LKML Archive on lore.kernel.org
 help / color / Atom feed
From: Bernd Edlinger <bernd.edlinger@hotmail.de>
To: "Eric W. Biederman" <ebiederm@xmission.com>
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: Sun, 29 Mar 2020 06:31:11 +0200
Message-ID: <AM6PR03MB51700B30078BDCB6C6A4E0CAE4CA0@AM6PR03MB5170.eurprd03.prod.outlook.com> (raw)
In-Reply-To: <87a7448q7t.fsf@x220.int.ebiederm.org>

On 3/25/20 3:27 PM, Eric W. Biederman wrote:
> 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.
> 

Okay, I can add big fat warning comments, yeah.

> 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.
> 

Not sure I understand this comment correct.
You refer to this block here:

> @@ -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);

I think the logic is correct, but instead if an if-then-else,
I need the big-fat-warning-comment followed by if-unsafe-goto-mutex-unlock
kind of thing, so it looks more like the other places, right?


> 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.
> 

Please elaborate your idea a bit.

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

No problem, I can update this patch and if you like take it to your tree,
otherwise it is of course not the most important issue in the world ;-)


Thanks
Bernd.

  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
2020-03-29  4:31     ` Bernd Edlinger [this message]
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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=AM6PR03MB51700B30078BDCB6C6A4E0CAE4CA0@AM6PR03MB5170.eurprd03.prod.outlook.com \
    --to=bernd.edlinger@hotmail.de \
    --cc=aarcange@redhat.com \
    --cc=adobriyan@gmail.com \
    --cc=akpm@linux-foundation.org \
    --cc=anshuman.khandual@arm.com \
    --cc=avagin@gmail.com \
    --cc=bigeasy@linutronix.de \
    --cc=christian.brauner@ubuntu.com \
    --cc=christian@kellner.me \
    --cc=corbet@lwn.net \
    --cc=cyphar@cyphar.com \
    --cc=david@redhat.com \
    --cc=dhowells@redhat.com \
    --cc=duyuyang@gmail.com \
    --cc=ebiederm@xmission.com \
    --cc=frederic@kernel.org \
    --cc=gregkh@linuxfoundation.org \
    --cc=jamorris@linux.microsoft.com \
    --cc=jannh@google.com \
    --cc=jgg@ziepe.ca \
    --cc=keescook@chromium.org \
    --cc=ktkhai@virtuozzo.com \
    --cc=ldv@altlinux.org \
    --cc=linux-api@vger.kernel.org \
    --cc=linux-doc@vger.kernel.org \
    --cc=linux-fsdevel@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --cc=mingo@kernel.org \
    --cc=oleg@redhat.com \
    --cc=peterz@infradead.org \
    --cc=shakeelb@google.com \
    --cc=stable@vger.kernel.org \
    --cc=tglx@linutronix.de \
    --cc=viro@zeniv.linux.org.uk \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

LKML Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/lkml/0 lkml/git/0.git
	git clone --mirror https://lore.kernel.org/lkml/1 lkml/git/1.git
	git clone --mirror https://lore.kernel.org/lkml/2 lkml/git/2.git
	git clone --mirror https://lore.kernel.org/lkml/3 lkml/git/3.git
	git clone --mirror https://lore.kernel.org/lkml/4 lkml/git/4.git
	git clone --mirror https://lore.kernel.org/lkml/5 lkml/git/5.git
	git clone --mirror https://lore.kernel.org/lkml/6 lkml/git/6.git
	git clone --mirror https://lore.kernel.org/lkml/7 lkml/git/7.git
	git clone --mirror https://lore.kernel.org/lkml/8 lkml/git/8.git
	git clone --mirror https://lore.kernel.org/lkml/9 lkml/git/9.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 lkml lkml/ https://lore.kernel.org/lkml \
		linux-kernel@vger.kernel.org
	public-inbox-index lkml

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-kernel


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git