linux-kernel - Re: [PATCH v5] fs: clear file privilege bits when mmap writing

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Message-ID: <CAGXu5jLF5-jbQ8tEHWnTZKqWj5_kmrqdKcJMb_B_HdN34RwCqA@mail.gmail.com>
Date:	Thu, 10 Dec 2015 11:47:18 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	Willy Tarreau <w@....eu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kara <jack@...e.cz>, yalin wang <yalin.wang2010@...il.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5] fs: clear file privilege bits when mmap writing

On Thu, Dec 10, 2015 at 11:33 AM, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Thu, Dec 10, 2015 at 07:16:11PM +0100, Willy Tarreau wrote:
>
>> > Is f_flags safe to write like this without holding a lock?
>>
>> Unfortunately I have no idea. I've seen places where it's written without
>> taking a lock such as in blkdev_open() and I don't think that this one is
>> called with a lock held.
>
> In any ->open() we obviously have nobody else able to find that struct file,
> let alone modify it, so there the damn thing is essentially caller-private
> and no locking is needed.

In open, sure, but what about under mm/memory.c where we're trying to
twiddle it from vma->file->f_flags as in my patch? That seemed like it
would want atomic safety.

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security
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