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

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Date:	Thu, 10 Dec 2015 08:10:22 +0100
From:	Willy Tarreau <w@....eu>
To:	Kees Cook <keescook@...omium.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, Jan Kara <jack@...e.cz>,
	yalin wang <yalin.wang2010@...il.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5] fs: clear file privilege bits when mmap writing

On Thu, Dec 10, 2015 at 08:06:35AM +0100, Willy Tarreau wrote:
> Hi Kees,
> 
> Why not add a new file flag instead ?
> 
> Something like this (editing your patch by hand to illustrate) :
(...)
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 3aa514254161..409bd7047e7e 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -913,3 +913,4 @@
>  #define FL_OFDLCK       1024    /* lock is "owned" by struct file */
>  #define FL_LAYOUT       2048    /* outstanding pNFS layout */
> +#define FL_DROP_PRIVS   4096    /* lest something weird decides that 2 is OK */

Crap, these ones are for locks, we need to use O_* instead
But anyway you get the idea, I mean there are probably many spare bits
overthere.

Another option I was thinking about was to change f_mode and detect the
change on close. But I don't know what to compare it against.

Willy

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