linux-kernel - Re: [PATCH 5/6] KVM: x86/mmu: Protect kvm->memslots with a mutex

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Date:   Thu, 29 Apr 2021 01:42:07 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Ben Gardon <bgardon@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Peter Xu <peterx@...hat.com>, Peter Shier <pshier@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>
Subject: Re: [PATCH 5/6] KVM: x86/mmu: Protect kvm->memslots with a mutex

On Thu, Apr 29, 2021, Sean Christopherson wrote:
> On Thu, Apr 29, 2021, Paolo Bonzini wrote:
> > it's not ugly and it's still relatively easy to explain.
> 
> LOL, that's debatable.
> 
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 2799c6660cce..48929dd5fb29 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -1377,16 +1374,17 @@ static int kvm_set_memslot(struct kvm *kvm,
> >  		goto out_slots;
> >  	update_memslots(slots, new, change);
> > -	slots = install_new_memslots(kvm, as_id, slots);
> > +	install_new_memslots(kvm, as_id, slots);
> >  	kvm_arch_commit_memory_region(kvm, mem, old, new, change);
> > -
> > -	kvfree(slots);
> >  	return 0;
> >  out_slots:
> > -	if (change == KVM_MR_DELETE || change == KVM_MR_MOVE)
> > +	if (change == KVM_MR_DELETE || change == KVM_MR_MOVE) {
> > +		slot = id_to_memslot(slots, old->id);
> > +		slot->flags &= ~KVM_MEMSLOT_INVALID;
> 
> Modifying flags on an SRCU-protect field outside of said protection is sketchy.
> It's probably ok to do this prior to the generation update, emphasis on
> "probably".  Of course, the VM is also likely about to be killed in this case...
> 
> >  		slots = install_new_memslots(kvm, as_id, slots);
> 
> This will explode if memory allocation for KVM_MR_MOVE fails.  In that case,
> the rmaps for "slots" will have been cleared by kvm_alloc_memslot_metadata().

Gah, that's all wrong, slots are the second duplicate and the clear happens on
the new slot, not the old slot with the same id.

Though I still think temporarily dropping the SRCU lock would be simpler.  If
performance is a concern, it could be mitigated by adding a capability to
preallocate the rmaps.

> > +	}
> >  	kvfree(slots);
> >  	return r;
> >  }
> 
> The SRCU index is already tracked in vcpu->srcu_idx, why not temporarily drop
> the SRCU lock if activate_shadow_mmu() needs to do work so that it can take
> slots_lock?  That seems simpler and I think would avoid modifying the common
> memslot code.
> 
> kvm_arch_async_page_ready() is the only path for reaching kvm_mmu_reload() that
> looks scary, but that should be impossible to reach with the correct MMU context.
> We could always and an explicit sanity check on the rmaps being avaiable.