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From: Bernd Edlinger <bernd.edlinger@hotmail.de>
To: "jannh@google.com" <jannh@google.com>,
	"Eric W. Biederman" <ebiederm@xmission.com>
Cc: Christian Brauner <christian.brauner@ubuntu.com>,
	Kees Cook <keescook@chromium.org>,
	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>,
	"gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
	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>,
	Arnd Bergmann <arnd@arndb.de>,
	"sargun@sargun.me" <sargun@sargun.me>
Subject: Re: [PATCH] pidfd: Stop taking cred_guard_mutex
Date: Wed, 11 Mar 2020 06:11:57 +0000
Message-ID: <5a8b2794-b498-af33-1327-ff2861cff83f@hotmail.de> (raw)
In-Reply-To: <AM6PR03MB5170AF454A8A9C37891B12B2E4FF0@AM6PR03MB5170.eurprd03.prod.outlook.com>

On 3/10/20 9:22 PM, Bernd Edlinger wrote:
> On 3/10/20 9:10 PM, Jann Horn wrote:
>> On Tue, Mar 10, 2020 at 9:00 PM Jann Horn <jannh@google.com> wrote:
>>> On Tue, Mar 10, 2020 at 8:29 PM Eric W. Biederman <ebiederm@xmission.com> wrote:
>>>> Jann Horn <jannh@google.com> writes:
>>>>> On Tue, Mar 10, 2020 at 7:54 PM Eric W. Biederman <ebiederm@xmission.com> wrote:
>>>>>> During exec some file descriptors are closed and the files struct is
>>>>>> unshared.  But all of that can happen at other times and it has the
>>>>>> same protections during exec as at ordinary times.  So stop taking the
>>>>>> cred_guard_mutex as it is useless.
>>>>>>
>>>>>> Furthermore he cred_guard_mutex is a bad idea because it is deadlock
>>>>>> prone, as it is held in serveral while waiting possibly indefinitely
>>>>>> for userspace to do something.
>> [...]
>>>>> If you make this change, then if this races with execution of a setuid
>>>>> program that afterwards e.g. opens a unix domain socket, an attacker
>>>>> will be able to steal that socket and inject messages into
>>>>> communication with things like DBus. procfs currently has the same
>>>>> race, and that still needs to be fixed, but at least procfs doesn't
>>>>> let you open things like sockets because they don't have a working
>>>>> ->open handler, and it enforces the normal permission check for
>>>>> opening files.
>>>>
>>>> It isn't only exec that can change credentials.  Do we need a lock for
>>>> changing credentials?
>> [...]
>>>> If we need a lock around credential change let's design and build that.
>>>> Having a mismatch between what a lock is designed to do, and what
>>>> people use it for can only result in other bugs as people get confused.
>>>
>>> Hmm... what benefits do we get from making it a separate lock? I guess
>>> it would allow us to make it a per-task lock instead of a
>>> signal_struct-wide one? That might be helpful...
>>
>> But actually, isn't the core purpose of the cred_guard_mutex to guard
>> against concurrent credential changes anyway? That's what almost
>> everyone uses it for, and it's in the name...
>>
> 
> The main reason d'etre of exec_update_mutex is to get a consitent
> view of task->mm and task credentials.
> > The reason why you want the cred_guard_mutex, is that some action
> is changing the resulting credentials that the execve is about
> to install, and that is the data flow in the opposite direction.
> 

So in other words, you need the exec_update_mutex when you
access another thread's credentials and possibly the mmap at the
same time.

You need the cred_guard_mutex when you *change* the credentials
of another thread.  (Where you cannot be sure that the other thread
just started to execve something)

You need no mutex at all when you are just accessing or
even changing the credentials of the current thread.  (If another
thread is doing execve, your task will be killed, and wether
or not the credentials were changed does not matter any more)

