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

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Message-ID: <CAGXu5jLAK8SYDcrCbJhb4jRtLVW9xjaNi-k68-QV-8_FqZrdqA@mail.gmail.com>
Date:	Thu, 10 Dec 2015 13:45:09 -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 12:27 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Thu, Dec 10, 2015 at 11:47:18AM -0800, Kees Cook wrote:
>
>> 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.
>
> Sigh...  Again, I'm not at all convinced that this is the right approach,

I'm open to any suggestions. Every path I've tried has been ultimately
blocked by mmap_sem. :(

> but generally you need ->f_lock.  And in situations where the bit can
> go only off->on, check it lockless, skip the whole thing entirely if it's
> already set and grab the spinlock otherwise.

And I can take f_lock safely under mmap_sem?

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security
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