linux-kernel - Re: [PATCH v9 09/17] x86/split_lock: Handle #AC exception for split lock

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <3ec328dc-2763-9da5-28d6-e28970262c58@redhat.com>
Date:   Wed, 16 Oct 2019 11:40:39 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     Fenghua Yu <fenghua.yu@...el.com>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>, H Peter Anvin <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Radim Krcmar <rkrcmar@...hat.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Xiaoyao Li <xiaoyao.li@...el.com>,
        Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        x86 <x86@...nel.org>, kvm@...r.kernel.org
Subject: Re: [PATCH v9 09/17] x86/split_lock: Handle #AC exception for split
 lock

On 25/09/19 20:09, Sean Christopherson wrote:
> We're trying to sort out the trainwreck, but there's an additional wrinkle
> that I'd like your input on.

That's not exactly a wrinkle...

>   - Remove KVM loading of MSR_TEST_CTRL, i.e. KVM *never* writes the CPU's
>     actual MSR_TEST_CTRL.  KVM still emulates MSR_TEST_CTRL so that the
>     guest can do WRMSR and handle its own #AC faults, but KVM doesn't
>     change the value in hardware.
> 
>       * Allowing guest to enable split-lock detection can induce #AC on
>         the host after it has been explicitly turned off, e.g. the sibling
>         hyperthread hits an #AC in the host kernel, or worse, causes a
>         different process in the host to SIGBUS.
> 
>       * Allowing guest to disable split-lock detection opens up the host
>         to DoS attacks.
> 
>   - KVM advertises split-lock detection to guest/userspace if and only if
>     split_lock_detect_disabled is zero.
> 
>   - Add a pr_warn_once() in KVM that triggers if split locks are disabled
>     after support has been advertised to a guest.
> 
> Does this sound sane?

Not really, unfortunately.  Just never advertise split-lock detection to
guests.  If the host has enabled split-lock detection, trap #AC and
forward it to the host handler---which would disable split lock
detection globally and reenter the guest.

Paolo