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

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Message-ID: <20180107172637.GA9772@1wt.eu>
Date:   Sun, 7 Jan 2018 18:26:37 +0100
From:   Willy Tarreau <w@....eu>
To:     Alan Cox <gnomes@...rguk.ukuu.org.uk>
Cc:     Avi Kivity <avi@...lladb.com>,
        "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 02:36:28PM +0000, Alan Cox wrote:
> What I struggle to see is why I'd want to nominate specific processes for
> this except in very special cases (like your packet generator). Even then
> it would make me nervous as the packet generator if that trusted is
> effectively CAP_SYS_RAWIO or close to it and can steal any ssh keys or
> similar on that guest.

Sure but we can also say that a process with CAP_SYS_RAWIO can manipulate
the hardware using iopl() and reprogram memory controllers, PCI bridges
and various stuff to have direct access wherever it wants. That's why I
thought that grouping the risks reduces the attack surface in the end.

> I still prefer cgroups because once you include the branch predictions it
> suddenly becomes very interesting to be able to say 'this pile of stuff
> trusts itself' and avoid user/user protection costs while keeping
> user/kernel.

To be honnest, I don't know what it would imply in terms of management
(for the admin). Also, I'm really focused on the extra work to add to
syscalls, which should remain very minimalistic. Checking a flag on the
current task sounds reasonable. I don't know how far we might have to
go with cgroups. I remember a very long time ago you once todl me "we
have fast syscalls", I'd like this statement to remain true for those
who continue to rely on this property ;-)

Willy