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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 16 Jun 2021 13:37:29 -0700
Message-ID: <CAHk-=whvNaYffE8Eaa4-jjYbF_r1u1sh5LF5zXFhdEP8hxySMQ@mail.gmail.com>
Subject: Re: [PATCH 1/2] alpha/ptrace: Record and handle the absence of switch_stack
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     Michael Schmitz <schmitzmic@gmail.com>,
        linux-arch <linux-arch@vger.kernel.org>,
        Jens Axboe <axboe@kernel.dk>, Oleg Nesterov <oleg@redhat.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Richard Henderson <rth@twiddle.net>,
        Ivan Kokshaysky <ink@jurassic.park.msu.ru>,
        Matt Turner <mattst88@gmail.com>,
        alpha <linux-alpha@vger.kernel.org>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        linux-m68k <linux-m68k@lists.linux-m68k.org>,
        Arnd Bergmann <arnd@kernel.org>,
        Ley Foon Tan <ley.foon.tan@intel.com>,
        Tejun Heo <tj@kernel.org>, Kees Cook <keescook@chromium.org>
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On Wed, Jun 16, 2021 at 1:00 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> And even for debugging, I think it would be both easier and cheaper to
> just add a magic word to the entry stack instead.

IOW, just add a

      unsigned long magic;

to "struct switch_stack", and then make the stack switch code push that value.

That would be cheap enough to be just unconditional, but you could
make it depend on a debug config option too, of course.

It helps if 'xyz' is some constant that is easyish to generate. It
might not be a constant - maybe it could be the address of that
'magic' field itself, so you'd just generate it with

    stq $r,($r)

and it would be equally easy to just validate at lookup for that WARN_ON_ONCE():

    WARN_ON_ONCE(switch_stack->magic != (unsigned long)&switch_stack->magic);

or whatever.

It's for debugging, not security. So it doesn't have to be some kind
of super-great magic number, just something easy to generate and check
(that isn't a common value like "0" that trivially exist on the stack
anyway).

             Linus