Re: [PATCH] vhost/vsock: don't check owner in vhost_vsock_stop() while releasing

[Anirudh Rayabharam <mail@anirudhrb.com>]

On Mon, Feb 21, 2022 at 07:26:28PM +0100, Stefano Garzarella wrote:
> On Mon, Feb 21, 2022 at 11:33:11PM +0530, Anirudh Rayabharam wrote:
> > On Mon, Feb 21, 2022 at 05:44:20PM +0100, Stefano Garzarella wrote:
> > > On Mon, Feb 21, 2022 at 09:44:39PM +0530, Anirudh Rayabharam wrote:
> > > > On Mon, Feb 21, 2022 at 02:59:30PM +0100, Stefano Garzarella wrote:
> > > > > On Mon, Feb 21, 2022 at 12:49 PM Stefano Garzarella <sgarzare@redhat.com> wrote:
> > > > > >
> > > > > > vhost_vsock_stop() calls vhost_dev_check_owner() to check the device
> > > > > > ownership. It expects current->mm to be valid.
> > > > > >
> > > > > > vhost_vsock_stop() is also called by vhost_vsock_dev_release() when
> > > > > > the user has not done close(), so when we are in do_exit(). In this
> > > > > > case current->mm is invalid and we're releasing the device, so we
> > > > > > should clean it anyway.
> > > > > >
> > > > > > Let's check the owner only when vhost_vsock_stop() is called
> > > > > > by an ioctl.
> > > > > >
> > > > > > Fixes: 433fc58e6bf2 ("VSOCK: Introduce vhost_vsock.ko")
> > > > > > Cc: stable@vger.kernel.org
> > > > > > Reported-by: syzbot+1e3ea63db39f2b4440e0@syzkaller.appspotmail.com
> > > > > > Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> > > > > > ---
> > > > > >  drivers/vhost/vsock.c | 14 ++++++++------
> > > > > >  1 file changed, 8 insertions(+), 6 deletions(-)
> > > > >
> > > > > Reported-and-tested-by: syzbot+0abd373e2e50d704db87@syzkaller.appspotmail.com
> > > >
> > > > I don't think this patch fixes "INFO: task hung in vhost_work_dev_flush"
> > > > even though syzbot says so. I am able to reproduce the issue locally
> > > > even with this patch applied.
> > > 
> > > Are you using the sysbot reproducer or another test?
> > > In that case, can you share it?
> > 
> > I am using the syzbot reproducer.
> > 
> > > 
> > > From the stack trace it seemed to me that the worker accesses a zone that
> > > has been cleaned (iotlb), so it is invalid and fails.
> > 
> > Would the thread hang in that case? How?
> 
> Looking at this log [1] it seems that the process is blocked on the
> wait_for_completion() in vhost_work_dev_flush().
> 
> Since we're not setting the backend to NULL to stop the worker, it's likely
> that the worker will keep running, preventing the flush work from
> completing.

The log shows that the worker thread is stuck in iotlb_access_ok(). How
will setting the backend to NULL stop it? During my debugging I found
that the worker is stuck in this while loop:

1361         while (len > s) {                                                                     
1362                 map = vhost_iotlb_itree_first(umem, addr, last);                        
1363                 if (map == NULL || map->start > addr) {                                 
1364                         vhost_iotlb_miss(vq, addr, access);     
1365                         return false;                      
1366                 } else if (!(map->perm & access)) {        
1367                         /* Report the possible access violation by
1368                          * request another translation from userspace.    
1369                          */                                           
1370                         return false;                                 
1371                 }                      
1372                                          
1373                 pr_info("iotlb_access_ok: after msize=%llu, mstart=%llu\n",
1374                                 map->size, map->start);                    
1375                 size = map->size - addr + map->start;                      
1376                                                             
1377                 if (orig_addr == addr && size >= len)       
1378                         vhost_vq_meta_update(vq, map, type);                      
1379                                                                                   
1380                 s += size;                                                        
1381                 addr += size;                                                     
1382         }

> 
> [1] https://syzkaller.appspot.com/text?tag=CrashLog&x=153f0852700000
>