linux-kernel - Re: [PATCH v6 00/10] Retpoline: Avoid speculative indirect calls in kernel

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Date:   Mon, 8 Jan 2018 11:16:24 +0000
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Paul Turner <pjt@...gle.com>, David Woodhouse <dwmw@...zon.co.uk>
CC:     Andi Kleen <ak@...ux.intel.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>,
        Thomas Gleixner <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>,
        One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
Subject: Re: [PATCH v6 00/10] Retpoline: Avoid speculative indirect calls in
 kernel

On 08/01/18 10:42, Paul Turner wrote:
> A sequence for efficiently refilling the RSB is:
>     mov $8, %rax;
>     .align 16;
>    3: call 4f;
>   3p: pause; call 3p;
>      .align 16;
>   4: call 5f;
>   4p: pause; call 4p;
>      .align 16;
>    5: dec %rax;
>       jnz 3b;
>       add $(16*8), %rsp;
> This implementation uses 8 loops, with 2 calls per iteration.  This is
> marginally faster than a single call per iteration.  We did not
> observe useful benefit (particularly relative to text size) from
> further unrolling.  This may also be usefully split into smaller (e.g.
> 4 or 8 call)  segments where we can usefully pipeline/intermix with
> other operations.  It includes retpoline type traps so that if an
> entry is consumed, it cannot lead to controlled speculation.  On my
> test system it took ~43 cycles on average.  Note that non-zero
> displacement calls should be used as these may be optimized to not
> interact with the RSB due to their use in fetching RIP for 32-bit
> relocations.

Guidance from both Intel and AMD still states that 32 calls are required
in general.  Is your above code optimised for a specific processor which
you know the RSB to be smaller on?

~Andrew