linux-kernel - Re: [PATCH v14] exec: Fix dead-lock in de_thread with ptrace

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Date: Mon, 22 Jan 2024 13:30:50 -0800
From: Kees Cook <keescook@...omium.org>
To: Bernd Edlinger <bernd.edlinger@...mail.de>
Cc: Oleg Nesterov <oleg@...hat.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Andy Lutomirski <luto@...capital.net>,
	Will Drewry <wad@...omium.org>,
	Christian Brauner <brauner@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.com>, Serge Hallyn <serge@...lyn.com>,
	James Morris <jamorris@...ux.microsoft.com>,
	Randy Dunlap <rdunlap@...radead.org>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Yafang Shao <laoar.shao@...il.com>, Helge Deller <deller@....de>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Adrian Reber <areber@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>, Jens Axboe <axboe@...nel.dk>,
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	"Joel Fernandes (Google)" <joel@...lfernandes.org>,
	"Matthew Wilcox (Oracle)" <willy@...radead.org>,
	Hans Liljestrand <ishkamiel@...il.com>
Subject: Re: [PATCH v14] exec: Fix dead-lock in de_thread with ptrace_attach

On Mon, Jan 22, 2024 at 02:24:37PM +0100, Bernd Edlinger wrote:
> The main concern was when a set-suid program is executed by execve.
> Then it makes a difference if the current thread is traced before the
> execve or not.  That means if the current thread is already traced,
> the decision, which credentials will be used is different than otherwise.
> 
> So currently there are two possbilities, either the trace happens
> before the execve, and the suid-bit will be ignored, or the trace
> happens after the execve, but it is checked that the now potentially
> more privileged credentials allow the tracer to proceed.
> 
> With this patch we will have a third prossibility, that is in order
> to avoid the possible dead-lock we allow the suid-bit to take effect,
> but only if the tracer's privileges allow both to attach the current
> credentials and the new credentials.  But I would only do that as
> a last resort, to avoid the possible dead-lock, and not unless a dead-lock
> is really expected to happen.

Instead of doing this special cred check (which I am worried could
become fragile -- I'd prefer all privilege checks happen in the same
place and in the same way...), could we just fail the ptrace_attach of
the execve?

-- 
Kees Cook