From a12578e1477cbfb547256ed8dee6d5142a59cdcd Mon Sep 17 00:00:00 2001 From: Fuad Tabba Date: Tue, 29 Jul 2025 15:54:41 -0700 Subject: 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: Gavin Shan Reviewed-by: Shivank Garg Acked-by: David Hildenbrand Co-developed-by: Ackerley Tng Signed-off-by: Ackerley Tng Signed-off-by: Fuad Tabba Reviewed-by: Xiaoyao Li Signed-off-by: Sean Christopherson Message-ID: <20250729225455.670324-11-seanjc@google.com> Signed-off-by: Paolo Bonzini --- virt/kvm/guest_memfd.c | 70 ++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 70 insertions(+) (limited to 'virt') diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c index a99e11b8b77f..67e7cd7210ef 100644 --- a/virt/kvm/guest_memfd.c +++ b/virt/kvm/guest_memfd.c @@ -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]"; -- cgit v1.2.3