Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
From:   ebiederm@xmission.com (Eric W. Biederman)
To:     Kirill Tkhai <ktkhai@virtuozzo.com>
Cc:     Bernd Edlinger <bernd.edlinger@hotmail.de>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Kees Cook <keescook@chromium.org>,
        Jann Horn <jannh@google.com>, Jonathan Corbet <corbet@lwn.net>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Oleg Nesterov <oleg@redhat.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Andrei Vagin <avagin@gmail.com>,
        Ingo Molnar <mingo@kernel.org>,
        "Peter Zijlstra \(Intel\)" <peterz@infradead.org>,
        Yuyang Du <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>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Shakeel Butt <shakeelb@google.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Christian Kellner <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>
References: <AM6PR03MB5170EB4427BF5C67EE98FF09E4E60@AM6PR03MB5170.eurprd03.prod.outlook.com>
        <AM6PR03MB5170B976E6387FDDAD59A118E4E70@AM6PR03MB5170.eurprd03.prod.outlook.com>
        <202003021531.C77EF10@keescook>
        <20200303085802.eqn6jbhwxtmz4j2x@wittgenstein>
        <AM6PR03MB5170285B336790D3450E2644E4E40@AM6PR03MB5170.eurprd03.prod.outlook.com>
        <87v9nlii0b.fsf@x220.int.ebiederm.org>
        <AM6PR03MB5170609D44967E044FD1BE40E4E40@AM6PR03MB5170.eurprd03.prod.outlook.com>
        <87a74xi4kz.fsf@x220.int.ebiederm.org>
        <AM6PR03MB51705AA3009B4986BB6EF92FE4E50@AM6PR03MB5170.eurprd03.prod.outlook.com>
        <87r1y8dqqz.fsf@x220.int.ebiederm.org>
        <AM6PR03MB517053AED7DC89F7C0704B7DE4E50@AM6PR03MB5170.eurprd03.prod.outlook.com>
        <AM6PR03MB51703B44170EAB4626C9B2CAE4E20@AM6PR03MB5170.eurprd03.prod.outlook.com>
        <87tv32cxmf.fsf_-_@x220.int.ebiederm.org>
        <87v9ne5y4y.fsf_-_@x220.int.ebiederm.org>
        <87zhcq4jdj.fsf_-_@x220.int.ebiederm.org>
        <f37a5d68-9674-533f-ee9c-a49174605710@virtuozzo.com>
        <87d09hn4kt.fsf@x220.int.ebiederm.org>
        <dbce35c7-c060-cfd8-bde1-98fd9a0747a9@virtuozzo.com>
Date:   Thu, 12 Mar 2020 09:38:09 -0500
In-Reply-To: <dbce35c7-c060-cfd8-bde1-98fd9a0747a9@virtuozzo.com> (Kirill
        Tkhai's message of "Thu, 12 Mar 2020 16:45:37 +0300")
Message-ID: <87lfo5lju6.fsf@x220.int.ebiederm.org>
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux)
MIME-Version: 1.0
Content-Type: text/plain
Subject: Re: [PATCH v2 5/5] exec: Add a exec_update_mutex to replace cred_guard_mutex
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Kirill Tkhai <ktkhai@virtuozzo.com> writes:

