Subject: Re: KASAN: use-after-free Read in vhost_chr_write_iter

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 22 May 2018 06:53:27 +0300

On Tue, May 22, 2018 at 11:50:29AM +0800, Jason Wang wrote:
> 
> 
> On 2018年05月21日 22:42, Michael S. Tsirkin wrote:
> > On Mon, May 21, 2018 at 10:38:10AM +0800, Jason Wang wrote:
> > > On 2018年05月18日 17:24, Jason Wang wrote:
> > > > On 2018年05月17日 21:45, DaeRyong Jeong wrote:
> > > > > We report the crash: KASAN: use-after-free Read in vhost_chr_write_iter
> > > > > 
> > > > > This crash has been found in v4.17-rc1 using RaceFuzzer (a modified
> > > > > version of Syzkaller), which we describe more at the end of this
> > > > > report. Our analysis shows that the race occurs when invoking two
> > > > > syscalls concurrently, write$vnet and ioctl$VHOST_RESET_OWNER.
> > > > > 
> > > > > 
> > > > > Analysis:
> > > > > We think the concurrent execution of vhost_process_iotlb_msg() and
> > > > > vhost_dev_cleanup() causes the crash.
> > > > > Both of functions can run concurrently (please see call sequence below),
> > > > > and possibly, there is a race on dev->iotlb.
> > > > > If the switch occurs right after vhost_dev_cleanup() frees
> > > > > dev->iotlb, vhost_process_iotlb_msg() still sees the non-null value
> > > > > and it
> > > > > keep executing without returning -EFAULT. Consequently, use-after-free
> > > > > occures
> > > > > 
> > > > > 
> > > > > Thread interleaving:
> > > > > CPU0 (vhost_process_iotlb_msg)                CPU1 (vhost_dev_cleanup)
> > > > > (In the case of both VHOST_IOTLB_UPDATE and
> > > > > VHOST_IOTLB_INVALIDATE)
> > > > > =====                            =====
> > > > >                              vhost_umem_clean(dev->iotlb);
> > > > > if (!dev->iotlb) {
> > > > >              ret = -EFAULT;
> > > > >                  break;
> > > > > }
> > > > >                              dev->iotlb = NULL;
> > > > > 
> > > > > 
> > > > > Call Sequence:
> > > > > CPU0
> > > > > =====
> > > > > vhost_net_chr_write_iter
> > > > >      vhost_chr_write_iter
> > > > >          vhost_process_iotlb_msg
> > > > > 
> > > > > CPU1
> > > > > =====
> > > > > vhost_net_ioctl
> > > > >      vhost_net_reset_owner
> > > > >          vhost_dev_reset_owner
> > > > >              vhost_dev_cleanup
> > > > Thanks a lot for the analysis.
> > > > 
> > > > This could be addressed by simply protect it with dev mutex.
> > > > 
> > > > Will post a patch.
> > > > 
> > > Could you please help to test the attached patch? I've done some smoking
> > > test.
> > > 
> > > Thanks
> > > >From 88328386f3f652e684ee33dc4cf63dcaed871aea Mon Sep 17 00:00:00 2001
> > > From: Jason Wang<[email-masked]>
> > > Date: Fri, 18 May 2018 17:33:27 +0800
> > > Subject: [PATCH] vhost: synchronize IOTLB message with dev cleanup
> > > 
> > > DaeRyong Jeong reports a race between vhost_dev_cleanup() and
> > > vhost_process_iotlb_msg():
> > > 
> > > Thread interleaving:
> > > CPU0 (vhost_process_iotlb_msg)			CPU1 (vhost_dev_cleanup)
> > > (In the case of both VHOST_IOTLB_UPDATE and
> > > VHOST_IOTLB_INVALIDATE)
> > > =====						=====
> > > 						vhost_umem_clean(dev->iotlb);
> > > if (!dev->iotlb) {
> > > 	        ret = -EFAULT;
> > > 		        break;
> > > }
> > > 						dev->iotlb = NULL;
> > > 
> > > The reason is we don't synchronize between them, fixing by protecting
> > > vhost_process_iotlb_msg() with dev mutex.
> > > 
> > > Reported-by: DaeRyong Jeong<[email-masked]>
> > > Fixes: 6b1e6cc7855b0 ("vhost: new device IOTLB API")
> > > Reported-by: DaeRyong Jeong<[email-masked]>
> > Long terms we might want to move iotlb into vqs
> > so that messages can be processed in parallel.
> > Not sure how to do it yet.
> > 
> 
> Then we probably need to extend IOTLB msg to have a queue idx. But I thinkit
> was probably only help if we split tx/rx into separate processes.
> 
> Thanks

3 mutex locks on each access isn't pretty even if done by
a single process, but yes - might be more important for scsi.

-- 
MST