linux-kernel - Re: [RFC PATCH] x86/split_lock: Disable SLD if an unaware (out-of-tree) module enables VMX

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Date:   Mon, 6 Apr 2020 05:50:10 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        linux-kernel@...r.kernel.org,
        "Kenneth R. Crudup" <kenny@...ix.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Jessica Yu <jeyu@...nel.org>,
        Rasmus Villemoes <rasmus.villemoes@...vas.dk>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Xiaoyao Li <xiaoyao.li@...el.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Thomas Hellstrom <thellstrom@...are.com>,
        Tony Luck <tony.luck@...el.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        David Laight <David.Laight@...lab.com>,
        Doug Covelli <dcovelli@...are.com>
Subject: Re: [RFC PATCH] x86/split_lock: Disable SLD if an unaware
 (out-of-tree) module enables VMX

On Fri, Apr 03, 2020 at 09:30:07AM -0700, Sean Christopherson wrote:
> Hook into native CR4 writes to disable split-lock detection if CR4.VMXE
> is toggled on by an SDL-unaware entity, e.g. an out-of-tree hypervisor
> module.  Most/all VMX-based hypervisors blindly reflect #AC exceptions
> into the guest, or don't intercept #AC in the first place.  With SLD
> enabled, this results in unexpected #AC faults in the guest, leading to
> crashes in the guest and other undesirable behavior.

Out of tree modules do not matter, so we should not add code just to
work around broken third party code.  If you really feel strongly just
make sure something they rely on for their hacks stops being exported
and they are properly broken.