> On 12.03.2020 15:24, Eric W. Biederman wrote:
>> Kirill Tkhai <ktkhai@virtuozzo.com> writes:
>> 
>>> On 09.03.2020 00:38, Eric W. Biederman wrote:
>>>>
>>>> The cred_guard_mutex is problematic.  The cred_guard_mutex is held
>>>> over the userspace accesses as the arguments from userspace are read.
>>>> The cred_guard_mutex is held of PTRACE_EVENT_EXIT as the the other
>>>> threads are killed.  The cred_guard_mutex is held over
>>>> "put_user(0, tsk->clear_child_tid)" in exit_mm().
>>>>
>>>> Any of those can result in deadlock, as the cred_guard_mutex is held
>>>> over a possible indefinite userspace waits for userspace.
>>>>
>>>> Add exec_update_mutex that is only held over exec updating process
>>>> with the new contents of exec, so that code that needs not to be
>>>> confused by exec changing the mm and the cred in ways that can not
>>>> happen during ordinary execution of a process.
>>>>
>>>> The plan is to switch the users of cred_guard_mutex to
>>>> exec_udpate_mutex one by one.  This lets us move forward while still
>>>> being careful and not introducing any regressions.
>>>>
>>>> Link: https://lore.kernel.org/lkml/20160921152946.GA24210@dhcp22.suse.cz/
>>>> Link: https://lore.kernel.org/lkml/AM6PR03MB5170B06F3A2B75EFB98D071AE4E60@AM6PR03MB5170.eurprd03.prod.outlook.com/
>>>> Link: https://lore.kernel.org/linux-fsdevel/20161102181806.GB1112@redhat.com/
>>>> Link: https://lore.kernel.org/lkml/20160923095031.GA14923@redhat.com/
>>>> Link: https://lore.kernel.org/lkml/20170213141452.GA30203@redhat.com/
>>>> Ref: 45c1a159b85b ("Add PTRACE_O_TRACEVFORKDONE and PTRACE_O_TRACEEXIT facilities.")
>>>> Ref: 456f17cd1a28 ("[PATCH] user-vm-unlock-2.5.31-A2")
>>>> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
>>>> ---
>>>>  fs/exec.c                    | 9 +++++++++
>>>>  include/linux/sched/signal.h | 9 ++++++++-
>>>>  init/init_task.c             | 1 +
>>>>  kernel/fork.c                | 1 +
>>>>  4 files changed, 19 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/fs/exec.c b/fs/exec.c
>>>> index d820a7272a76..ffeebb1f167b 100644
>>>> --- a/fs/exec.c
>>>> +++ b/fs/exec.c
>>>> @@ -1014,6 +1014,7 @@ static int exec_mmap(struct mm_struct *mm)
>>>>  {
>>>>  	struct task_struct *tsk;
>>>>  	struct mm_struct *old_mm, *active_mm;
>>>> +	int ret;
>>>>  
>>>>  	/* Notify parent that we're no longer interested in the old VM */
>>>>  	tsk = current;
>>>> @@ -1034,6 +1035,11 @@ static int exec_mmap(struct mm_struct *mm)
>>>>  			return -EINTR;
>>>>  		}
>>>>  	}
>>>> +
>>>> +	ret = mutex_lock_killable(&tsk->signal->exec_update_mutex);
>>>> +	if (ret)
>>>> +		return ret;
>>>
>>> You missed old_mm->mmap_sem unlock. See here:
>> 
>> Duh.  Thank you.
>> 
>> I actually need to switch the lock ordering here, and I haven't yet
>> because my son was sick yesterday.
>
> There is some fundamental problem with your patch, since the below fires in 100% cases
> on current linux-next:

Thank you.

I have just backed this out of linux-next for now because it is clearly
flawed.

You make some good points about the recursion.  I will go back to the
drawing board and see what I can work out.


> [   22.838717] kernel BUG at fs/exec.c:1474!
>
> diff --git a/fs/exec.c b/fs/exec.c
> index 47582cd97f86..0f77f8c94905 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1470,8 +1470,10 @@ static void free_bprm(struct linux_binprm *bprm)
>  {
>  	free_arg_pages(bprm);
>  	if (bprm->cred) {
> -		if (!bprm->mm)
> +		if (!bprm->mm) {
> +			BUG_ON(!mutex_is_locked(&current->signal->exec_update_mutex));
>  			mutex_unlock(&current->signal->exec_update_mutex);
> +		}
>  		mutex_unlock(&current->signal->cred_guard_mutex);
>  		abort_creds(bprm->cred);
>  	}
> @@ -1521,6 +1523,7 @@ void install_exec_creds(struct linux_binprm *bprm)
>  	 * credentials; any time after this it may be unlocked.
>  	 */
>  	security_bprm_committed_creds(bprm);
> +	BUG_ON(!mutex_is_locked(&current->signal->exec_update_mutex));
>  	mutex_unlock(&current->signal->exec_update_mutex);
>  	mutex_unlock(&current->signal->cred_guard_mutex);
>  }
>
> ---------------------------------------------------------------------------------------------
>
> First time the mutex is unlocked in:
>
> exec_binprm()->search_binary_handler()->.load_binary->install_exec_creds()
>
> Then exec_binprm()->search_binary_handler()->.load_binary->flush_old_exec() clears mm:
>
>         bprm->mm = NULL;        
>
> Second time the mutex is unlocked in free_bprm():
>
> 	if (bprm->cred) {
>                 if (!bprm->mm)
>                         mutex_unlock(&current->signal->exec_update_mutex);
>
> My opinion is we should not relay on side indicators like bprm->mm. Better you may
> introduce struct linux_binprm::exec_update_mutex_is_locked. So the next person dealing
> with this after you won't waste much time on diving into this. Also, if someone decides
> to change the place, where bprm->mm is set into NULL, this person will bump into hell
> of dependences between unrelated components like your newly introduced mutex.
>
> So, I'm strongly for *struct linux_binprm::exec_update_mutex_is_locked*, since this improves
> modularity.

Am I wrong or is that also a problem with cred_guard_mutex?

Eric