linux-kernel - Re: [PATCH v2] devtmpfs: mount with noexec and nosuid

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Date:	Tue, 20 Nov 2012 23:53:53 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Kees Cook <keescook@...omium.org>
Cc:	linux-kernel@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	ellyjones@...omium.org, Kay Sievers <kay@...y.org>,
	Roland Eggner <edvx1@...temanalysen.net>
Subject: Re: [PATCH v2] devtmpfs: mount with noexec and nosuid

> > 1. Why not just have your userspace mount -o remount the file system this
> > way already in early boot. (and if you trojanned boot that early then any
> > supposed security gain is already lost)
> 
> That's certainly possible, but I am hoping to avoid adding any extra
> boot time. The kernel is responsible for this mount, so its flags
> should be configurable, resulting in no time penalty anywhere.

You just broke my bullshitometer

It's a single syscall from your init binary, its microseconds.

> > 2. If you want to do this right then you need to work out what you are
> > trying to prevent. Your devtmpfs can force file permissions on the
> > underlying device nodes by having its own operation handling for chmod.
> >
> > At that point you can force permissions on anything that you want to
> > avoid floating around that filesystem with other rights, while not
> > touching it on device or directory nodes where the meaning is different.
> >
> > In its current form however it appears to be a kernel implementation of
> > "mount is too hard".
> 
> This change also stops mmap() with PROT_EXEC which a chmod handler
> wouldn't be able to do.

You don't want to stop mmap with PROT_EXEC on /dev/mem as that breaks a
load of stuff, you want to stop people adding stuff to that file system
and executing it.

Think about it more carefully - if I've got access to /dev/mem you
already lost.

The patch is as is nonsense. It's doable in all kernels (including
existing legacy ones) as a single syscall in your init code. That's
portable back compatible and works all over the place. Your patch
introduces a pointless config option to make things less compatible than
before. It's bogus.

Alan


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