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

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Message-ID: <d71a1b44-9b17-4b49-3a38-530c739e8d82@scylladb.com>
Date:   Sun, 7 Jan 2018 14:51:59 +0200
From:   Avi Kivity <avi@...lladb.com>
To:     Theodore Ts'o <tytso@....edu>,
        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 01/07/2018 02:29 PM, Theodore Ts'o wrote:
> On Sun, Jan 07, 2018 at 11:16:28AM +0200, Avi Kivity wrote:
>> I think capabilities will work just as well with cgroups. The container
>> manager will set CAP_PAYLOAD to payload containers; and if those run an init
>> system or a container manager themselves, they'll drop CAP_PAYLOAD for all
>> process/sub-containers but their payloads.
> The reason why cgroups are better is Spectre can be used to steal
> information from within the same privilege level --- e.g., you could
> use Javascript to steal a user's Coindesk credentials or Lastpass
> data, which is going to be *way* more lucrative than trying to mine
> cryptocurrency in the sly in a user's browser.  :-)
>
> As a result, you probably want Spectre mitigations to be enabled in a
> root process --- which means capabilities aren't the right answer.
>
>

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).

On a containerized server that happens to run just one application, init 
will retain CAP_PAYLOAD, as well as the process in the container (if the 
sysadmin makes it so).

On a containerized server that happens to run just one application, 
which itself runs an init system, the two inits will retain CAP_PAYLOAD, 
as well as the application process (if the sysadmin makes it so).