linux-kernel - Re: Kernel stack read with PTRACE_EVENT_EXIT and io

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Message-ID: <YNJFr5xUOm91Vy1r@zeniv-ca.linux.org.uk>
Date:   Tue, 22 Jun 2021 20:18:55 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Michael Schmitz <schmitzmic@...il.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        Jens Axboe <axboe@...nel.dk>, Oleg Nesterov <oleg@...hat.com>,
        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>, Tejun Heo <tj@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Andreas Schwab <schwab@...ux-m68k.org>
Subject: Re: Kernel stack read with PTRACE_EVENT_EXIT and io_uring threads

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).