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

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Date:   Tue, 15 Jun 2021 14:30:59 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Michael Schmitz <schmitzmic@...il.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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: Kernel stack read with PTRACE_EVENT_EXIT and io_uring threads

Michael Schmitz <schmitzmic@...il.com> writes:

> Hi Eric,
>
> On 15/06/21 4:26 am, Eric W. Biederman wrote:
>> Michael Schmitz <schmitzmic@...il.com> writes:
>>
>>> On second thought, I'm not certain what adding another empty stack frame would
>>> achieve here.
>>>
>>> On m68k, 'frame' already is a new stack frame, for running the new thread
>>> in. This new frame does not have any user context at all, and it's explicitly
>>> wiped anyway.
>>>
>>> Unless we save all user context on the stack, then push that context to a new
>>> save frame, and somehow point get_signal to look there for IO threads
>>> (essentially what Eric suggested), I don't see how this could work?
>>>
>>> I must be missing something.
>> It is only designed to work well enough so that ptrace will access
>> something well defined when ptrace accesses io_uring tasks.
>>
>> The io_uring tasks are special in that they are user process
>> threads that never run in userspace.  So as long as everything
>> ptrace can read is accessible on that process all is well.
> OK, I'm testing a patch that would save extra context in sys_io_uring_setup,
> which ought to ensure that for m68k.

I had to update ret_from_kernel_thread to pop that state to get Linus's
change to boot.  Apparently kernel_threads exiting needs to be handled.

>> Having stared a bit longer at the code I think the short term
>> fix for both of PTRACE_EVENT_EXIT and io_uring is to guard
>> them both with CONFIG_HAVE_ARCH_TRACEHOOK.

Which does not work because nios2 which looks susceptible
sets CONFIG_HAVE_ARCH_TRACEHOOK.

A further look shows that there is also PTRACE_EVENT_EXEC that
needs to be handled so execve and execveat need to be wrapped
as well.

Do you happen to know if there is userspace that will run
in qemu-system-m68k that can be used for testing?

Eric