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

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Message-ID: <20231019141514.GCZTE58qPOvcJCiBp3@fat_crate.local>
Date:   Thu, 19 Oct 2023 16:15:14 +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 11:59:28PM -0700, Josh Poimboeuf wrote:
> One last idea, since the return thunk is used everywhere (even non-ABI
> compliant functions) it might be possible the "call check_thunks" (and
> its call to warn_printk) is clobbering some registers which some code
> (exception handling entry code?) doesn't appreciate.

Yeah, that is still unclean, I'd say. gcc doesn't know that we patch in
a CALL insn in the alternative. What should work is to have

	alternative_call

there which alternates between two calls and gcc knows there's a call so
it can act accordingly wrt callee-* regs.

Considering how __x86_return_thunk is there only until alternatives have
run, we could do something like

	ALTERNATIVE_CALL nop, check_thunks

where nop is a function which doesn't do anything.

I say "ALTERNATIVE_CALL" because we don't have a _CALL asm macro yet.

And then in check_thunks() we can do all kinds of screaming, tainting
and setting mitigation status to vulnerable, etc.

Anyway something along those lines.

Thx.

-- 
Regards/Gruss,
    Boris.

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