linux-kernel - Re: [PATCH v9] exec: Fix dead-lock in de_thread with ptrace

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Date:   Sat, 12 Jun 2021 09:02:43 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Bernd Edlinger <bernd.edlinger@...mail.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...capital.net>,
        Will Drewry <wad@...omium.org>, Shuah Khan <shuah@...nel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Michal Hocko <mhocko@...e.com>,
        Serge Hallyn <serge@...lyn.com>,
        James Morris <jamorris@...ux.microsoft.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Charles Haithcock <chaithco@...hat.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Yafang Shao <laoar.shao@...il.com>,
        Helge Deller <deller@....de>,
        YiFei Zhu <yifeifz2@...inois.edu>,
        Adrian Reber <areber@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jens Axboe <axboe@...nel.dk>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-kselftest@...r.kernel.org,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH v9] exec: Fix dead-lock in de_thread with ptrace_attach

On Fri, Jun 11, 2021 at 05:55:09PM +0200, Bernd Edlinger wrote:
> This introduces signal->unsafe_execve_in_progress,
> which is used to fix the case when at least one of the
> sibling threads is traced, and therefore the trace
> process may dead-lock in ptrace_attach, but de_thread
> will need to wait for the tracer to continue execution.
> 
> The solution is to detect this situation and allow
> ptrace_attach to continue, while de_thread() is still
> waiting for traced zombies to be eventually released.
> When the current thread changed the ptrace status from
> non-traced to traced, we can simply abort the whole
> execve and restart it by returning -ERESTARTSYS.
> This needs to be done before changing the thread leader,
> because the PTRACE_EVENT_EXEC needs to know the old
> thread pid.
> 
> Although it is technically after the point of no return,
> we just have to reset bprm->point_of_no_return here,
> since at this time only the other threads have received
> a fatal signal, not the current thread.
> 
> >From the user's point of view the whole execve was
> simply delayed until after the ptrace_attach.
> 
> Other threads die quickly since the cred_guard_mutex
> is released, but a deadly signal is already pending.
> In case the mutex_lock_killable misses the signal,
> ->unsafe_execve_in_progress makes sure they release
> the mutex immediately and return with -ERESTARTNOINTR.
> 
> This means there is no API change, unlike the previous
> version of this patch which was discussed here:
> 
> https://lore.kernel.org/lkml/b6537ae6-31b1-5c50-f32b-8b8332ace882@hotmail.de/
> 
> See tools/testing/selftests/ptrace/vmaccess.c
> for a test case that gets fixed by this change.
> 
> Note that since the test case was originally designed to
> test the ptrace_attach returning an error in this situation,
> the test expectation needed to be adjusted, to allow the
> API to succeed at the first attempt.
> 
> Signed-off-by: Bernd Edlinger <bernd.edlinger@...mail.de>
> ---
>  fs/exec.c                                 | 37 +++++++++++++++++++++++++++++--
>  fs/proc/base.c                            |  6 +++++
>  include/linux/sched/signal.h              | 13 +++++++++++
>  kernel/ptrace.c                           |  9 ++++++++
>  kernel/seccomp.c                          | 12 +++++++---
>  tools/testing/selftests/ptrace/vmaccess.c | 25 ++++++++++++++-------
>  6 files changed, 89 insertions(+), 13 deletions(-)
> 

<formletter>

This is not the correct way to submit patches for inclusion in the
stable kernel tree.  Please read:
    https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
for how to do this properly.

</formletter>