linux-kernel - Re: [PATCH] KVM: x86: Add GPA limit check to kvm_arch_vcpu_pre_fault

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Message-ID: <ZpbVVyp3YvCJp3Am@google.com>
Date: Tue, 16 Jul 2024 13:17:27 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: isaku.yamahata@...el.com
Cc: kvm@...r.kernel.org, isaku.yamahata@...il.com, 
	Paolo Bonzini <pbonzini@...hat.com>, rick.p.edgecombe@...el.com, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: Add GPA limit check to kvm_arch_vcpu_pre_fault_memory()

On Mon, Jul 15, 2024, isaku.yamahata@...el.com wrote:
> From: Isaku Yamahata <isaku.yamahata@...el.com>
> 
> Add GPA limit check to kvm_arch_vcpu_pre_fault_memory() with guest
> maxphyaddr and kvm_mmu_max_gfn().
> 
> The KVM page fault handler decides which level of TDP to use, 4-level TDP
> or 5-level TDP based on guest maxphyaddr (CPUID[0x80000008].EAX[7:0]), the
> host maxphyaddr, and whether the host supports 5-level TDP or not.  The
> 4-level TDP can map GPA up to 48 bits, and the 5-level TDP can map GPA up
> to 52 bits.  If guest maxphyaddr <= 48, KVM uses 4-level TDP even when the
> host supports 5-level TDP.
> 
> If we pass GPA > beyond the TDP mappable limit to the TDP MMU fault handler
> (concretely GPA > 48-bits with 4-level TDP), it will operate on GPA without
> upper bits, (GPA & ((1UL < 48) - 1)), not the specified GPA.  It is not
> expected behavior.  It wrongly maps GPA without upper bits with the page
> for GPA with upper bits.
> 
> KVM_PRE_FAULT_MEMORY calls x86 KVM page fault handler, kvm_tdp_page_fault()
> with a user-space-supplied GPA without the limit check so that the user
> space can trigger WARN_ON_ONCE().  Check the GPA limit to fix it.

Which WARN?

> - For non-TDX case (DEFAULT_VM, SW_PROTECTED_VM, or SEV):
>   When the host supports 5-level TDP, KVM decides to use 4-level TDP if
>   cpuid_maxphyaddr() <= 48.  cpuid_maxhyaddr() check prevents
>   KVM_PRE_FAULT_MEMORY from passing GFN beyond mappable GFN.

Hardening against cpuid_maxphyaddr() should be out of scope.  We don't enforce
it for guest faults, e.g. KVM doesn't kill the guest if allow_smaller_maxphyaddr
is false and the GPA is supposed to be illegal.  And trying to enforce it here is
a fool's errand since userspace can simply do KVM_SET_CPUID2 to circumvent the
restriction.

> - For TDX case:
>   We'd like to exclude shared bit (or gfn_direct_mask in [1]) from GPA
>   passed to the TDP MMU so that the TDP MMU can handle Secure-EPT or
>   Shared-EPT (direct or mirrored in [1]) without explicitly
>   setting/clearing the GPA (except setting up the TDP iterator,
>   tdp_iter_refresh_sptep()).  We'd like to make kvm_mmu_max_gfn() per VM
>   for TDX to be 52 or 47 independent of the guest maxphyaddr with other
>   patches.
> 
> Fixes: 6e01b7601dfe ("KVM: x86: Implement kvm_arch_vcpu_pre_fault_memory()")
> Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
> ---
>  arch/x86/kvm/mmu/mmu.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 4e0e9963066f..6ee5af55cee1 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4756,6 +4756,11 @@ long kvm_arch_vcpu_pre_fault_memory(struct kvm_vcpu *vcpu,
>  	u64 end;
>  	int r;
>  
> +	if (range->gpa >= (1UL << cpuid_maxphyaddr(vcpu)))
> +		return -E2BIG;
> +	if (gpa_to_gfn(range->gpa) > kvm_mmu_max_gfn())
> +		return -E2BIG;


Related to my thoughts on making kvm_mmu_max_gfn() and rejecting aliased memslots,
I think we should add a common helper that's used by kvm_arch_prepare_memory_region()
and kvm_arch_vcpu_pre_fault_memory() to reject GPAs that are disallowed.

https://lore.kernel.org/all/ZpbKqG_ZhCWxl-Fc@google.com

> +
>  	/*
>  	 * reload is efficient when called repeatedly, so we can do it on
>  	 * every iteration.
> 
> base-commit: c8b8b8190a80b591aa73c27c70a668799f8db547
> -- 
> 2.45.2
>