linux-kernel - Re: [PATCH v3 1/3] KVM: Do not reset dirty GFNs in a memslot not enabling dirty tracking

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Message-ID: <aKzKw70r5bRnv0FC@google.com>
Date: Mon, 25 Aug 2025 13:42:43 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Yan Zhao <yan.y.zhao@...el.com>
Cc: pbonzini@...hat.com, peterx@...hat.com, rick.p.edgecombe@...el.com, 
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v3 1/3] KVM: Do not reset dirty GFNs in a memslot not
 enabling dirty tracking

On Fri, Aug 22, 2025, Yan Zhao wrote:
> Do not allow resetting dirty GFNs in memslots that do not enable dirty
> tracking.
> 
> vCPUs' dirty rings are shared between userspace and KVM. After KVM sets
> dirtied entries in the dirty rings, userspace is responsible for
> harvesting/resetting these entries and calling the ioctl
> KVM_RESET_DIRTY_RINGS to inform KVM to advance the reset_index in the dirty
> rings and invoke kvm_arch_mmu_enable_log_dirty_pt_masked() to clear the
> SPTEs' dirty bits or perform write protection of the GFNs.
> 
> Although KVM does not set dirty entries for GFNs in a memslot that does not
> enable dirty tracking, userspace can write arbitrary data into the dirty
> ring. This makes it possible for misbehaving userspace to specify that it
> has harvested a GFN from such a memslot. When this happens, KVM will be
> asked to clear dirty bits or perform write protection for GFNs in a memslot
> that does not enable dirty tracking, which is undesirable.
> 
> For TDX, this unexpected resetting of dirty GFNs could cause inconsistency
> between the mirror SPTE and the external SPTE in hardware (e.g., the mirror
> SPTE has no write bit while the external SPTE is writable). When
> kvm_dirty_log_manual_protect_and_init_set() is true and huge pages are
> enabled in TDX, this could even lead to kvm_mmu_slot_gfn_write_protect()
> being called and trigger KVM_BUG_ON() due to permission reduction changes
> in the huge mirror SPTEs.
> 

Sounds like this needs a Fixes and Cc: stable?

> Signed-off-by: Yan Zhao <yan.y.zhao@...el.com>
> ---
>  virt/kvm/dirty_ring.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/virt/kvm/dirty_ring.c b/virt/kvm/dirty_ring.c
> index 02bc6b00d76c..b38b4b7d7667 100644
> --- a/virt/kvm/dirty_ring.c
> +++ b/virt/kvm/dirty_ring.c
> @@ -63,7 +63,13 @@ static void kvm_reset_dirty_gfn(struct kvm *kvm, u32 slot, u64 offset, u64 mask)
>  
>  	memslot = id_to_memslot(__kvm_memslots(kvm, as_id), id);
>  
> -	if (!memslot || (offset + __fls(mask)) >= memslot->npages)
> +	/*
> +	 * Userspace can write arbitrary data into the dirty ring, making it
> +	 * possible for misbehaving userspace to try to reset an out-of-memslot
> +	 * GFN or a GFN in a memslot that isn't being dirty-logged.
> +	 */
> +	if (!memslot || (offset + __fls(mask)) >= memslot->npages ||
> +	    !kvm_slot_dirty_track_enabled(memslot))

Maybe check for dirty tracking being enabled before checking the range?  Purely
because checking if _any_  gfn can be recorded seems like something that should
be checked before a specific gfn can be recorded.  I.e.

	if (!memslot || !kvm_slot_dirty_track_enabled(memslot) ||
	    (offset + __fls(mask)) >= memslot->npages)
	    
>  		return;
>  
>  	KVM_MMU_LOCK(kvm);
> -- 
> 2.43.2
>