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Message-ID: <b0074626-8e98-4573-8047-08916fbb5537@citrix.com>
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
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