Provide defined memory content modes so that KVM can make guarantees about memory content after setting memory attributes, according to userspace requests.
Suggested-by: Sean Christoperson <[email protected]> Signed-off-by: Ackerley Tng <[email protected]> --- Documentation/virt/kvm/api.rst | 40 +++++++++++++++++++++++++++++ include/uapi/linux/kvm.h | 4 +++ virt/kvm/guest_memfd.c | 58 ++++++++++++++++++++++++++++++++++++++++-- 3 files changed, 100 insertions(+), 2 deletions(-) diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst index 15148c80cfdb6..3ec92f8606099 100644 --- a/Documentation/virt/kvm/api.rst +++ b/Documentation/virt/kvm/api.rst @@ -6571,6 +6571,8 @@ Errors: EAGAIN Some page within requested range had unexpected refcounts. The offset of the page will be returned in `error_offset`. ENOMEM Ran out of memory trying to track private/shared state + EOPNOTSUPP There is no way for KVM to guarantee in-memory contents as + requested. ========== =============================================================== KVM_SET_MEMORY_ATTRIBUTES2 is an extension to @@ -6619,6 +6621,44 @@ on the shared pages, such as refcounts taken by get_user_pages(), and try the ioctl again. A possible source of these long term refcounts is if the guest_memfd memory was pinned in IOMMU page tables. +By default, KVM makes no guarantees about the in-memory values after +memory is convert to/from shared/private. Optionally, userspace may +instruct KVM to ensure the contents of memory are zeroed or preserved, +e.g. to enable in-place sharing of data, or as an optimization to +avoid having to re-zero memory when userspace could have relied on the +trusted entity to guarantee the memory will be zeroed as part of the +entire conversion process. + +The content modes available are as follows: + +``KVM_SET_MEMORY_ATTRIBUTES2_ZERO`` + + On conversion, KVM guarantees all entities that have "allowed" + access to the memory will read zeros. E.g. on private to shared + conversion, both trusted and untrusted code will read zeros. + + Zeroing is currently only guaranteed for private-to-shared + conversions, as KVM in general is untrusted and thus cannot + guarantee the guest (or any trusted entity) will read zeros after + conversion. Note, some CoCo implementations do zero memory contents + such that the guest reads zeros after conversion, and the guest may + choose to rely on that behavior. However, that's a contract between + the trusted CoCo entity and the guest, not between KVM and the + guest. + +``KVM_SET_MEMORY_ATTRIBUTES2_PRESERVE`` + + On conversion, KVM guarantees memory contents will be preserved with + respect to the last written unencrypted value. As a concrete + example, if the host writes ``0xbeef`` to shared memory and converts + the memory to private, the guest will also read ``0xbeef``, even if + the in-memory data is encrypted as part of the conversion. And vice + versa, if the guest writes ``0xbeef`` to private memory and then + converts the memory to shared, the host (and guest) will read + ``0xbeef`` (if the memory is accessible). + +TODO: Document CAP after CAP discussion. + See also: :ref: `KVM_SET_MEMORY_ATTRIBUTES`. .. _kvm_run: diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h index 29baaa60de35a..0fc9ad4ea0d93 100644 --- a/include/uapi/linux/kvm.h +++ b/include/uapi/linux/kvm.h @@ -1642,6 +1642,10 @@ struct kvm_memory_attributes { /* Available with