linux-kernel - RE: [PATCH 06/18] x86, barrier: stop speculation for failed access

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Date:   Mon, 8 Jan 2018 12:33:19 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Alan Cox' <gnomes@...rguk.ukuu.org.uk>
CC:     'Linus Torvalds' <torvalds@...ux-foundation.org>,
        Willy Tarreau <w@....eu>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        Andi Kleen <ak@...ux.intel.com>, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Peter Zijlstra" <peterz@...radead.org>,
        Network Development <netdev@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: RE: [PATCH 06/18] x86, barrier: stop speculation for failed access_ok

From: Alan Cox
> Sent: 08 January 2018 12:13
...
> > Try over 35% slowdown....
> > Given that AWS instance runs known code (user and kernel) why do we
> > need to worry about any of these sideband attacks?
> 
> You may not need to. Amazon themselves obviously need to worry that no
> other VM steals your data (or vice versa) but above that (and with raw
> hardware appliances) if you control all the code you run then the nopti
> and other disables may be useful (At the end of the day as with anything
> else you do your own risk assessment).

I believe AWS allows VM kernels to load user-written device drivers
so the security of other VMs cannot rely on whether a VM is booted
with PTI=yes or PTI=no.

	David