Re: [PATCH Part2 v6 25/49] KVM: SVM: Disallow registering memory range from HugeTLB for SNP guest

Vlastimil Babka
2022-08-04T13:37:28Z

On 6/21/22 01:07, Ashish Kalra wrote:
> From: Brijesh Singh <brijesh.singh@amd.com>
>
> While creating the VM, userspace call the KVM_MEMORY_ENCRYPT_REG_REGION
> ioctl to register the memory regions for the guest. This registered
> memory region is typically used as a guest RAM. Later, the guest may
> issue the page state change (PSC) request that will require splitting
> the large page into smaller page. If the memory is allocated from the
> HugeTLB then hypervisor will not be able to split it.
>
> Do not allow registering the memory range backed by the HugeTLB until
> the hypervisor support is added to handle the case.
>
> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
> ---
> arch/x86/kvm/svm/sev.c | 37 +++++++++++++++++++++++++++++++++++++
> 1 file changed, 37 insertions(+)
>
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 9e6fc7a94ed7..41b83aa6b5f4 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -17,6 +17,7 @@
> #include <linux/misc_cgroup.h>
> #include <linux/processor.h>
> #include <linux/trace_events.h>
> +#include <linux/hugetlb.h>
>
> #include <asm/pkru.h>
> #include <asm/trapnr.h>
> @@ -2007,6 +2008,35 @@ int sev_mem_enc_ioctl(struct kvm *kvm, void __user *argp)
> return r;
> }
>
> +static bool is_range_hugetlb(struct kvm *kvm, struct kvm_enc_region *range)
> +{
> + struct vm_area_struct *vma;
> + u64 start, end;
> + bool ret = true;
> +
> + start = range->addr;
> + end = start + range->size;
> +
> + mmap_read_lock(kvm->mm);
> +
> + do {
> + vma = find_vma_intersection(kvm->mm, start, end);
> + if (!vma)
> + goto unlock;
> +
> + if (is_vm_hugetlb_page(vma))
> + goto unlock;
> +
> + start = vma->vm_end;
> + } while (end > vma->vm_end);

Note it's more efficient to only find the first vma and then iterate using
vma->vm_next. But the details will change when maple tree is merged, and
likely this patch won't exist after rebasing to UPM anyway...

> + ret = false;
> +
> +unlock:
> + mmap_read_unlock(kvm->mm);
> + return ret;
> +}
> +
> int sev_mem_enc_register_region(struct kvm *kvm,
> struct kvm_enc_region *range)
> {
> @@ -2024,6 +2054,13 @@ int sev_mem_enc_register_region(struct kvm *kvm,
> if (range->addr > ULONG_MAX || range->size > ULONG_MAX)
> return -EINVAL;
>
> + /*
> + * SEV-SNP does not support the backing pages from the HugeTLB. Verify
> + * that the registered memory range is not from the HugeTLB.
> + */
> + if (sev_snp_guest(kvm) && is_range_hugetlb(kvm, range))
> + return -EINVAL;
> +
> region = kzalloc(sizeof(*region), GFP_KERNEL_ACCOUNT);
> if (!region)
> return -ENOMEM;