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Date:   Fri, 23 Jul 2021 12:40:25 +0800
From:   Jisheng Zhang <jszhang3@mail.ustc.edu.cn>
To:     tongtiangen <tongtiangen@huawei.com>
Cc:     Andreas Schwab <schwab@linux-m68k.org>,
        Atish Patra <atishp@atishpatra.org>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        linux-riscv <linux-riscv@lists.infradead.org>,
        "linux-kernel@vger.kernel.org List" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH -next v2] riscv: add VMAP_STACK overflow detection
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On Fri, 23 Jul 2021 12:29:25 +0800
Jisheng Zhang <jszhang3@mail.ustc.edu.cn> wrote:

> On Fri, 23 Jul 2021 09:36:47 +0800
> tongtiangen <tongtiangen@huawei.com> wrote:
>=20
> > On 2021/7/23 7:54, Jisheng Zhang wrote: =20
> > > On Thu, 22 Jul 2021 17:42:52 +0200
> > > Andreas Schwab <schwab@linux-m68k.org> wrote:
> > >   =20
> > >> On Jul 22 2021, Jisheng Zhang wrote:
> > >>   =20
> > >>> I think we need to pin the stack before calling get_wchan(), could =
you please
> > >>> try below patch?   =20
> > >>
> > >> Thanks, this fixes the crash for me.
> > >>
> > >> Andreas.
> > >>   =20
> > >
> > > Thanks for testing. I will send out formal patch later
> > >
> > > Thanks
> > >
> > > .
> > >   =20
> >=20
> > Hi all:
> > I tried to reproduced this crash in openSUSE code repo=EF=BC=88=20
> > https://github.com/opensuse/kernel =EF=BC=89, but not reproduced succes=
sfully.
> >=20
> >  From the patch of problem repair, the crash is due to task->stack is=20
> > released before calling get_wchan, the task state of maybe TASK_DEAD.
> >=20
> > VMAP_STACK is used to detect kernel stack overflow, there is no=20
> > connection between the two, it makes me a little confused. =20
>=20
> I believe the bug exists from the first day of riscv mainlined.
>=20
> Since THREAD_INFO_IN_TASK=3Dy in riscv, so when task stack can be freed
> before being destroyed.

typo: task stack can be freed before task is destroyed

>=20
> When VMAP_STACK=3Dn, 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.
>=20
> When VMAP_STACK=3Dy, 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.
>=20
> In summary, the bug isn't related with VMAP_STACK, but VMAP_STACK makes
> the bug observable.
>=20
> Thanks
>=20
>=20
>=20
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