linux-kernel - Re: [PATCH MANUALSEL 5.15 4/7] KVM: x86/mmu: do not allow readers to acquire references to invalid roots

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Date:   Wed, 27 Apr 2022 18:20:32 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sasha Levin <sashal@...nel.org>, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, kvm@...r.kernel.org
Subject: Re: [PATCH MANUALSEL 5.15 4/7] KVM: x86/mmu: do not allow readers to
 acquire references to invalid roots

On 4/27/22 17:54, Sasha Levin wrote:
> From: Paolo Bonzini <pbonzini@...hat.com>
> 
> [ Upstream commit 614f6970aa70242a3f8a8051b01244c029f77b2a ]
> 
> Remove the "shared" argument of for_each_tdp_mmu_root_yield_safe, thus ensuring
> that readers do not ever acquire a reference to an invalid root.  After this
> patch, all readers except kvm_tdp_mmu_zap_invalidated_roots() treat
> refcount=0/valid, refcount=0/invalid and refcount=1/invalid in exactly the
> same way.  kvm_tdp_mmu_zap_invalidated_roots() is different but it also
> does not acquire a reference to the invalid root, and it cannot see
> refcount=0/invalid because it is guaranteed to run after
> kvm_tdp_mmu_invalidate_all_roots().
> 
> Opportunistically add a lockdep assertion to the yield-safe iterator.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> Signed-off-by: Sasha Levin <sashal@...nel.org>
> ---
>   arch/x86/kvm/mmu/tdp_mmu.c | 9 +++++----
>   1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 853780eb033b..7e854313ec3b 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -155,14 +155,15 @@ static struct kvm_mmu_page *tdp_mmu_next_root(struct kvm *kvm,
>   	for (_root = tdp_mmu_next_root(_kvm, NULL, _shared, _only_valid);	\
>   	     _root;								\
>   	     _root = tdp_mmu_next_root(_kvm, _root, _shared, _only_valid))	\
> -		if (kvm_mmu_page_as_id(_root) != _as_id) {			\
> +		if (kvm_lockdep_assert_mmu_lock_held(_kvm, _shared) &&		\
> +		    kvm_mmu_page_as_id(_root) != _as_id) {			\
>   		} else
>   
>   #define for_each_valid_tdp_mmu_root_yield_safe(_kvm, _root, _as_id, _shared)	\
>   	__for_each_tdp_mmu_root_yield_safe(_kvm, _root, _as_id, _shared, true)
>   
> -#define for_each_tdp_mmu_root_yield_safe(_kvm, _root, _as_id, _shared)		\
> -	__for_each_tdp_mmu_root_yield_safe(_kvm, _root, _as_id, _shared, false)
> +#define for_each_tdp_mmu_root_yield_safe(_kvm, _root, _as_id)			\
> +	__for_each_tdp_mmu_root_yield_safe(_kvm, _root, _as_id, false, false)
>   
>   #define for_each_tdp_mmu_root(_kvm, _root, _as_id)				\
>   	list_for_each_entry_rcu(_root, &_kvm->arch.tdp_mmu_roots, link,		\
> @@ -828,7 +829,7 @@ bool __kvm_tdp_mmu_zap_gfn_range(struct kvm *kvm, int as_id, gfn_t start,
>   {
>   	struct kvm_mmu_page *root;
>   
> -	for_each_tdp_mmu_root_yield_safe(kvm, root, as_id, false)
> +	for_each_tdp_mmu_root_yield_safe(kvm, root, as_id)
>   		flush = zap_gfn_range(kvm, root, start, end, can_yield, flush,
>   				      false);
>   

Sorry no, this is a NACK.

Paolo