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Subject: Re: Kernel stack read with PTRACE_EVENT_EXIT and io_uring threads
To:     Linus Torvalds <torvalds@linux-foundation.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     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>,
        Daniel Jacobowitz <drow@nevyn.them.org>,
        Kees Cook <keescook@chromium.org>
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From:   Michael Schmitz <schmitzmic@gmail.com>
Message-ID: <6e47eff8-d0a4-8390-1222-e975bfbf3a65@gmail.com>
Date:   Mon, 14 Jun 2021 14:05:38 +1200
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Hi Linus,

On 14/06/21 10:18 am, Linus Torvalds wrote:
> On Sun, Jun 13, 2021 at 2:55 PM Eric W. Biederman <ebiederm@xmission.com> wrote:
>> The alpha_switch_to will remove the extra registers from the stack and
>> then call ret which if I understand alpha assembly correctly is
>> equivalent to jumping to where $26 points.  Which is
>> ret_from_kernel_thread (as setup by copy_thread).
>>
>> Which leaves ret_from_kernel_thread and everything it calls without
>> the extra context saved on the stack.
> Uhhuh. Right you are, I think. It's been ages since I worked on that
> code and my alpha handbook is somewhere else, but yes, when
> alpha_switch_to() has context-switched to the new PCB state, it will
> then pop those registers in the new context and return.
>
> So we do set up the right stack frame for the worker thread, but as
> you point out, it then gets used up immediately when running. So by
> the time the IO worker thread calls get_signal(), it's no longer
> useful.
>
> How very annoying.
>
> The (obviously UNTESTED) patch might be something like the attached.
>
> I wouldn't be surprised if m68k has the exact same thing for the exact
> same reason, but I didn't check..

m68k is indeed similar, it has:

        if (unlikely(p->flags & (PF_KTHREAD | PF_IO_WORKER))) {
                 /* kernel thread */
                 memset(frame, 0, sizeof(struct fork_frame));
                 frame->regs.sr = PS_S;
                 frame->sw.a3 = usp; /* function */
                 frame->sw.d7 = arg;
                 frame->sw.retpc = (unsigned long)ret_from_kernel_thread;
                 p->thread.usp = 0;
                 return 0;
         }

so a similar patch should be possible.

Cheers,

     Michael


>
>                    Linus