linux-kernel - Re: [PATCH 3/3] x86/bugs: Remove support for Spectre v2 LFENCE "retpolines"

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Date: Fri, 12 Apr 2024 21:49:33 +0100
From: Andrew Cooper <andrew.cooper3@...rix.com>
To: Josh Poimboeuf <jpoimboe@...nel.org>, x86@...nel.org
Cc: linux-kernel@...r.kernel.org,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
 Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
 Thomas Gleixner <tglx@...utronix.de>,
 Alexandre Chartre <alexandre.chartre@...cle.com>,
 Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
 Peter Zijlstra <peterz@...radead.org>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Sean Christopherson <seanjc@...gle.com>,
 Dave Hansen <dave.hansen@...ux.intel.com>,
 Nikolay Borisov <nik.borisov@...e.com>, KP Singh <kpsingh@...nel.org>,
 Waiman Long <longman@...hat.com>, Borislav Petkov <bp@...en8.de>,
 Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 3/3] x86/bugs: Remove support for Spectre v2 LFENCE
 "retpolines"

On 12/04/2024 7:10 pm, Josh Poimboeuf wrote:
> I found several bugs where code assumes that X86_FEATURE_RETPOLINE
> actually means retpolines (imagine that!).

Yeah :(   One could also imagine a past where that was pointed out, or
just read about it in the archives.

>   In fact that feature also
> includes the original AMD LFENCE "retpolines", which aren't in fact
> retpolines.
>
> Really, those "retpolines" should just be removed.  They're already
> considered vulnerable due to the fact that the speculative window after
> the indirect branch can still be long enough to do multiple dependent
> loads.  And recent tooling makes such gadgets easier to find.

There are two Atom CPUs which are not repotline safe, and for which
Intel released a statement saying "use lfence/jmp" on these.

I'm still trying to find it...

~Andrew