> 
> Bernd.
> 

  reply index

Thread overview: 203+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-01 11:27 [PATCH] exec: Fix a deadlock in ptrace Bernd Edlinger
2020-03-01 15:13 ` Aleksa Sarai
2020-03-01 15:58   ` Christian Brauner
2020-03-01 17:46     ` Bernd Edlinger
2020-03-01 18:20       ` Christian Brauner
2020-03-01 17:24   ` Bernd Edlinger
2020-03-01 18:21 ` Jann Horn
2020-03-01 18:52   ` Christian Brauner
2020-03-01 19:00     ` Bernd Edlinger
2020-03-01 20:00     ` Jann Horn
2020-03-01 20:34       ` [PATCHv2] " Bernd Edlinger
2020-03-02  6:38         ` Eric W. Biederman
2020-03-02 15:43           ` Bernd Edlinger
2020-03-02 15:57             ` Eric W. Biederman
2020-03-02 16:02               ` Bernd Edlinger
2020-03-02 16:17                 ` Eric W. Biederman
2020-03-02 16:43                   ` Jann Horn
2020-03-02 17:01                     ` Bernd Edlinger
2020-03-02 17:37                       ` Jann Horn
2020-03-02 17:42                         ` christian
2020-03-02 18:08                           ` Jann Horn
2020-03-02 20:10                             ` [PATCHv3] " Bernd Edlinger
2020-03-02 20:28                               ` Bernd Edlinger
2020-03-02 17:13                   ` [PATCHv2] " Bernd Edlinger
2020-03-02 21:49                     ` Eric W. Biederman
2020-03-02 22:00                       ` Bernd Edlinger
2020-03-02 22:18                       ` [PATCHv4] " Bernd Edlinger
2020-03-03  2:26                         ` Kees Cook
2020-03-03  4:54                           ` Bernd Edlinger
2020-03-03  5:29                             ` Kees Cook
2020-03-03  8:08                               ` Bernd Edlinger
2020-03-03  8:34                                 ` Christian Brauner
2020-03-03  8:43                                   ` Christian Brauner
2020-03-04 15:30                                 ` Christian Brauner
2020-03-03  8:58                           ` Christian Brauner
2020-03-03 10:34                             ` Bernd Edlinger
2020-03-03 11:23                               ` Bernd Edlinger
2020-03-03 14:20                                 ` Christian Brauner
2020-03-03 13:02                             ` [PATCHv5] " Bernd Edlinger
2020-03-03 15:18                               ` Eric W. Biederman
2020-03-03 16:48                                 ` Bernd Edlinger
2020-03-03 17:01                                   ` Christian Brauner
2020-03-03 17:20                                     ` Christian Brauner
2020-03-03 20:08                                   ` Eric W. Biederman
2020-03-04 14:37                                     ` Bernd Edlinger
2020-03-04 16:33                                       ` Eric W. Biederman
2020-03-04 21:49                                         ` Bernd Edlinger
2020-03-04 21:56                                         ` [PATCHv6] " Bernd Edlinger
2020-03-05 18:36                                           ` Bernd Edlinger
2020-03-05 21:14                                             ` [PATCH 0/2] Infrastructure to allow fixing exec deadlocks Eric W. Biederman
2020-03-05 21:15                                               ` [PATCH 1/2] exec: Properly mark the point of no return Eric W. Biederman
2020-03-05 22:34                                                 ` Bernd Edlinger
2020-03-06  5:19                                                   ` Eric W. Biederman
2020-03-05 22:56                                                 ` Bernd Edlinger
2020-03-06  5:09                                                   ` Eric W. Biederman
2020-03-06 16:26                                                     ` Bernd Edlinger
2020-03-06 17:16                                                       ` Eric W. Biederman
2020-03-05 21:16                                               ` [PATCH 2/2] exec: Add a exec_update_mutex to replace cred_guard_mutex Eric W. Biederman
2020-03-05 21:51                                                 ` Bernd Edlinger
2020-03-06  5:17                                                   ` Eric W. Biederman
2020-03-06 11:46                                                     ` Bernd Edlinger
2020-03-06 21:18                                                       ` Eric W. Biederman
2020-03-06 19:16                                                     ` Bernd Edlinger
2020-03-06 21:58                                                       ` Eric W. Biederman
2020-03-06 22:29                                                         ` Eric W. Biederman
2020-03-07  1:03                                                           ` Eric W. Biederman
2020-03-08 12:58                                                             ` [PATCH] exec: make de_thread alloc new signal struct earlier Bernd Edlinger
2020-03-08 18:12                                                               ` Eric W. Biederman
2020-03-05 22:31                                               ` [PATCH 0/2] Infrastructure to allow fixing exec deadlocks Bernd Edlinger
2020-03-06  5:06                                                 ` Eric W. Biederman
2020-03-08 21:34                                               ` [PATCH 0/5] " Eric W. Biederman
2020-03-08 21:35                                                 ` [PATCH v2 1/5] exec: Only compute current once in flush_old_exec Eric W. Biederman
2020-03-09 13:56                                                   ` Bernd Edlinger
2020-03-09 17:34                                                     ` Eric W. Biederman
2020-03-09 17:56                                                       ` Bernd Edlinger
2020-03-09 19:27                                                         ` Bernd Edlinger
2020-03-10 20:17                                                   ` Kees Cook
2020-03-10 21:12                                                   ` Christian Brauner
2020-03-08 21:36                                                 ` [PATCH v2 2/5] exec: Factor unshare_sighand out of de_thread and call it separately Eric W. Biederman
2020-03-09 19:28                                                   ` Bernd Edlinger
2020-03-10 20:29                                                   ` Kees Cook
2020-03-10 20:34                                                     ` Bernd Edlinger
2020-03-10 20:57                                                       ` Kees Cook
2020-03-10 21:21                                                   ` Christian Brauner
