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Date:   Tue, 22 Jun 2021 20:18:55 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Michael Schmitz <schmitzmic@gmail.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        linux-arch <linux-arch@vger.kernel.org>,
        Jens Axboe <axboe@kernel.dk>, Oleg Nesterov <oleg@redhat.com>,
        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>, Tejun Heo <tj@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        Andreas Schwab <schwab@linux-m68k.org>
Subject: Re: Kernel stack read with PTRACE_EVENT_EXIT and io_uring threads
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On Wed, Jun 23, 2021 at 08:04:11AM +1200, Michael Schmitz wrote:

> All syscalls that _do_ save the switch stack are currently called through
> wrappers which pull the syscall arguments out of the saved pt_regs on the
> stack (pushing the switch stack after the SAVE_ALL saved stuff buries the
> syscall arguments on the stack, see comment about m68k_clone(). We'd have to
> push the switch stack _first_ when entering system_call to leave the syscall
> arguments in place, but that will require further changes to the syscall
> exit path (currently shared with the interrupt exit path). Not to mention
> the register offset calculations in arch/m68k/kernel/ptrace.c, and perhaps a
> few other dependencies that don't come to mind immediately.
> 
> We have both pt_regs and switch_stack in uapi/asm/ptrace.h, but the ordering
> of the two is only mentioned in a comment. Can we reorder them on the stack,
> as long as we don't change the struct definitions proper?
> 
> This will take a little more time to work out and test - certainly not
> before the weekend. I'll send a corrected version of my debug patch before
> that.

This is insane, *especially* on m68k where you have the mess with different
frame layouts and associated ->stkadj crap (see mangle_kernel_stack() for
the (very) full barfbag).