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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 08:36:15 +0200
Message-ID: <AM6PR03MB5170480FE4462D0DFF04B14BE4CA0@AM6PR03MB5170.eurprd03.prod.outlook.com> (raw)
In-Reply-To: <AM6PR03MB51700B30078BDCB6C6A4E0CAE4CA0@AM6PR03MB5170.eurprd03.prod.outlook.com>

On 3/29/20 6:31 AM, Bernd Edlinger wrote:
> On 3/25/20 3:27 PM, Eric W. Biederman wrote:
>> Bernd Edlinger <bernd.edlinger@hotmail.de> writes:
>> 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.
> 

So how about that:

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 221759e..2d6b5cd 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -395,6 +395,17 @@ static int ptrace_attach(struct task_struct *task, long req
        if (mutex_lock_interruptible(&task->signal->exec_guard_mutex))
                goto out;
 
+       /*
+        * BIG FAT WARNING - Fragile code ahead.
+        * Please do not insert any code between these two
+        * if statements.  It may happen that execve has to
+        * release the exec_guard_mutex in order to prevent
+        * deadlocks.  In that case unsafe_execve_in_progress
+        * will be set.  If that happens you cannot assume that
+        * the usual guarantees implied by exec_guard_mutex
+        * are valid.  Just return -EAGAIN in that case and
+        * unlock the mutex immediately.
+        */
        retval = -EAGAIN;
        if (unlikely(task->signal->unsafe_execve_in_progress))
                goto unlock_creds;

Is that cool enough :-)


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
2020-03-29  6:36       ` Bernd Edlinger [this message]
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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