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Subject: Re: [PATCH v3 5/5] exec: Add a exec_update_mutex to replace
 cred_guard_mutex
To:     Bernd Edlinger <bernd.edlinger@hotmail.de>,
        "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     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>
 <87r1y8dqqz.fsf@x220.int.ebiederm.org>
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 <AM6PR03MB51703B44170EAB4626C9B2CAE4E20@AM6PR03MB5170.eurprd03.prod.outlook.com>
 <87tv32cxmf.fsf_-_@x220.int.ebiederm.org>
 <87v9ne5y4y.fsf_-_@x220.int.ebiederm.org>
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 <f37a5d68-9674-533f-ee9c-a49174605710@virtuozzo.com>
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 <dbce35c7-c060-cfd8-bde1-98fd9a0747a9@virtuozzo.com>
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 <AM6PR03MB51705D8A5631B53844CE447CE4F60@AM6PR03MB5170.eurprd03.prod.outlook.com>
From:   Kirill Tkhai <ktkhai@virtuozzo.com>
Message-ID: <13c4d333-9c33-8036-3142-dac22c392c60@virtuozzo.com>
Date:   Wed, 18 Mar 2020 15:22:19 +0300
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On 18.03.2020 00:53, Bernd Edlinger wrote:
> On 3/17/20 9:56 AM, Kirill Tkhai wrote:
>> On 14.03.2020 12:11, Bernd Edlinger 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>
>>> Signed-off-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
>>> ---
>>>  fs/exec.c                    | 17 ++++++++++++++---
>>>  include/linux/binfmts.h      |  8 +++++++-
>>>  include/linux/sched/signal.h |  9 ++++++++-
>>>  init/init_task.c             |  1 +
>>>  kernel/fork.c                |  1 +
>>>  5 files changed, 31 insertions(+), 5 deletions(-)
>>>
>>> v3: this update fixes lock-order and adds an explicit data member in linux_binprm
>>>
>>> diff --git a/fs/exec.c b/fs/exec.c
>>> index d820a72..11974a1 100644
>>> --- a/fs/exec.c
>>> +++ b/fs/exec.c
>>> @@ -1014,12 +1014,17 @@ 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;
>>>  	old_mm = current->mm;
>>>  	exec_mm_release(tsk, old_mm);
>>>  
>>> +	ret = mutex_lock_killable(&tsk->signal->exec_update_mutex);
>>> +	if (ret)
>>> +		return ret;
>>> +
>>>  	if (old_mm) {
>>>  		sync_mm_rss(old_mm);
>>>  		/*
>>> @@ -1031,9 +1036,11 @@ static int exec_mmap(struct mm_struct *mm)
>>>  		down_read(&old_mm->mmap_sem);
>>>  		if (unlikely(old_mm->core_state)) {
>>>  			up_read(&old_mm->mmap_sem);
>>> +			mutex_unlock(&tsk->signal->exec_update_mutex);
>>>  			return -EINTR;
>>>  		}
>>>  	}
>>> +
>>>  	task_lock(tsk);
>>>  	active_mm = tsk->active_mm;
>>>  	membarrier_exec_mmap(mm);
>>> @@ -1288,11 +1295,12 @@ int flush_old_exec(struct linux_binprm * bprm)
>>>  		goto out;
>>>  
>>>  	/*
>>> -	 * After clearing bprm->mm (to mark that current is using the
>>> -	 * prepared mm now), we have nothing left of the original
>>> +	 * After setting bprm->called_exec_mmap (to mark that current is
>>> +	 * using the prepared mm now), we have nothing left of the original
>>>  	 * process. If anything from here on returns an error, the check
>>>  	 * in search_binary_handler() will SEGV current.
>>>  	 */
>>> +	bprm->called_exec_mmap = 1;
>>
>> The two below is non-breaking pair:
>>
>> exec_mmap(bprm->mm);
>> bprm->called_exec_mmap = 1;
>>
>> Why not move this into exec_mmap(), so nobody definitely inserts something
>> between them?
>>
> 
> Hmm, could be done, but then I would probably need a different name than
> "called_exec_mmap".
> 
> How about adding a nice function comment to exec_mmap that calls out the
> changed behaviour that the exec_update_mutex is taken unless the function
> fails?

Not sure, I understand correct.

Could you post this like a small patch hunk (on top of anything you want)?

