linux-kernel - Re: [tip: x86/bugs] x86/retpoline: Ensure default return thunk isn't used at runtime

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Message-ID: <20231018183915.xwamzzqjf6gehaou@treble>
Date:   Wed, 18 Oct 2023 11:39:15 -0700
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        linux-tip-commits@...r.kernel.org,
        David Kaplan <david.kaplan@....com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
        David Howells <dhowells@...hat.com>
Subject: Re: [tip: x86/bugs] x86/retpoline: Ensure default return thunk isn't
 used at runtime

On Wed, Oct 18, 2023 at 08:22:23PM +0200, Borislav Petkov wrote:
> On Wed, Oct 18, 2023 at 11:14:31AM -0700, Josh Poimboeuf wrote:
> > > > > WARN_ONCE() is not enough considering that if this fires, it means we're
> > > > > not really properly protected against one of those RET-speculation
> > > > > things.
> > > > > 
> > > > > It needs to be warning constantly but then still allow booting. I.e,
> > > > > a ratelimited warn of sorts but I don't think we have that... yet.
> 
> 
>      ^
> -----|		this here.
> 
> 
> > > > I'm not sure a rate-limited WARN() would be a good thing.  Either the
> > > > user is regularly checking dmesg (most likely in some automated fashion)
> > > > or they're not.  If the latter, a rate-limited WARN() would wrap dmesg
> > > > pretty quickly.
> > > 
> > > Well, freezing the box without any mention about why it happens is not
> > > viable either. So for lack of a better solution, overflowing dmesg is
> > > all we could do.
> > 
> > Why not just WARN_ONCE() then?
> 
> See above....^

And see my reply to that?  Not trying to be daft, I just didn't see how
your reply was responsive.

A single WARN_ONCE() has the benefit of not overflowing dmesg, while
also making the warning available to those looking at dmesg (or the
taint flag), as those who care should be.

A rate-limited WARN() is problematic, as it overflows dmesg (and
possibly wrapping other logs), potentially obscuring other important
data.

> > Ok.  A revert is fine for now, but either way we do need to get to the
> > bottom of why objtool is messing up.  Can you share the config?
> 
> Attached.
> 
> And as said, you need gcc 13.

Thanks, I'll see if I can recreate.

-- 
Josh