2020-03-08 21:36                                                 ` [PATCH v2 3/5] exec: Move cleanup of posix timers on exec out of de_thread Eric W. Biederman
2020-03-09 19:30                                                   ` Bernd Edlinger
2020-03-09 19:59                                                   ` Christian Brauner
2020-03-09 20:06                                                     ` Eric W. Biederman
2020-03-09 20:17                                                       ` Christian Brauner
2020-03-09 20:48                                                         ` Eric W. Biederman
2020-03-10  8:55                                                           ` Christian Brauner
2020-03-10 18:52                                                             ` [PATCH] pidfd: Stop taking cred_guard_mutex Eric W. Biederman
2020-03-10 19:15                                                               ` Christian Brauner
2020-03-10 19:16                                                               ` Jann Horn
2020-03-10 19:27                                                                 ` Eric W. Biederman
2020-03-10 20:00                                                                   ` Jann Horn
2020-03-10 20:10                                                                     ` Jann Horn
2020-03-10 20:22                                                                       ` Bernd Edlinger
2020-03-11  6:11                                                                         ` Bernd Edlinger [this message]
2020-03-11 14:56                                                                           ` Jann Horn
2020-03-10 20:57                                                                       ` Eric W. Biederman
2020-03-10 21:29                                                                         ` Christian Brauner
2020-03-11 18:49                                                                         ` Kees Cook
2020-03-14  9:12                                                                           ` [PATCH] pidfd: Use new infrastructure to fix deadlocks in execve Bernd Edlinger
2020-03-10 20:16                                                           ` [PATCH v2 3/5] exec: Move cleanup of posix timers on exec out of de_thread Kees Cook
2020-03-10 20:31                                                   ` Kees Cook
2020-03-10 20:57                                                   ` Jann Horn
2020-03-10 21:05                                                     ` Eric W. Biederman
2020-03-10 21:22                                                   ` Christian Brauner
2020-03-08 21:38                                                 ` [PATCH v2 4/5] exec: Move exec_mmap right after de_thread in flush_old_exec Eric W. Biederman
2020-03-09 19:34                                                   ` Bernd Edlinger
2020-03-09 19:45                                                     ` Eric W. Biederman
2020-03-09 19:52                                                       ` Bernd Edlinger
2020-03-09 19:58                                                         ` Eric W. Biederman
2020-03-09 20:03                                                           ` Bernd Edlinger
2020-03-09 20:35                                                             ` Eric W. Biederman
2020-03-10 20:44                                                   ` Kees Cook
2020-03-10 21:20                                                     ` Eric W. Biederman
2020-03-10 20:47                                                   ` Kees Cook
2020-03-10 21:09                                                     ` Eric W. Biederman
2020-03-08 21:38                                                 ` [PATCH v2 5/5] exec: Add a exec_update_mutex to replace cred_guard_mutex Eric W. Biederman
2020-03-09 13:45                                                   ` Bernd Edlinger
2020-03-09 17:40                                                     ` Eric W. Biederman
2020-03-09 18:01                                                       ` Bernd Edlinger
2020-03-09 18:10                                                         ` Eric W. Biederman
2020-03-09 18:24                                                           ` Eric W. Biederman
2020-03-09 18:36                                                             ` Eric W. Biederman
2020-03-09 18:47                                                               ` Bernd Edlinger
2020-03-09 19:02                                                                 ` Eric W. Biederman
2020-03-09 19:24                                                                   ` Bernd Edlinger
2020-03-09 19:35                                                                     ` Eric W. Biederman
2020-03-09 19:39                                                                     ` Eric W. Biederman
2020-03-10 13:43                                                                       ` [PATCH 0/4] Use new infrastructure to fix deadlocks in execve Bernd Edlinger
2020-03-10 15:35                                                                         ` Eric W. Biederman
2020-03-10 17:44                                                                           ` [PATCH 0/4] Use new infrastructure in more simple cases Bernd Edlinger
2020-03-10 17:45                                                                           ` [PATCH 1/4] kernel/kcmp.c: Use new infrastructure to fix deadlocks in execve Bernd Edlinger
2020-03-10 19:01                                                                             ` Eric W. Biederman
2020-03-10 19:42                                                                               ` Bernd Edlinger
2020-03-10 17:45                                                                           ` [PATCH 2/4] proc: " Bernd Edlinger
2020-03-11 18:59                                                                             ` Kees Cook
2020-03-11 19:10                                                                             ` Kees Cook
2020-03-11 19:38                                                                               ` Bernd Edlinger
2020-03-10 17:45                                                                           ` [PATCH 3/4] proc: io_accounting: " Bernd Edlinger
2020-03-10 19:06                                                                             ` Eric W. Biederman
