linux-kernel - Re: [PATCH] KVM: x86/mmu: Properly dereference rcu-protected TDP MMU sptep iterator

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Message-ID: <CANgfPd9VVAdP0umt6Odz_-f+nUmKfsNa0hqvUzuto6=G6b=M+A@mail.gmail.com>
Date:   Wed, 3 Nov 2021 09:36:23 -0700
From:   Ben Gardon <bgardon@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86/mmu: Properly dereference rcu-protected TDP MMU
 sptep iterator

On Wed, Nov 3, 2021 at 9:18 AM Sean Christopherson <seanjc@...gle.com> wrote:
>
> Wrap the read of iter->sptep in tdp_mmu_map_handle_target_level() with
> rcu_dereference().  Shadow pages in the TDP MMU, and thus their SPTEs,
> are protected by rcu.
>
> This fixes a Sparse warning at tdp_mmu.c:900:51:
>   warning: incorrect type in argument 1 (different address spaces)
>   expected unsigned long long [usertype] *sptep
>   got unsigned long long [noderef] [usertype] __rcu *[usertype] sptep
>
> Fixes: 7158bee4b475 ("KVM: MMU: pass kvm_mmu_page struct to make_spte")
> Cc: Ben Gardon <bgardon@...gle.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: Ben Gardon <bgardon@...gle.com>

This change looks good to me. It also reminds me that the struct
kvm_mmu_pages are also protected by RCU.
I wonder if it would be worth creating something similar to tdp_ptep_t
for the struct kvm_mmu_pages to add RCU annotations to them as well.

> ---
>  arch/x86/kvm/mmu/tdp_mmu.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 7c5dd83e52de..a54c3491af42 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -897,7 +897,7 @@ static int tdp_mmu_map_handle_target_level(struct kvm_vcpu *vcpu,
>                                           struct kvm_page_fault *fault,
>                                           struct tdp_iter *iter)
>  {
> -       struct kvm_mmu_page *sp = sptep_to_sp(iter->sptep);
> +       struct kvm_mmu_page *sp = sptep_to_sp(rcu_dereference(iter->sptep));
>         u64 new_spte;
>         int ret = RET_PF_FIXED;
>         bool wrprot = false;
> --
> 2.33.1.1089.g2158813163f-goog
>