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

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Date:   Thu, 29 Apr 2021 00:40:43 +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, 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().

> +	}
>  	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.