KVM_CAP_MEMORY_ATTRIBUTES2 */ #define KVM_SET_MEMORY_ATTRIBUTES2 _IOWR(KVMIO, 0xd2, struct kvm_memory_attributes2) +#define KVM_SET_MEMORY_ATTRIBUTES2_MODE_UNSPECIFIED 0 +#define KVM_SET_MEMORY_ATTRIBUTES2_ZERO (1ULL << 0) +#define KVM_SET_MEMORY_ATTRIBUTES2_PRESERVE (1ULL << 1) + struct kvm_memory_attributes2 { union { __u64 address; diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c index f23acbca28e54..6bfcb2ed12c61 100644 --- a/virt/kvm/guest_memfd.c +++ b/virt/kvm/guest_memfd.c @@ -674,8 +674,50 @@ int __weak kvm_arch_gmem_apply_content_mode_preserve(struct kvm *kvm, return -EOPNOTSUPP; } +static int kvm_gmem_apply_content_mode_folio(struct kvm *kvm, + struct folio *folio, + uint64_t content_mode) +{ + switch (content_mode) { + case KVM_SET_MEMORY_ATTRIBUTES2_MODE_UNSPECIFIED: + return kvm_arch_gmem_apply_content_mode_unspecified(kvm, folio); + case KVM_SET_MEMORY_ATTRIBUTES2_ZERO: + return kvm_arch_gmem_apply_content_mode_zero(kvm, folio); + case KVM_SET_MEMORY_ATTRIBUTES2_PRESERVE: + return kvm_arch_gmem_apply_content_mode_preserve(kvm, folio); + default: + WARN_ONCE(1, "Unexpected policy requested."); + return -EOPNOTSUPP; + } +} + +static void kvm_gmem_apply_content_mode(struct inode *inode, pgoff_t start, + pgoff_t end, struct kvm *kvm, + uint64_t content_mode) +{ + struct address_space *mapping = inode->i_mapping; + struct folio_batch fbatch; + int i; + + folio_batch_init(&fbatch); + while (filemap_get_folios(mapping, &start, end - 1, &fbatch)) { + + for (i = 0; i < folio_batch_count(&fbatch); ++i) { + struct folio *folio = fbatch.folios[i]; + int ret; + + ret = kvm_gmem_apply_content_mode_folio(kvm, folio, + content_mode); + WARN_ON_ONCE(ret); + } + + folio_batch_release(&fbatch); + } +} + static int __kvm_gmem_set_attributes(struct inode *inode, pgoff_t start, size_t nr_pages, uint64_t attrs, + struct kvm *kvm, uint64_t content_mode, pgoff_t *err_index) { struct address_space *mapping = inode->i_mapping; @@ -689,6 +731,12 @@ static int __kvm_gmem_set_attributes(struct inode *inode, pgoff_t start, filemap_invalidate_lock(mapping); + if (content_mode && + !(kvm_gmem_supported_content_modes(kvm) & content_mode)) { + r = -EOPNOTSUPP; + goto out; + } + mas_init(&mas, mt, start); if (kvm_gmem_range_has_attributes(mt, start, nr_pages, attrs)) { @@ -715,6 +763,8 @@ static int __kvm_gmem_set_attributes(struct inode *inode, pgoff_t start, kvm_gmem_invalidate_begin(inode, start, end); + kvm_gmem_apply_content_mode(inode, start, end, kvm, content_mode); + mas_store_prealloc(&mas, xa_mk_value(attrs)); kvm_gmem_invalidate_end(inode, start, end); @@ -736,7 +786,11 @@ static long kvm_gmem_set_attributes(struct file *file, void __user *argp) if (copy_from_user(&attrs, argp, sizeof(attrs))) return -EFAULT; - if (attrs.flags) + if (attrs.flags & ~(KVM_SET_MEMORY_ATTRIBUTES2_ZERO | + KVM_SET_MEMORY_ATTRIBUTES2_PRESERVE)) + return -EINVAL; + if ((attrs.flags & KVM_SET_MEMORY_ATTRIBUTES2_ZERO) && + (attrs.flags & KVM_SET_MEMORY_ATTRIBUTES2_PRESERVE)) return -EINVAL; if (attrs.error_offset) return -EINVAL; @@ -758,7 +812,7 @@ static long kvm_gmem_set_attributes(struct file *file, void __user *argp) nr_pages = attrs.size >> PAGE_SHIFT; index = attrs.offset >> PAGE_SHIFT; r = __kvm_gmem_set_attributes(inode, index, nr_pages, attrs.attributes, - &err_index); + f->kvm, attrs.flags, &err_index); if (r) { attrs.error_offset = err_index << PAGE_SHIFT; -- 2.53.0.851.ga537e3e6e9-goog
