linux-kernel - Re: [PATCH RESEND] ptrace: make ptrace() fail if the tracee changed its pid unexpectedly

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Date:   Mon, 26 Apr 2021 17:04:00 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Jan Kratochvil <jan.kratochvil@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Pedro Alves <palves@...hat.com>,
        Simon Marchi <simon.marchi@...icios.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RESEND] ptrace: make ptrace() fail if the tracee changed
 its pid unexpectedly

----- On Apr 26, 2021, at 11:45 AM, Oleg Nesterov oleg@...hat.com wrote:

> Suppose we have 2 threads, the group-leader L and a sub-theread T,
> both parked in ptrace_stop(). Debugger tries to resume both threads
> and does
> 
>	ptrace(PTRACE_CONT, T);
>	ptrace(PTRACE_CONT, L);
> 
> If the sub-thread T execs in between, the 2nd PTRACE_CONT doesn not
> resume the old leader L, it resumes the post-exec thread T which was
> actually now stopped in PTHREAD_EVENT_EXEC. In this case the
> PTHREAD_EVENT_EXEC event is lost, and the tracer can't know that the
> tracee changed its pid.
> 
> This patch makes ptrace() fail in this case until debugger does wait()
> and consumes PTHREAD_EVENT_EXEC which reports old_pid. This affects all
> ptrace requests except the "asynchronous" PTRACE_INTERRUPT/KILL.
> 
> The patch doesn't add the new PTRACE_ option to not complicate the API,
> and I _hope_ this won't cause any noticeable regression:
> 
>	- If debugger uses PTRACE_O_TRACEEXEC and the thread did an exec
>	  and the tracer does a ptrace request without having consumed
>	  the exec event, it's 100% sure that the thread the ptracer
>	  thinks it is targeting does not exist anymore, or isn't the
>	  same as the one it thinks it is targeting.
> 
>	- To some degree this patch adds nothing new. In the scenario
>	  above ptrace(L) can fail with -ESRCH if it is called after the
>	  execing sub-thread wakes the leader up and before it "steals"
>	  the leader's pid.

Hi Oleg,

Is this something that should also target stable kernels ? AFAIU this change
won't break debuggers more that they are already in this scenario. Or maybe
it makes them fail in more obvious ways ?

Thanks,

Mathieu
 
-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com