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Subject: Re: Kernel stack read with PTRACE_EVENT_EXIT and io_uring threads
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        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>, Kees Cook <keescook@chromium.org>
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From:   Michael Schmitz <schmitzmic@gmail.com>
Message-ID: <5929e116-fa61-b211-342a-c706dcb834ca@gmail.com>
Date:   Tue, 15 Jun 2021 10:26:33 +1200
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Hi Eric,

On 15/06/21 4:26 am, Eric W. Biederman wrote:
> Michael Schmitz <schmitzmic@gmail.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.
> 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.

Fair enough :-)

Cheers,

     Michael

>
> Today CONFIG_HAVE_ARCH_TRACEHOOK guards access to /proc/self/syscall.
> Which out of necessity ensures that user context is always readable.
> Which seems to solve both the PTRACE_EVENT_EXIT and the io_uring
> problems.
>
> What I especially like about that is there are a lot of other reasons
> to encourage architectures in a CONFIG_HAVE_ARCH_TRACEHOOK direction.
> I think the biggies are getting architectures to store the extra
> saved state on context switch into some place in task_struct
> and to implement the regset view of registers.
>
> Hmm. This is odd. CONFIG_HAVE_ARCH_TRACEHOOK is supposed to imply
> CORE_DUMP_USE_REGSET.  But alpha, csky, h8300, m68k, microblaze, nds32
> don't implement CORE_DUMP_USE_REGSET but nds32 implements
> CONFIG_ARCH_HAVE_TRACEHOOK.
>
> I will keep digging and see what clean code I can come up with.
>
> Eric