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

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Message-ID: <CAGXu5jJWe8F-uj+ukPNMeo0W02YA4ssmLyBX4VTJ1nmeB-HxHw@mail.gmail.com>
Date:	Thu, 10 Dec 2015 14:00:07 -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 1:56 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Thu, Dec 10, 2015 at 01:45:09PM -0800, Kees Cook wrote:
>> > 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?
>
> Are you asking whether it's safe to take a spinlock under an rwsem?

I keep getting various surprises while trying to implement this
change, so yeah, I just want to make sure I won't waste my time adding
taking the spinlock to the patch.

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security
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