linux-kernel - Re: Proposal: CAP_PAYLOAD to reduce Meltdown and Spectre mitigation costs

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Message-ID: <20180107180643.GG2404@thunk.org>
Date:   Sun, 7 Jan 2018 13:06:43 -0500
From:   Theodore Ts'o <tytso@....edu>
To:     Avi Kivity <avi@...lladb.com>
Cc:     Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Proposal: CAP_PAYLOAD to reduce Meltdown and Spectre mitigation
 costs

On Sun, Jan 07, 2018 at 02:51:59PM +0200, Avi Kivity wrote:
> 
> I don't see the connection. The browser wouldn't run with CAP_PAYLOAD set.
> 
> In a desktop system, only init retains CAP_PAYLOAD.
> 
> On a server that runs one application (and some supporting processes), only
> init and that one application have CAP_PAYLOAD (if the sysadmin makes it
> so).

In the classical (as defined by the withdrawn Posix draft spec)
capaibilities model, if you have a setuid root process it gets all the
capabilities, and capabilities are used to limit what privileges a
root process.  Hence using strict capabilities, any setuid root
process would have CAP_PAYLOAD.

Linux has extensions which allow you to have capability bound which
capabilities that can be obtained by a process, so you _could_ make it
work, but it just seems like an bad fit, since it's not strictly
speaking a root-owned privilege.  It's more like a configuration
setting, and so modulating it via cgroups attribute seems to make a
lot more sense --- it's certainly (IMHO) less confusing than trying to
ab(use) the capabilities system and its extensions in this fashion.

	    		 	    	- Ted