2020-03-10 20:19                                                                               ` Bernd Edlinger
2020-03-10 21:25                                                                                 ` Eric W. Biederman
2020-03-11 19:08                                                                             ` Kees Cook
2020-03-11 19:48                                                                               ` Bernd Edlinger
2020-03-11 19:48                                                                               ` Eric W. Biederman
2020-03-10 17:45                                                                           ` [PATCH 4/4] perf: " Bernd Edlinger
2020-03-10 13:43                                                                       ` [PATCH 1/4] exec: Fix a deadlock in ptrace Bernd Edlinger
2020-03-10 15:13                                                                         ` Eric W. Biederman
2020-03-10 15:17                                                                           ` Bernd Edlinger
2020-03-10 21:00                                                                         ` Kees Cook
2020-03-10 13:44                                                                       ` [PATCH 2/4] selftests/ptrace: add test cases for dead-locks Bernd Edlinger
2020-03-10 21:36                                                                         ` Kees Cook
2020-03-10 22:41                                                                         ` Dmitry V. Levin
2020-03-10 13:44                                                                       ` [PATCH 3/4] mm: docs: Fix a comment in process_vm_rw_core Bernd Edlinger
2020-03-11 18:53                                                                         ` Kees Cook
2020-03-10 13:44                                                                       ` [PATCH 4/4] kernel: doc: remove outdated comment cred.c Bernd Edlinger
2020-03-11 18:54                                                                         ` Kees Cook
2020-03-09 19:33                                                                   ` [PATCH v2 5/5] exec: Add a exec_update_mutex to replace cred_guard_mutex Dmitry V. Levin
2020-03-09 19:42                                                                     ` Eric W. Biederman
2020-03-10 20:55                                                                   ` Kees Cook
2020-03-10 21:02                                                                     ` Eric W. Biederman
2020-03-10 21:21                                                   ` Jann Horn
2020-03-10 21:30                                                     ` Eric W. Biederman
2020-03-10 23:21                                                       ` Jann Horn
2020-03-11  0:15                                                         ` Eric W. Biederman
2020-03-11  6:33                                                           ` Bernd Edlinger
2020-03-11 16:29                                                             ` Eric W. Biederman
2020-03-11 13:18                                                   ` Qian Cai
2020-03-12 10:27                                                   ` Kirill Tkhai
2020-03-12 12:24                                                     ` Eric W. Biederman
2020-03-12 13:45                                                       ` Kirill Tkhai
2020-03-12 14:38                                                         ` Eric W. Biederman
2020-03-12 15:23                                                           ` Kirill Tkhai
2020-03-13  1:05                                                           ` Bernd Edlinger
2020-03-13  9:13                                                             ` Kirill Tkhai
2020-03-14  9:11                                                               ` [PATCH v3 " Bernd Edlinger
2020-03-17  8:56                                                                 ` Kirill Tkhai
2020-03-17 21:53                                                                   ` Bernd Edlinger
2020-03-18 12:22                                                                     ` Kirill Tkhai
2020-03-18 20:06                                                                       ` Bernd Edlinger
2020-03-19  7:13                                                                         ` Kirill Tkhai
2020-03-19  7:19                                                                           ` Bernd Edlinger
2020-03-19  9:11                                                                           ` [PATCH v4 3/5] " Bernd Edlinger
2020-03-19  9:13                                                                             ` Bernd Edlinger
2020-03-19  9:19                                                                               ` Greg Kroah-Hartman
2020-03-19  9:20                                                                                 ` Bernd Edlinger
2020-03-21 22:53                                                                                 ` Bernd Edlinger
2020-03-14  9:12                                                               ` [PATCH 0/2] exec: Fix dead-lock in de_thread with ptrace_attach Bernd Edlinger
2020-03-14  9:12                                                               ` [PATCH 1/2] " Bernd Edlinger
2020-03-14  9:13                                                               ` [PATCH 2/2] doc: Update documentation of ->exec_*_mutex Bernd Edlinger
2020-03-14  9:57                                                               ` [PATCH v2 5/5] exec: Add a exec_update_mutex to replace cred_guard_mutex Bernd Edlinger
2020-03-14 10:02                                                                 ` Bernd Edlinger
2020-03-17  8:58                                                                   ` Kirill Tkhai
2020-03-09 13:58                                                 ` [PATCH 0/5] Infrastructure to allow fixing exec deadlocks Bernd Edlinger
2020-03-03 16:50                                 ` [PATCHv5] exec: Fix a deadlock in ptrace Christian Brauner
2020-03-02 12:28         ` [PATCHv2] " Oleg Nesterov
2020-03-02 15:56           ` Bernd Edlinger
2020-03-02  7:47       ` [PATCH] " Christian Brauner
2020-03-02  7:48         ` Christian Brauner

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