the virtqueue from being cleared
 whilst still in use

On Tue, 08 Mar 2022, Michael S. Tsirkin wrote:

> On Tue, Mar 08, 2022 at 12:45:19PM +0100, Greg KH wrote:
> > On Tue, Mar 08, 2022 at 05:55:58AM -0500, Michael S. Tsirkin wrote:
> > > On Tue, Mar 08, 2022 at 10:57:42AM +0100, Greg KH wrote:
> > > > On Tue, Mar 08, 2022 at 09:15:27AM +0000, Lee Jones wrote:
> > > > > On Tue, 08 Mar 2022, Greg KH wrote:
> > > > > 
> > > > > > On Tue, Mar 08, 2022 at 08:10:06AM +0000, Lee Jones wrote:
> > > > > > > On Mon, 07 Mar 2022, Greg KH wrote:
> > > > > > > 
> > > > > > > > On Mon, Mar 07, 2022 at 07:17:57PM +0000, Lee Jones wrote:
> > > > > > > > > vhost_vsock_handle_tx_kick() already holds the mutex during its call
> > > > > > > > > to vhost_get_vq_desc().  All we have to do here is take the same lock
> > > > > > > > > during virtqueue clean-up and we mitigate the reported issues.
> > > > > > > > > 
> > > > > > > > > Also WARN() as a precautionary measure.  The purpose of this is to
> > > > > > > > > capture possible future race conditions which may pop up over time.
> > > > > > > > > 
> > > > > > > > > Link: https://syzkaller.appspot.com/bug?extid=279432d30d825e63ba00
> > > > > > > > > 
> > > > > > > > > Cc: <stable@...r.kernel.org>
> > > > > > > > > Reported-by: syzbot+adc3cb32385586bec859@...kaller.appspotmail.com
> > > > > > > > > Signed-off-by: Lee Jones <lee.jones@...aro.org>
> > > > > > > > > ---
> > > > > > > > >  drivers/vhost/vhost.c | 10 ++++++++++
> > > > > > > > >  1 file changed, 10 insertions(+)
> > > > > > > > > 
> > > > > > > > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > > > > > > > > index 59edb5a1ffe28..ef7e371e3e649 100644
> > > > > > > > > --- a/drivers/vhost/vhost.c
> > > > > > > > > +++ b/drivers/vhost/vhost.c
> > > > > > > > > @@ -693,6 +693,15 @@ void vhost_dev_cleanup(struct vhost_dev *dev)
> > > > > > > > >  	int i;
> > > > > > > > >  
> > > > > > > > >  	for (i = 0; i < dev->nvqs; ++i) {
> > > > > > > > > +		/* No workers should run here by design. However, races have
> > > > > > > > > +		 * previously occurred where drivers have been unable to flush
> > > > > > > > > +		 * all work properly prior to clean-up.  Without a successful
> > > > > > > > > +		 * flush the guest will malfunction, but avoiding host memory
> > > > > > > > > +		 * corruption in those cases does seem preferable.
> > > > > > > > > +		 */
> > > > > > > > > +		WARN_ON(mutex_is_locked(&dev->vqs[i]->mutex));
> > > > > > > > 
> > > > > > > > So you are trading one syzbot triggered issue for another one in the
> > > > > > > > future?  :)
> > > > > > > > 
> > > > > > > > If this ever can happen, handle it, but don't log it with a WARN_ON() as
> > > > > > > > that will trigger the panic-on-warn boxes, as well as syzbot.  Unless
> > > > > > > > you want that to happen?
> > > > > > > 
> > > > > > > No, Syzbot doesn't report warnings, only BUGs and memory corruption.
> > > > > > 
> > > > > > Has it changed?  Last I looked, it did trigger on WARN_* calls, which
> > > > > > has resulted in a huge number of kernel fixes because of that.
> > > > > 
> > > > > Everything is customisable in syzkaller, so maybe there are specific
> > > > > builds which panic_on_warn enabled, but none that I'm involved with
> > > > > do.
> > > > 
> > > > Many systems run with panic-on-warn (i.e. the cloud), as they want to
> > > > drop a box and restart it if anything goes wrong.
> > > > 
> > > > That's why syzbot reports on WARN_* calls.  They should never be
> > > > reachable by userspace actions.
> > > > 
> > > > > Here follows a topical example.  The report above in the Link: tag
> > > > > comes with a crashlog [0].  In there you can see the WARN() at the
> > > > > bottom of vhost_dev_cleanup() trigger many times due to a populated
> > > > > (non-flushed) worker list, before finally tripping the BUG() which
> > > > > triggers the report:
> > > > > 
> > > > > [0] https://syzkaller.appspot.com/text?tag=CrashLog&x=16a61fce700000
> > > > 
> > > > Ok, so both happens here.  But don't add a warning for something that
> > > > can't happen.  Just handle it and move on.  It looks like you are
> > > > handling it in this code, so please drop the WARN_ON().
> > > > 
> > > > thanks,
> > > > 
> > > > greg k-h
> > > 
> > > Hmm. Well this will mean if we ever reintroduce the bug then
> > > syzkaller will not catch it for us :( And the bug is there,
> > > it just results in a hard to reproduce error for userspace.
> > 
> > Is this an error you can recover from in the kernel?
> >  What is userspace
> > supposed to know with this information when it sees it?
> 
> IIUC we are talking about a use after free here since we somehow
> managed to have a pointer to the device in a worker while
> device is being destroyed.
> 
> That's the point of the warning as use after free is hard to debug. You
> ask can we recover from a use after free? 
> 
> As regards to the added lock, IIUC it kind of shifts the use after free
> window to later and since we zero out some of the memory just before we
> free it, it's a bit more likely to recover.  I would still like to see
> some more analysis on why the situation is always better than it was
> before though.

With the locks in place, the UAF should not occur.

The issue here is that you have 2 different tasks processing the
same area of memory (via pointers to structs).  In these scenarios you
should always provide locking and/or reference counting to prevent
memory corruption or UAF.

-- 
Lee Jones [李琼斯]
Principal Technical Lead - Developer Services
Linaro.org │ Open source software for Arm SoCs
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