linux-kernel - Re: [PATCH v3 2/3] KVM: x86: Introduce allow list for MSR emulation

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Date:   Mon, 3 Aug 2020 14:23:08 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Alexander Graf <graf@...zon.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        KarimAllah Raslan <karahmed@...zon.de>,
        Aaron Lewis <aaronlewis@...gle.com>, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/3] KVM: x86: Introduce allow list for MSR emulation

On Mon, Aug 03, 2020 at 10:50:53PM +0200, Alexander Graf wrote:
> 
> On 03.08.20 13:37, Vitaly Kuznetsov wrote:
> >>+static int kvm_vm_ioctl_clear_msr_allowlist(struct kvm *kvm)
> >>+{
> >>+     mutex_lock(&kvm->lock);
> >>+     kvm->arch.msr_allowlist_ranges_count = 0;
> >>+     mutex_unlock(&kvm->lock);
> >
> >Are we also supposed to kfree() bitmaps here?
> 
> Phew. Yes, because without the kfree() we're leaking memory. Unfortunately
> if I just put in a kfree() here, we may allow a concurrently executing vCPU
> to access already free'd memory.
> 
> So I'll also add locking around the range check. Let's hope it won't regress
> performance too much.

What about using KVM's SRCU to protect the list?  The only thing I'm not 100%
on is whether holding kvm->lock across synchronize_srcu() is safe from a lock
inversion perspective.  I'm pretty sure KVM doesn't try to acquire kvm->lock
after grabbing SRCU, but that's hard to audit and there aren't any existing
flows that invoke synchronize_srcu() while holding kvm->lock.