linux-kernel - Re: [patch 00/38] x86/retbleed: Call depth tracking mitigation

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Date:   Tue, 19 Jul 2022 00:22:26 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Andrew Cooper <Andrew.Cooper3@...rix.com>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Johannes Wikner <kwikner@...z.ch>,
        Alyssa Milburn <alyssa.milburn@...ux.intel.com>,
        Jann Horn <jannh@...gle.com>, "H.J. Lu" <hjl.tools@...il.com>,
        Joao Moreira <joao.moreira@...el.com>,
        Joseph Nuzman <joseph.nuzman@...el.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Juergen Gross <jgross@...e.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, samitolvanen@...gle.com
Subject: Re: [patch 00/38] x86/retbleed: Call depth tracking mitigation

On Mon, Jul 18 2022 at 23:18, Peter Zijlstra wrote:
> On Mon, Jul 18, 2022 at 10:44:14PM +0200, Thomas Gleixner wrote:
>> And we need input from the Clang folks because their CFI work also puts
>> stuff in front of the function entry, which nicely collides.
>
> Right, I need to go look at the latest kCFI patches, that sorta got
> side-tracked for working on all the retbleed muck :/
>
> Basically kCFI wants to preface every (indirect callable) function with:
>
> __cfi_\func:
> 	int3
>         movl $0x12345678, %rax
>         int3
>         int3
> \func:
>         endbr
> \func_direct:
>
> Ofc, we can still put the whole:
>
> 	sarq	$5, PER_CPU_VAR(__x86_call_depth);
> 	jmp	\func_direct
>
> thing in front of that. But it does somewhat destroy the version I had
> that only needs the 10 bytes padding for the sarq.

Right, because it needs the jump. I was just chatting with Jaoa about
that over IRC.

The jump slow things down. Jaoa has ideas and will reply soonish.

Thanks,

        tglx