Re: [PATCH next] vhost_task: after freeing vhost_task it should not be accessed in vhost_task_fn

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, May 01, 2024 at 10:57:38AM -0500, Mike Christie wrote:
> On 5/1/24 2:50 AM, Hillf Danton wrote:
> > On Wed, 1 May 2024 02:01:20 -0400 Michael S. Tsirkin <mst@redhat.com>
> >>
> >> and then it failed testing.
> >>
> > So did my patch [1] but then the reason was spotted [2,3]
> > 
> > [1] https://lore.kernel.org/lkml/20240430110209.4310-1-hdanton@sina.com/
> > [2] https://lore.kernel.org/lkml/20240430225005.4368-1-hdanton@sina.com/
> > [3] https://lore.kernel.org/lkml/000000000000a7f8470617589ff2@google.com/
> 
> Just to make sure I understand the conclusion.
> 
> Edward's patch that just swaps the order of the calls:
> 
> https://lore.kernel.org/lkml/tencent_546DA49414E876EEBECF2C78D26D242EE50A@qq.com/
> 
> fixes the UAF. I tested the same in my setup. However, when you guys tested it
> with sysbot, it also triggered a softirq/RCU warning.
> 
> The softirq/RCU part of the issue is fixed with this commit:
> 
> https://lore.kernel.org/all/20240427102808.29356-1-qiang.zhang1211@gmail.com/
> 
> commit 1dd1eff161bd55968d3d46bc36def62d71fb4785
> Author: Zqiang <qiang.zhang1211@gmail.com>
> Date:   Sat Apr 27 18:28:08 2024 +0800
> 
>     softirq: Fix suspicious RCU usage in __do_softirq()
> 
> The problem was that I was testing with -next master which has that patch.
> It looks like you guys were testing against bb7a2467e6be which didn't have
> the patch, and so that's why you guys still hit the softirq/RCU issue. Later
> when you added that patch to your patch, it worked with syzbot.
> 
> So is it safe to assume that the softirq/RCU patch above will be upstream
> when the vhost changes go in or is there a tag I need to add to my patches?

Two points:
- I do not want bisect broken. If you depend on this patch either I pick
  it too before your patch, or we defer until 1dd1eff161bd55968d3d46bc36def62d71fb4785
  is merged. You can also ask for that patch to be merged in this cycle.
- Do not assume - pls push somewhere a hash based on vhost that syzbot can test
  and confirm all is well. Thanks!