linux-kernel - Re: [PATCH] kernel/fork: stop playing lockless games for exe

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Message-ID: <52192c2f-c7b1-9c07-7ca2-10fc6bd347b0@redhat.com>
Date:   Mon, 14 Aug 2023 17:37:49 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Oleg Nesterov <oleg@...hat.com>, Mateusz Guzik <mjguzik@...il.com>
Cc:     linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
        brauner@...nel.org, ebiederm@...ssion.com,
        akpm@...ux-foundation.org, linux-mm@...ck.org, koct9i@...il.com,
        dave@...olabs.net
Subject: Re: [PATCH] kernel/fork: stop playing lockless games for exe_file
 replacement

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