linux-kernel - Re: [PATCH v18] exec: Fix dead-lock in de_thread with ptrace

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Message-ID: <87tsyozqdu.fsf@email.froward.int.ebiederm.org>
Date: Thu, 20 Nov 2025 09:15:57 -0600
From: "Eric W. Biederman" <ebiederm@...ssion.com>
To: Bernd Edlinger <bernd.edlinger@...mail.de>
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Subject: Re: [PATCH v18] exec: Fix dead-lock in de_thread with ptrace_attach

Bernd Edlinger <bernd.edlinger@...mail.de> writes:

> This introduces signal->exec_bprm, 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.

A small quibble it isn't a dead lock.  It isn't even really a live lock,
as it is possible to SIGKILL our way out.

Thinking about this there is a really silly and simple way we can deal
with this situation for PTRACE_ATTACH.  We can send SIGSTOP and wait for
the thread to stop before doing anything with cred_guard_mutex.

PTRACE_ATTACH already implies sending SIGSTOP so as long as we have
enough permissions to send SIGSTOP I don't see that being a problem.

The worst case I can see is that we get a case where we stop the
process, the permission check fails under cred_guard_mutex and
and ptrace attach has fails and has to send SIGCONT to undo it's
premature SIGSTOP.  That might almost be visible, but it would still
be legitimate because we can still check that we have permission to
send SIGSTOP.

Eric