linux-kernel - Re: Avoid speculative indirect calls in kernel

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Date:   Thu, 04 Jan 2018 16:15:07 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Andrew Cooper <andrew.cooper3@...rix.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "pavel@....cz" <pavel@....cz>,
        Tom Lendacky <thomas.lendacky@....com>
Cc:     "tim.c.chen@...ux.intel.com" <tim.c.chen@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "andi@...stfloor.org" <andi@...stfloor.org>,
        "gnomes@...rguk.ukuu.org.uk" <gnomes@...rguk.ukuu.org.uk>,
        "dave.hansen@...el.com" <dave.hansen@...el.com>,
        "gregkh@...ux-foundation.org" <gregkh@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: Avoid speculative indirect calls in kernel

On Thu, 2018-01-04 at 15:29 +0000, Woodhouse, David wrote:
> 
> > With the GCC -mindirect-branch=thunk-external support, and microcode,
> > Xen will make a boot-time choice between using Retpoline, Lfence (which
> > is the better AMD option, and more performant than retpoline), or IBRS
> > on Skylake and newer processors where it is strictly necessary, as well
> > as using IBPB whenever available.
> 
> I need to pull in the AMD lfence alternative for retpoline, giving us a
> 3-way choice of the existing retpoline thunk, "lfence; jmp *%\reg", and
> a bare "jmp *%\reg".

I think I can abuse X86_FEATURE_SYSCALL for that, right? So it would
look something like this:

 --- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -12,7 +12,7 @@
 
 ENTRY(__x86.indirect_thunk.\reg)
        CFI_STARTPROC
-       ALTERNATIVE "call 2f", __stringify(jmp *%\reg), X86_BUG_NO_RETPOLINE
+       ALTERNATIVE_2 "call 2f", __stringify(lfence;jmp *%\reg), X86_FEATURE_SYSCALL, __stringify(jmp *%\reg), X86_BUG_NO_RETPOLINE
 1:
        lfence
        ASM_UNREACHABLE


However, I would very much like to see a categorical statement from AMD
that the lfence is sufficient in all cases. Remember, Intel were saying
that too for a while, before finding that it was not *quite* good
enough.
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