Re: [PATCH v9 08/17] KVM: guest_memfd: Check that userspace_addr and fd+offset refer to same range

[David Hildenbrand <david@redhat.com>]

On 13.05.25 18:34, Fuad Tabba wrote:
> From: Ackerley Tng <ackerleytng@google.com>
> 
> On binding of a guest_memfd with a memslot, check that the slot's
> userspace_addr and the requested fd and offset refer to the same memory
> range.
> 
> This check is best-effort: nothing prevents userspace from later mapping
> other memory to the same provided in slot->userspace_addr and breaking
> guest operation.
> 
> Suggested-by: David Hildenbrand <david@redhat.com>
> Suggested-by: Sean Christopherson <seanjc@google.com>
> Suggested-by: Yan Zhao <yan.y.zhao@intel.com>
> Signed-off-by: Ackerley Tng <ackerleytng@google.com>
> Signed-off-by: Fuad Tabba <tabba@google.com>
> ---
>   virt/kvm/guest_memfd.c | 37 ++++++++++++++++++++++++++++++++++---
>   1 file changed, 34 insertions(+), 3 deletions(-)
> 
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index 8e6d1866b55e..2f499021df66 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -556,6 +556,32 @@ int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args)
>   	return __kvm_gmem_create(kvm, size, flags);
>   }
>   
> +static bool kvm_gmem_is_same_range(struct kvm *kvm,
> +				   struct kvm_memory_slot *slot,
> +				   struct file *file, loff_t offset)
> +{
> +	struct mm_struct *mm = kvm->mm;
> +	loff_t userspace_addr_offset;
> +	struct vm_area_struct *vma;
> +	bool ret = false;
> +
> +	mmap_read_lock(mm);
> +
> +	vma = vma_lookup(mm, slot->userspace_addr);
> +	if (!vma)
> +		goto out;
> +
> +	if (vma->vm_file != file)
> +		goto out;
> +
> +	userspace_addr_offset = slot->userspace_addr - vma->vm_start;
> +	ret = userspace_addr_offset + (vma->vm_pgoff << PAGE_SHIFT) == offset;

You'd probably have to iterate over the whole range (which might span 
multiple VMAs), but reading the discussion, I'm fine with dropping this 
patch for now.

I think it's more important to document that thoroughly: what does it 
mean when we use GUEST_MEMFD_FLAG_SUPPORT_SHARED and then pass that fd 
in a memslot.

Skimming over patch #15, I assume this is properly documented in there.

-- 
Cheers,

David / dhildenb