linux-kernel - Re: [PATCH v3 10/13] x86/retpoline/pvops: Convert assembler indirect jumps

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Message-ID: <3c1bcc6d-d509-c48e-32ab-a24596f03e2f@citrix.com>
Date:   Thu, 4 Jan 2018 15:18:02 +0000
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Juergen Gross <jgross@...e.com>,
        David Woodhouse <dwmw@...zon.co.uk>, <ak@...ux.intel.com>
CC:     Paul Turner <pjt@...gle.com>, LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>, <tglx@...utronix.de>,
        Kees Cook <keescook@...gle.com>,
        Rik van Riel <riel@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...capital.net>,
        Jiri Kosina <jikos@...nel.org>, <gnomes@...rguk.ukuu.org.uk>
Subject: Re: [PATCH v3 10/13] x86/retpoline/pvops: Convert assembler indirect
 jumps

On 04/01/18 15:02, Juergen Gross wrote:
> On 04/01/18 15:37, David Woodhouse wrote:
>> Convert pvops invocations to use non-speculative call sequences, when
>> CONFIG_RETPOLINE is enabled.
>>
>> There is scope for future optimisation here — once the pvops methods are
>> actually set, we could just turn the damn things into *direct* jumps.
>> But this is perfectly sufficient for now, without that added complexity.
> I don't see the need to modify the pvops calls.
>
> All indirect calls are replaced by either direct calls or other code
> long before any user code is active.
>
> For modules the replacements are in place before the module is being
> used.

When booting virtualised, sibling hyperthreads can arrange VM-to-VM SP2
attacks.

One mitigation though is to consider if there is any interesting data to
leak that early during boot.

~Andrew