linux-kernel - Re: [PATCH 1/2] alpha/ptrace: Record and handle the absence of switch

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Date:   Wed, 16 Jun 2021 13:00:52 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Michael Schmitz <schmitzmic@...il.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        Jens Axboe <axboe@...nel.dk>, Oleg Nesterov <oleg@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Richard Henderson <rth@...ddle.net>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Matt Turner <mattst88@...il.com>,
        alpha <linux-alpha@...r.kernel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>,
        Arnd Bergmann <arnd@...nel.org>,
        Ley Foon Tan <ley.foon.tan@...el.com>,
        Tejun Heo <tj@...nel.org>, Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH 1/2] alpha/ptrace: Record and handle the absence of switch_stack

On Wed, Jun 16, 2021 at 11:32 AM Eric W. Biederman
<ebiederm@...ssion.com> wrote:
>
> Prevent security holes by recording when all of the registers are
> available so generic code changes do not result in security holes
> on alpha.

Please no, not this way. ldl/stc is extremely expensive on some alpha cpus.

I really think thatTIF_ALLREGS_SAVED bit isn't worth it, except
perhaps for debugging.

And even for debugging, I think it would be both easier and cheaper to
just add a magic word to the entry stack instead.

              Linus