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From: ebiederm@xmission.com (Eric W. Biederman)
To: Bernd Edlinger <bernd.edlinger@hotmail.de>
Cc: "gregkh\@linuxfoundation.org" <gregkh@linuxfoundation.org>,
	Kirill Tkhai <ktkhai@virtuozzo.com>,
	Christian Brauner <christian.brauner@ubuntu.com>,
	Kees Cook <keescook@chromium.org>,
	"jannh\@google.com" <jannh@google.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Andrew Morton <akpm@linux-foundation.org>,
	"adobriyan\@gmail.com" <adobriyan@gmail.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Oleg Nesterov <oleg@redhat.com>,
	Frederic Weisbecker <frederic@kernel.org>,
	"avagin\@gmail.com" <avagin@gmail.com>,
	Ingo Molnar <mingo@kernel.org>,
	"Peter Zijlstra \(Intel\)" <peterz@infradead.org>,
	"duyuyang\@gmail.com" <duyuyang@gmail.com>,
	David Hildenbrand <david@redhat.com>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Anshuman Khandual <anshuman.khandual@arm.com>,
	David Howells <dhowells@redhat.com>,
	James Morris <jamorris@linux.microsoft.com>,
	Shakeel Butt <shakeelb@google.com>,
	Jason Gunthorpe <jgg@ziepe.ca>,
	"christian\@kellner.me" <christian@kellner.me>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Aleksa Sarai <cyphar@cyphar.com>,
	"Dmitry V. Levin" <ldv@altlinux.org>,
	"linux-doc\@vger.kernel.org" <linux-doc@vger.kernel.org>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-fsdevel\@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
	"linux-mm\@kvack.org" <linux-mm@kvack.org>,
	"stable\@vger.kernel.org" <stable@vger.kernel.org>,
	"linux-api\@vger.kernel.org" <linux-api@vger.kernel.org>
Subject: Re: [PATCH v6 15/16] exec: Fix dead-lock in de_thread with ptrace_attach
Date: Wed, 25 Mar 2020 09:27:18 -0500
Message-ID: <87a7448q7t.fsf@x220.int.ebiederm.org> (raw)
In-Reply-To: <b6537ae6-31b1-5c50-f32b-8b8332ace882@hotmail.de> (Bernd Edlinger's message of "Sat, 21 Mar 2020 02:46:39 +0000")

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

> This removes the last users of cred_guard_mutex
> and replaces it with a new mutex exec_guard_mutex,
> and a boolean unsafe_execve_in_progress.
>
> This addresses 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 make
> ptrace_attach and similar functions return -EAGAIN,
> but only in a situation where a dead-lock is imminent.
>
> This means this is an API change, but only when the
> process is traced while execve happens in a
> multi-threaded application.
>
> See tools/testing/selftests/ptrace/vmaccess.c
> for a test case that gets fixed by this change.

Hmm.  The logic with unsafe_execve_in_progress is interesting.
I think I see what you are aiming for.

So far as you have hit what you are aiming for I think this is
a safe change as the only cases that will change are the cases
that would deadlock today.

At a minimum the code is subtle and I don't see big fat
warning comments that subtle code needs to keep people
from using it wrong.

Further while the change below to proc_pid_attr_write looks
like it is being treated the same as ptrace_attach.  When in
fact proc_pid_attr_write needs the no_new_privs and ptrace_attach
protection the same as exec.  As the updated cred won't be used in an
ongoing exec, exec does not need protection from proc_pid_attr_write,
other than deadlock protection.

Having the relevant lock be per task_struct lock would probably be a
better way to avoid deadlock with a concurrent proc_pid_attr_write.


So I am going to pass on these last two patches for now, and apply the
rest and get them into linux-next.

Eric


> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 6b13fc4..a428536 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -2680,14 +2680,17 @@ static ssize_t proc_pid_attr_write(struct file * file, const char __user * buf,
>  	}
>  
>  	/* Guard against adverse ptrace interaction */
> -	rv = mutex_lock_interruptible(&current->signal->cred_guard_mutex);
> +	rv = mutex_lock_interruptible(&current->signal->exec_guard_mutex);
>  	if (rv < 0)
>  		goto out_free;
>  
> -	rv = security_setprocattr(PROC_I(inode)->op.lsm,
> -				  file->f_path.dentry->d_name.name, page,
> -				  count);
> -	mutex_unlock(&current->signal->cred_guard_mutex);
> +	if (unlikely(current->signal->unsafe_execve_in_progress))
> +		rv = -EAGAIN;
> +	else
> +		rv = security_setprocattr(PROC_I(inode)->op.lsm,
> +					  file->f_path.dentry->d_name.name,
> +					  page, count);
> +	mutex_unlock(&current->signal->exec_guard_mutex);
>  out_free:
>  	kfree(page);
>  out:

  reply index

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <077b63b7-6f5e-aa8e-bf96-a586b481cc46@hotmail.de>
2020-03-20 20:24 ` [PATCH v6 01/16] exec: Only compute current once in flush_old_exec Bernd Edlinger
2020-03-20 20:24 ` [PATCH v6 02/16] exec: Factor unshare_sighand out of de_thread and call it separately Bernd Edlinger
2020-03-20 20:25 ` [PATCH v6 03/16] exec: Move cleanup of posix timers on exec out of de_thread Bernd Edlinger
2020-03-20 20:25 ` [PATCH v6 04/16] exec: Move exec_mmap right after de_thread in flush_old_exec Bernd Edlinger
2020-03-20 20:25 ` [PATCH v6 05/16] exec: Add exec_update_mutex to replace cred_guard_mutex Bernd Edlinger
2020-03-23 10:51   ` Kirill Tkhai
2020-03-20 20:26 ` [PATCH v6 06/16] exec: Fix a deadlock in strace Bernd Edlinger
2020-03-20 20:26 ` [PATCH v6 07/16] selftests/ptrace: add test cases for dead-locks Bernd Edlinger
2020-03-20 20:26 ` [PATCH v6 08/16] mm: docs: Fix a comment in process_vm_rw_core Bernd Edlinger
2020-03-20 20:26 ` [PATCH v6 09/16] kernel: doc: remove outdated comment cred.c Bernd Edlinger
2020-03-20 20:27 ` [PATCH v6 10/16] kernel/kcmp.c: Use new infrastructure to fix deadlocks in execve Bernd Edlinger
2020-03-25 15:41   ` Christian Brauner
2020-03-20 20:27 ` [PATCH v6 11/16] proc: " Bernd Edlinger
2020-03-20 20:27 ` [PATCH v6 12/16] proc: io_accounting: " Bernd Edlinger
2020-03-20 20:27 ` [PATCH v6 13/16] perf: " Bernd Edlinger
2020-03-21  2:46 ` [PATCH v6 14/16] pidfd: " Bernd Edlinger
2020-03-25 15:40   ` Christian Brauner
2020-03-21  2:46 ` [PATCH v6 15/16] exec: Fix dead-lock in de_thread with ptrace_attach Bernd Edlinger
2020-03-25 14:27   ` Eric W. Biederman [this message]
2020-03-29  4:31     ` Bernd Edlinger
2020-03-29  6:36       ` Bernd Edlinger
2020-03-30 18:26         ` [PATCH v7 " Bernd Edlinger
2020-03-21  2:47 ` [PATCH v6 16/16] doc: Update documentation of ->exec_*_mutex Bernd Edlinger

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