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
| ||
|
Date: Thu, 12 May 2022 20:33:43 +0000 From: Jon Kohler <jon@...anix.com> To: Jim Mattson <jmattson@...gle.com> CC: Sean Christopherson <seanjc@...gle.com>, Jon Kohler <jon@...anix.com>, Jonathan Corbet <corbet@....net>, Paolo Bonzini <pbonzini@...hat.com>, Vitaly Kuznetsov <vkuznets@...hat.com>, Wanpeng Li <wanpengli@...cent.com>, Joerg Roedel <joro@...tes.org>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>, Kees Cook <keescook@...omium.org>, Andrea Arcangeli <aarcange@...hat.com>, Josh Poimboeuf <jpoimboe@...hat.com>, Kim Phillips <kim.phillips@....com>, Lukas Bulwahn <lukas.bulwahn@...il.com>, Peter Zijlstra <peterz@...radead.org>, Ashok Raj <ashok.raj@...el.com>, KarimAllah Ahmed <karahmed@...zon.de>, David Woodhouse <dwmw@...zon.co.uk>, "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, "kvm @ vger . kernel . org" <kvm@...r.kernel.org>, Waiman Long <longman@...hat.com> Subject: Re: [PATCH v4] x86/speculation, KVM: remove IBPB on vCPU load > On May 12, 2022, at 4:27 PM, Jim Mattson <jmattson@...gle.com> wrote: > > On Thu, May 12, 2022 at 1:07 PM Sean Christopherson <seanjc@...gle.com> wrote: >> >> On Thu, May 12, 2022, Jon Kohler wrote: >>> >>> >>>> On May 12, 2022, at 3:35 PM, Sean Christopherson <seanjc@...gle.com> wrote: >>>> >>>> On Thu, May 12, 2022, Sean Christopherson wrote: >>>>> On Thu, May 12, 2022, Jon Kohler wrote: >>>>>> Remove IBPB that is done on KVM vCPU load, as the guest-to-guest >>>>>> attack surface is already covered by switch_mm_irqs_off() -> >>>>>> cond_mitigation(). >>>>>> >>>>>> The original commit 15d45071523d ("KVM/x86: Add IBPB support") was simply >>>>>> wrong in its guest-to-guest design intention. There are three scenarios >>>>>> at play here: >>>>> >>>>> Jim pointed offline that there's a case we didn't consider. When switching between >>>>> vCPUs in the same VM, an IBPB may be warranted as the tasks in the VM may be in >>>>> different security domains. E.g. the guest will not get a notification that vCPU0 is >>>>> being swapped out for vCPU1 on a single pCPU. >>>>> >>>>> So, sadly, after all that, I think the IBPB needs to stay. But the documentation >>>>> most definitely needs to be updated. >>>>> >>>>> A per-VM capability to skip the IBPB may be warranted, e.g. for container-like >>>>> use cases where a single VM is running a single workload. >>>> >>>> Ah, actually, the IBPB can be skipped if the vCPUs have different mm_structs, >>>> because then the IBPB is fully redundant with respect to any IBPB performed by >>>> switch_mm_irqs_off(). Hrm, though it might need a KVM or per-VM knob, e.g. just >>>> because the VMM doesn't want IBPB doesn't mean the guest doesn't want IBPB. >>>> >>>> That would also sidestep the largely theoretical question of whether vCPUs from >>>> different VMs but the same address space are in the same security domain. It doesn't >>>> matter, because even if they are in the same domain, KVM still needs to do IBPB. >>> >>> So should we go back to the earlier approach where we have it be only >>> IBPB on always_ibpb? Or what? >>> >>> At minimum, we need to fix the unilateral-ness of all of this :) since we’re >>> IBPB’ing even when the user did not explicitly tell us to. >> >> I think we need separate controls for the guest. E.g. if the userspace VMM is >> sufficiently hardened then it can run without "do IBPB" flag, but that doesn't >> mean that the entire guest it's running is sufficiently hardened. >> >>> That said, since I just re-read the documentation today, it does specifically >>> suggest that if the guest wants to protect *itself* it should turn on IBPB or >>> STIBP (or other mitigations galore), so I think we end up having to think >>> about what our “contract” is with users who host their workloads on >>> KVM - are they expecting us to protect them in any/all cases? >>> >>> Said another way, the internal guest areas of concern aren’t something >>> the kernel would always be able to A) identify far in advance and B) >>> always solve on the users behalf. There is an argument to be made >>> that the guest needs to deal with its own house, yea? >> >> The issue is that the guest won't get a notification if vCPU0 is replaced with >> vCPU1 on the same physical CPU, thus the guest doesn't get an opportunity to emit >> IBPB. Since the host doesn't know whether or not the guest wants IBPB, unless the >> owner of the host is also the owner of the guest workload, the safe approach is to >> assume the guest is vulnerable. > > Exactly. And if the guest has used taskset as its mitigation strategy, > how is the host to know? Yea thats fair enough. I posed a solution on Sean’s response just as this email came in, would love to know your thoughts (keying off MSR bitmap).
Powered by blists - more mailing lists