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

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Message-ID: <20231018182223.GFZTAiX4BJ6FT8bHzz@fat_crate.local>
Date:   Wed, 18 Oct 2023 20:22:23 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Josh Poimboeuf <jpoimboe@...nel.org>
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 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....^

> 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.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette

View attachment "config" of type "text/plain" (127560 bytes)