Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <52192c2f-c7b1-9c07-7ca2-10fc6bd347b0@redhat.com>
Date:   Mon, 14 Aug 2023 17:37:49 +0200
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101
 Thunderbird/102.13.0
Subject: Re: [PATCH] kernel/fork: stop playing lockless games for exe_file
 replacement
Content-Language: en-US
To:     Oleg Nesterov <oleg@redhat.com>, Mateusz Guzik <mjguzik@gmail.com>
Cc:     linux-kernel@vger.kernel.org, torvalds@linux-foundation.org,
        brauner@kernel.org, ebiederm@xmission.com,
        akpm@linux-foundation.org, linux-mm@kvack.org, koct9i@gmail.com,
        dave@stgolabs.net
References: <20230813123333.1705833-1-mjguzik@gmail.com>
 <20230814150530.GB17738@redhat.com> <20230814152038.GA2367@redhat.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <20230814152038.GA2367@redhat.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 14.08.23 17:20, Oleg Nesterov wrote:
> On 08/14, Oleg Nesterov wrote:
>>
>> On 08/13, Mateusz Guzik wrote:
>>>
>>> fe69d560b5bd ("kernel/fork: always deny write access to current MM
>>> exe_file") added another lock trip to synchronize the state of exe_file
>>> against fork, further defeating the point of xchg.
>>>
>>> As such I think the atomic here only adds complexity for no benefit.
>>>
>>> Just write-lock around the replacement.
>>
>> Well, I tend to agree but can't really comment because I forgot everything
>> about these code paths.
>>
>> But I have to admit that I don't understand the code in replace_mm_exe_file()
>> without this patch...
>>
>> 	old_exe_file = xchg(&mm->exe_file, new_exe_file);
>> 	if (old_exe_file) {
>> 		/*
>> 		 * Don't race with dup_mmap() getting the file and disallowing
>> 		 * write access while someone might open the file writable.
>> 		 */
>> 		mmap_read_lock(mm);
>> 		allow_write_access(old_exe_file);
>> 		fput(old_exe_file);
>> 		mmap_read_unlock(mm);
>> 	}
>>
>> Can someone please explain me which exactly race this mmap_read_lock() tries
>> to avoid and how ?
> 
> OK, I seem to understand... without mmap_read_lock() it is possible that
> 
> 	- dup_mm_exe_file() sees mm->exe_file = old_exe_file
> 
> 	- replace_mm_exe_file() does allow_write_access(old_exe_file)
> 
> 	- another process does get_write_access(old_exe_file)
> 
> 	- dup_mm_exe_file()->deny_write_access() fails
> 
> Right?

 From what I recall, yes.

-- 
Cheers,

David / dhildenb