[PATCH -next v2] riscv: add VMAP_STACK overflow detection

[Jisheng Zhang]

On Fri, 23 Jul 2021 12:29:25 +0800
Jisheng Zhang <jszhang3 at mail.ustc.edu.cn> wrote:

> On Fri, 23 Jul 2021 09:36:47 +0800
> tongtiangen <tongtiangen at huawei.com> wrote:
> 
> > On 2021/7/23 7:54, Jisheng Zhang wrote:  
> > > On Thu, 22 Jul 2021 17:42:52 +0200
> > > Andreas Schwab <schwab at linux-m68k.org> wrote:
> > >    
> > >> On Jul 22 2021, Jisheng Zhang wrote:
> > >>    
> > >>> I think we need to pin the stack before calling get_wchan(), could you please
> > >>> try below patch?    
> > >>
> > >> Thanks, this fixes the crash for me.
> > >>
> > >> Andreas.
> > >>    
> > >
> > > Thanks for testing. I will send out formal patch later
> > >
> > > Thanks
> > >
> > > .
> > >    
> > 
> > Hi all:
> > I tried to reproduced this crash in openSUSE code repo（ 
> > https://github.com/opensuse/kernel ）, but not reproduced successfully.
> > 
> >  From the patch of problem repair, the crash is due to task->stack is 
> > released before calling get_wchan, the task state of maybe TASK_DEAD.
> > 
> > VMAP_STACK is used to detect kernel stack overflow, there is no 
> > connection between the two, it makes me a little confused.  
> 
> I believe the bug exists from the first day of riscv mainlined.
> 
> Since THREAD_INFO_IN_TASK=y in riscv, so when task stack can be freed
> before being destroyed.

typo: task stack can be freed before task is destroyed

> 
> When VMAP_STACK=n, task's stack is allocated from linear mapping. When
> task stack is freed, the corresponding mapping still exists, and since
> get_wchan() only read, no harm is observed so far.
> 
> When VMAP_STACK=y, task's stack is allocated from vmalloc area. When
> task stack is freed, the corresponding mapping may not exist, so I expect
> MMU fault here, thus the kernel panic.
> 
> In summary, the bug isn't related with VMAP_STACK, but VMAP_STACK makes
> the bug observable.
> 
> Thanks
> 
> 
> 
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