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From:   Joel Fernandes <joel@joelfernandes.org>
Date:   Tue, 25 Apr 2023 07:06:08 -0400
Message-ID: <CAEXW_YQEarLt7YGQZdwmcSyZcGRCGKf89ovxjQdXBO-TgXAk-w@mail.gmail.com>
Subject: Re: BUG : PowerPC RCU: torture test failed with __stack_chk_fail
To:     Zhouyi Zhou <zhouzhouyi@gmail.com>,
        Christophe Leroy <christophe.leroy@c-s.fr>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Segher Boessenkool <segher@kernel.crashing.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        rcu <rcu@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>, lance@osuosl.org,
        "Paul E. McKenney" <paulmck@kernel.org>
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On Tue, Apr 25, 2023 at 6:58=E2=80=AFAM Zhouyi Zhou <zhouzhouyi@gmail.com> =
wrote:
>
> hi
>
> On Tue, Apr 25, 2023 at 6:13=E2=80=AFPM Peter Zijlstra <peterz@infradead.=
org> wrote:
> >
> > On Mon, Apr 24, 2023 at 02:55:11PM -0400, Joel Fernandes wrote:
> > > This is amazing debugging Boqun, like a boss! One comment below:
> > >
> > > > > > Or something simple I haven't thought of? :)
> > > > >
> > > > > At what points can r13 change?  Only when some particular functio=
ns are
> > > > > called?
> > > > >
> > > >
> > > > r13 is the local paca:
> > > >
> > > >         register struct paca_struct *local_paca asm("r13");
> > > >
> > > > , which is a pointer to percpu data.
> > > >
> > > > So if a task schedule from one CPU to anotehr CPU, the value gets
> > > > changed.
> > >
> > > It appears the whole issue, per your analysis, is that the stack
> > > checking code in gcc should not cache or alias r13, and must read its
> > > most up-to-date value during stack checking, as its value may have
> > > changed during a migration to a new CPU.
> > >
> > > Did I get that right?
> > >
> > > IMO, even without a reproducer, gcc on PPC should just not do that,
> > > that feels terribly broken for the kernel. I wonder what clang does,
> > > I'll go poke around with compilerexplorer after lunch.
> > >
> > > Adding +Peter Zijlstra as well to join the party as I have a feeling
> > > he'll be interested. ;-)
> >
> > I'm a little confused; the way I understand the whole stack protector
> > thing to work is that we push a canary on the stack at call and on
> > return check it is still valid. Since in general tasks randomly migrate=
,
> > the per-cpu validation canary should be the same on all CPUs.
> >
> > Additionally, the 'new' __srcu_read_{,un}lock_nmisafe() functions use
> > raw_cpu_ptr() to get 'a' percpu sdp, preferably that of the local cpu,
> > but no guarantees.
> >
> > Both cases use r13 (paca) in a racy manner, and in both cases it should
> > be safe.
> New test results today: both gcc build from git (git clone
> git://gcc.gnu.org/git/gcc.git) and Ubuntu 22.04 gcc-12.1.0
> are immune from the above issue. We can see the assembly code on
> http://140.211.169.189/0425/srcu_gp_start_if_needed-gcc-12.txt
>
> while
> Both native gcc on PPC vm (gcc version 9.4.0), and gcc cross compiler
> on my x86 laptop (gcc version 10.4.0) will reproduce the bug.

Do you know what fixes the issue? I would not declare victory yet. My
feeling is something changes in timing, or compiler codegen which
hides the issue. So the issue is still there but it is just a matter
of time before someone else reports it.

Out of curiosity for PPC folks, why cannot 64-bit PPC use per-task
canary? Michael, is this an optimization? Adding Christophe as well
since it came in a few years ago via the following commit:

commit 06ec27aea9fc84d9c6d879eb64b5bcf28a8a1eb7
Author: Christophe Leroy <christophe.leroy@c-s.fr>
Date:   Thu Sep 27 07:05:55 2018 +0000

    powerpc/64: add stack protector support

    On PPC64, as register r13 points to the paca_struct at all time,
    this patch adds a copy of the canary there, which is copied at
    task_switch.
    That new canary is then used by using the following GCC options:
    -mstack-protector-guard=3Dtls
    -mstack-protector-guard-reg=3Dr13
    -mstack-protector-guard-offset=3Doffsetof(struct paca_struct, canary))

    Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
    Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

 - Joel