Re: [PATCH 09/12] KVM: guest_memfd: move check for already-populated page to common code

[Michael Roth <michael.roth@amd.com>]

On Thu, Jul 11, 2024 at 06:27:52PM -0400, Paolo Bonzini wrote:
> Do not allow populating the same page twice with startup data.  In the
> case of SEV-SNP, for example, the firmware does not allow it anyway,
> since the launch-update operation is only possible on pages that are
> still shared in the RMP.
> 
> Even if it worked, kvm_gmem_populate()'s callback is meant to have side
> effects such as updating launch measurements, and updating the same
> page twice is unlikely to have the desired results.
> 
> Races between calls to the ioctl are not possible because kvm_gmem_populate()
> holds slots_lock and the VM should not be running.  But again, even if
> this worked on other confidential computing technology, it doesn't matter
> to guest_memfd.c whether this is an intentional attempt to do something
> fishy, or missing synchronization in userspace, or even something
> intentional.  One of the racers wins, and the page is initialized by

I think one of those "intentional"s was meant to be an "unintentional".

> either kvm_gmem_prepare_folio() or kvm_gmem_populate().
> 
> Anyway, out of paranoia, adjust sev_gmem_post_populate() anyway to use
> the same errno that kvm_gmem_populate() is using.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Assuming based on the discussion here that there will be some logic added
to enforce that KVM_PRE_FAULT_MEMORY can only happen after finalization, I
think the new checks make sense.

Reviewed-by: Michael Roth <michael.roth@amd.com>

> ---
>  arch/x86/kvm/svm/sev.c | 2 +-
>  virt/kvm/guest_memfd.c | 7 +++++++
>  2 files changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index df8818759698..397ef9e70182 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -2213,7 +2213,7 @@ static int sev_gmem_post_populate(struct kvm *kvm, gfn_t gfn_start, kvm_pfn_t pf
>  		if (ret || assigned) {
>  			pr_debug("%s: Failed to ensure GFN 0x%llx RMP entry is initial shared state, ret: %d assigned: %d\n",
>  				 __func__, gfn, ret, assigned);
> -			ret = -EINVAL;
> +			ret = ret ? -EINVAL : -EEXIST;
>  			goto err;
>  		}
>  
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index 509360eefea5..266810bb91c9 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -650,6 +650,13 @@ long kvm_gmem_populate(struct kvm *kvm, gfn_t start_gfn, void __user *src, long
>  			break;
>  		}
>  
> +		if (folio_test_uptodate(folio)) {
> +			folio_unlock(folio);
> +			folio_put(folio);
> +			ret = -EEXIST;
> +			break;
> +		}
> +
>  		folio_unlock(folio);
>  		if (!IS_ALIGNED(gfn, (1 << max_order)) ||
>  		    (npages - i) < (1 << max_order))
> -- 
> 2.43.0
> 
> 
>