Commit a12578e1 authored by Fuad Tabba's avatar Fuad Tabba Committed by Paolo Bonzini
Browse files

KVM: guest_memfd: Add plumbing to host to map guest_memfd pages 

Introduce the core infrastructure to enable host userspace to mmap()
guest_memfd-backed memory. This is needed for several evolving KVM use
cases:

* Non-CoCo VM backing: Allows VMMs like Firecracker to run guests
  entirely backed by guest_memfd, even for non-CoCo VMs [1]. This
  provides a unified memory management model and simplifies guest memory
  handling.

* Direct map removal for enhanced security: This is an important step
  for direct map removal of guest memory [2]. By allowing host userspace
  to fault in guest_memfd pages directly, we can avoid maintaining host
  kernel direct maps of guest memory. This provides additional hardening
  against Spectre-like transient execution attacks by removing a
  potential attack surface within the kernel.

* Future guest_memfd features: This also lays the groundwork for future
  enhancements to guest_memfd, such as supporting huge pages and
  enabling in-place sharing of guest memory with the host for CoCo
  platforms that permit it [3].

Enable the basic mmap and fault handling logic within guest_memfd, but
hold off on allow userspace to actually do mmap() until the architecture
support is also in place.

[1] https://github.com/firecracker-microvm/firecracker/tree/feature/secret-hiding
[2] https://lore.kernel.org/linux-mm/cc1bb8e9bc3e1ab637700a4d3defeec95b55060a.camel@amazon.com
[3] https://lore.kernel.org/all/c1c9591d-218a-495c-957b-ba356c8f8e09@redhat.com/T/#u



Reviewed-by: default avatarGavin Shan <gshan@redhat.com>
Reviewed-by: default avatarShivank Garg <shivankg@amd.com>
Acked-by: default avatarDavid Hildenbrand <david@redhat.com>
Co-developed-by: default avatarAckerley Tng <ackerleytng@google.com>
Signed-off-by: default avatarAckerley Tng <ackerleytng@google.com>
Signed-off-by: default avatarFuad Tabba <tabba@google.com>
Reviewed-by: default avatarXiaoyao Li <xiaoyao.li@intel.com>
Signed-off-by: default avatarSean Christopherson <seanjc@google.com>
Message-ID: <20250729225455.670324-11-seanjc@google.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent d1e54dd0

arch/x86/kvm/x86.c

+11 −0
Original line number Diff line number Diff line
@@ -13521,6 +13521,16 @@ bool kvm_arch_no_poll(struct kvm_vcpu *vcpu) 
}

EXPORT_SYMBOL_GPL(kvm_arch_no_poll);



#ifdef CONFIG_KVM_GUEST_MEMFD

/*

 * KVM doesn't yet support mmap() on guest_memfd for VMs with private memory

 * (the private vs. shared tracking needs to be moved into guest_memfd).

 */

bool kvm_arch_supports_gmem_mmap(struct kvm *kvm)

{

	return !kvm_arch_has_private_mem(kvm);

}



#ifdef CONFIG_HAVE_KVM_ARCH_GMEM_PREPARE

int kvm_arch_gmem_prepare(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn, int max_order)

{
@@ -13534,6 +13544,7 @@ void kvm_arch_gmem_invalidate(kvm_pfn_t start, kvm_pfn_t end) 
	kvm_x86_call(gmem_invalidate)(start, end);

}

#endif

#endif



int kvm_spec_ctrl_test_value(u64 value)

{

include/linux/kvm_host.h

+4 −0
Original line number Diff line number Diff line
@@ -726,6 +726,10 @@ static inline bool kvm_arch_has_private_mem(struct kvm *kvm) 
}

#endif



#ifdef CONFIG_KVM_GUEST_MEMFD

bool kvm_arch_supports_gmem_mmap(struct kvm *kvm);

#endif



#ifndef kvm_arch_has_readonly_mem

static inline bool kvm_arch_has_readonly_mem(struct kvm *kvm)

{

virt/kvm/guest_memfd.c

+70 −0
Original line number Diff line number Diff line
@@ -312,7 +312,72 @@ static pgoff_t kvm_gmem_get_index(struct kvm_memory_slot *slot, gfn_t gfn) 
	return gfn - slot->base_gfn + slot->gmem.pgoff;

}



static bool kvm_gmem_supports_mmap(struct inode *inode)

{

	return false;

}



static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf)

{

	struct inode *inode = file_inode(vmf->vma->vm_file);

	struct folio *folio;

	vm_fault_t ret = VM_FAULT_LOCKED;



	if (((loff_t)vmf->pgoff << PAGE_SHIFT) >= i_size_read(inode))

		return VM_FAULT_SIGBUS;



	folio = kvm_gmem_get_folio(inode, vmf->pgoff);

	if (IS_ERR(folio)) {

		int err = PTR_ERR(folio);



		if (err == -EAGAIN)

			return VM_FAULT_RETRY;



		return vmf_error(err);

	}



	if (WARN_ON_ONCE(folio_test_large(folio))) {

		ret = VM_FAULT_SIGBUS;

		goto out_folio;

	}



	if (!folio_test_uptodate(folio)) {

		clear_highpage(folio_page(folio, 0));

		kvm_gmem_mark_prepared(folio);

	}



	vmf->page = folio_file_page(folio, vmf->pgoff);



out_folio:

	if (ret != VM_FAULT_LOCKED) {

		folio_unlock(folio);

		folio_put(folio);

	}



	return ret;

}



static const struct vm_operations_struct kvm_gmem_vm_ops = {

	.fault = kvm_gmem_fault_user_mapping,

};



static int kvm_gmem_mmap(struct file *file, struct vm_area_struct *vma)

{

	if (!kvm_gmem_supports_mmap(file_inode(file)))

		return -ENODEV;



	if ((vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) !=

	    (VM_SHARED | VM_MAYSHARE)) {

		return -EINVAL;

	}



	vma->vm_ops = &kvm_gmem_vm_ops;



	return 0;

}



static struct file_operations kvm_gmem_fops = {

	.mmap		= kvm_gmem_mmap,

	.open		= generic_file_open,

	.release	= kvm_gmem_release,

	.fallocate	= kvm_gmem_fallocate,
@@ -391,6 +456,11 @@ static const struct inode_operations kvm_gmem_iops = { 
	.setattr	= kvm_gmem_setattr,

};



bool __weak kvm_arch_supports_gmem_mmap(struct kvm *kvm)

{

	return true;

}



static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags)

{

	const char *anon_name = "[kvm-gmem]";