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 [thread overview]
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(¤t->signal->cred_guard_mutex);
> + rv = mutex_lock_interruptible(¤t->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(¤t->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(¤t->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.
next prev parent reply other threads:[~2020-03-29 4:31 UTC|newest]
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
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