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Date:   Mon, 14 Aug 2023 17:20:38 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     Mateusz Guzik <mjguzik@gmail.com>
Cc:     linux-kernel@vger.kernel.org, torvalds@linux-foundation.org,
        brauner@kernel.org, ebiederm@xmission.com, david@redhat.com,
        akpm@linux-foundation.org, linux-mm@kvack.org, koct9i@gmail.com,
        dave@stgolabs.net
Subject: Re: [PATCH] kernel/fork: stop playing lockless games for exe_file
 replacement
Message-ID: <20230814152038.GA2367@redhat.com>
References: <20230813123333.1705833-1-mjguzik@gmail.com>
 <20230814150530.GB17738@redhat.com>
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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?

Or something else?

Well to me Mateusz's patch does make this logic more clear ;)

Oleg.