linux-kernel - Re: [PATCHv8] exec: Fix dead-lock in de_thread with ptrace

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Date:   Fri, 11 Jun 2021 06:42:48 +0200
From:   Bernd Edlinger <bernd.edlinger@...mail.de>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     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
Subject: Re: [PATCHv8] exec: Fix dead-lock in de_thread with ptrace_attach

On 6/10/21 11:36 PM, Andrew Morton wrote:
> On Thu, 10 Jun 2021 09:31:42 +0200 Bernd Edlinger <bernd.edlinger@...mail.de> 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.
> 
> Deadlocks are serious.  Is this exploitable by unprivileged userspace?
> 

Yes, in theory:

You need a program doing things like the second test case in vmaccess.c
and if the parent (also unprivileged) does simply not call PTRACE_ATTACH
and not waitpid.   This is an unprivileged process.

Now if A kernel process would try the PTRACE_ATTACH from the test case
it would freeze until the parent process calls waitpid, which it would
not do in this scenario. 

>> Signed-off-by: Bernd Edlinger <bernd.edlinger@...mail.de>
> 
> Was a -stable backport considered?
> 
> 

Yes, maybe after some time.


Thanks
Bernd.