linux-kernel - Re: [PATCH 4/7] x86,nospec: Annotate indirect calls/jumps

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Message-ID: <20180201152134.63e553pvoq7mbm4p@treble>
Date:   Thu, 1 Feb 2018 09:21:34 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     David Woodhouse <dwmw2@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, Dave Hansen <dave.hansen@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>
Subject: Re: [PATCH 4/7] x86,nospec: Annotate indirect calls/jumps

On Thu, Feb 01, 2018 at 04:13:48PM +0100, Peter Zijlstra wrote:
> On Thu, Feb 01, 2018 at 04:11:36PM +0100, Peter Zijlstra wrote:
> > On Thu, Feb 01, 2018 at 02:55:26PM +0000, David Woodhouse wrote:
> > > 
> > > 
> > > On Thu, 2018-02-01 at 15:34 +0100, Peter Zijlstra wrote:
> > > > 
> > > >   * These are the bare retpoline primitives for indirect jmp and call.
> > > >   * Do not use these directly; they only exist to make the ALTERNATIVE
> > > >   * invocation below less ugly.
> > > > @@ -102,9 +114,9 @@
> > > >  .macro JMP_NOSPEC reg:req
> > > >  #ifdef CONFIG_RETPOLINE
> > > >         ANNOTATE_NOSPEC_ALTERNATIVE
> > > > -       ALTERNATIVE_2 __stringify(jmp *\reg),                           \
> > > > +       ALTERNATIVE_2 __stringify(ANNOTATE_RETPOLINE_SAFE; jmp *\reg),  \
> > > >                 __stringify(RETPOLINE_JMP \reg), X86_FEATURE_RETPOLINE, \
> > > > -               __stringify(lfence; jmp *\reg), X86_FEATURE_RETPOLINE_AMD
> > > > +               __stringify(lfence; ANNOTATE_RETPOLINE_SAFE; jmp *\reg), X86_FEATURE_RETPOLINE_AMD
> > > >  #else
> > > >         jmp     *\reg
> > > >  #endif
> > > 
> > > The first one, yes. But the second one for the AMD retpoline is
> > > redundant, isn't it? Objtool isn't going to look there.
> > 
> > It was when I wrote it.. lemme try again.
> 
> Insta complaint:
> 
> arch/x86/entry/.tmp_entry_64.o: warning: objtool: .altinstr_replacement+0x19: indirect jump found in RETPOLINE build

Right, objtool was recently made smarter, such that it actually decodes
the ignored alternatives.  The check for that warning needs to also
check that insn->ignore isn't set.

-- 
Josh