> Bernd.
> 
> 
>>>  	bprm->mm = NULL;
>>>  
>>>  #ifdef CONFIG_POSIX_TIMERS
>>> @@ -1438,6 +1446,8 @@ static void free_bprm(struct linux_binprm *bprm)
>>>  {
>>>  	free_arg_pages(bprm);
>>>  	if (bprm->cred) {
>>> +		if (bprm->called_exec_mmap)
>>> +			mutex_unlock(&current->signal->exec_update_mutex);
>>>  		mutex_unlock(&current->signal->cred_guard_mutex);
>>>  		abort_creds(bprm->cred);
>>>  	}
>>> @@ -1487,6 +1497,7 @@ void install_exec_creds(struct linux_binprm *bprm)
>>>  	 * credentials; any time after this it may be unlocked.
>>>  	 */
>>>  	security_bprm_committed_creds(bprm);
>>> +	mutex_unlock(&current->signal->exec_update_mutex);
>>>  	mutex_unlock(&current->signal->cred_guard_mutex);
>>>  }
>>>  EXPORT_SYMBOL(install_exec_creds);
>>> @@ -1678,7 +1689,7 @@ int search_binary_handler(struct linux_binprm *bprm)
>>>  
>>>  		read_lock(&binfmt_lock);
>>>  		put_binfmt(fmt);
>>> -		if (retval < 0 && !bprm->mm) {
>>> +		if (retval < 0 && bprm->called_exec_mmap) {
>>>  			/* we got to flush_old_exec() and failed after it */
>>>  			read_unlock(&binfmt_lock);
>>>  			force_sigsegv(SIGSEGV);
>>> diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
>>> index b40fc63..a345d9f 100644
>>> --- a/include/linux/binfmts.h
>>> +++ b/include/linux/binfmts.h
>>> @@ -44,7 +44,13 @@ struct linux_binprm {
>>>  		 * exec has happened. Used to sanitize execution environment
>>>  		 * and to set AT_SECURE auxv for glibc.
>>>  		 */
>>> -		secureexec:1;
>>> +		secureexec:1,
>>> +		/*
>>> +		 * Set by flush_old_exec, when exec_mmap has been called.
>>> +		 * This is past the point of no return, when the
>>> +		 * exec_update_mutex has been taken.
>>> +		 */
>>> +		called_exec_mmap:1;
>>>  #ifdef __alpha__
>>>  	unsigned int taso:1;
>>>  #endif
>>> diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
>>> index 8805025..a29df79 100644
>>> --- a/include/linux/sched/signal.h
>>> +++ b/include/linux/sched/signal.h
>>> @@ -224,7 +224,14 @@ struct signal_struct {
>>>  
>>>  	struct mutex cred_guard_mutex;	/* guard against foreign influences on
>>>  					 * credential calculations
>>> -					 * (notably. ptrace) */
>>> +					 * (notably. ptrace)
>>> +					 * Deprecated do not use in new code.
>>> +					 * Use exec_update_mutex instead.
>>> +					 */
>>> +	struct mutex exec_update_mutex;	/* Held while task_struct is being
>>> +					 * updated during exec, and may have
>>> +					 * inconsistent permissions.
>>> +					 */
>>>  } __randomize_layout;
>>>  
>>>  /*
>>> diff --git a/init/init_task.c b/init/init_task.c
>>> index 9e5cbe5..bd403ed 100644
>>> --- a/init/init_task.c
>>> +++ b/init/init_task.c
>>> @@ -26,6 +26,7 @@
>>>  	.multiprocess	= HLIST_HEAD_INIT,
>>>  	.rlim		= INIT_RLIMITS,
>>>  	.cred_guard_mutex = __MUTEX_INITIALIZER(init_signals.cred_guard_mutex),
>>> +	.exec_update_mutex = __MUTEX_INITIALIZER(init_signals.exec_update_mutex),
>>>  #ifdef CONFIG_POSIX_TIMERS
>>>  	.posix_timers = LIST_HEAD_INIT(init_signals.posix_timers),
>>>  	.cputimer	= {
>>> diff --git a/kernel/fork.c b/kernel/fork.c
>>> index 8642530..036b692 100644
>>> --- a/kernel/fork.c
>>> +++ b/kernel/fork.c
>>> @@ -1594,6 +1594,7 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
>>>  	sig->oom_score_adj_min = current->signal->oom_score_adj_min;
>>>  
>>>  	mutex_init(&sig->cred_guard_mutex);
>>> +	mutex_init(&sig->exec_update_mutex);
>>>  
>>>  	return 0;
>>>  }
>>>
>>