Re: [PATCH RFC v5 10/53] KVM: guest_memfd: Add basic support for KVM_SET_MEMORY_ATTRIBUTES2

Liam R. Howlett
2026-05-07T03:46:43Z

On 26/04/28 04:25PM, Ackerley Tng via B4 Relay wrote:
> From: Ackerley Tng <ackerleytng@google.com>
>
> Introduce basic support for KVM_SET_MEMORY_ATTRIBUTES2 in guest_memfd,
> which just updates attributes tracked by guest_memfd.
>
> Validate input fields in general. Guard usage of KVM_SET_MEMORY_ATTRIBUTES2
> by making sure requested attributes are supported for this instance of kvm.
>
> A new KVM_SET_MEMORY_ATTRIBUTES2 is defined to support writes (unlike
> KVM_SET_MEMORY_ATTRIBUTES) in addition to reads so it can provide error
> details to userspace. This will be used in a later patch.
>
> The two ioctls use their corresponding structs with no overlap, but
> backward compatibility is baked in for future support of
> KVM_SET_MEMORY_ATTRIBUTES2 and struct kvm_memory_attributes2 in the VM
> ioctl.
>
> The process of setting memory attributes is set up such that the later half
> will not fail due to allocation. Any necessary checks are performed before
> the point of no return.
>
> Signed-off-by: Ackerley Tng <ackerleytng@google.com>
> Co-developed-by: Vishal Annapurve <vannapurve@google.com>
> Signed-off-by: Vishal Annapurve <vannapurve@google.com>
> Co-developed-by: Sean Christoperson <seanjc@google.com>
> Signed-off-by: Sean Christoperson <seanjc@google.com>
> ---
> include/uapi/linux/kvm.h | 13 ++++++
> virt/kvm/Kconfig | 1 +
> virt/kvm/guest_memfd.c | 114 +++++++++++++++++++++++++++++++++++++++++++++++
> virt/kvm/kvm_main.c | 12 +++++
> 4 files changed, 140 insertions(+)
>
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 6c8afa2047bf3..e6bbf68a83813 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -1648,6 +1648,19 @@ struct kvm_memory_attributes {
> __u64 flags;
> };
>
> +#define KVM_SET_MEMORY_ATTRIBUTES2 _IOWR(KVMIO, 0xd2, struct kvm_memory_attributes2)
> +
> +struct kvm_memory_attributes2 {
> + union {
> + __u64 address;
> + __u64 offset;
> + };
> + __u64 size;
> + __u64 attributes;
> + __u64 flags;
> + __u64 reserved[12];
> +};
> +
> #define KVM_MEMORY_ATTRIBUTE_PRIVATE (1ULL << 3)
>
> #define KVM_CREATE_GUEST_MEMFD _IOWR(KVMIO, 0xd4, struct kvm_create_guest_memfd)
> diff --git a/virt/kvm/Kconfig b/virt/kvm/Kconfig
> index 3fea89c45cfb4..e371e079e2c50 100644
> --- a/virt/kvm/Kconfig
> +++ b/virt/kvm/Kconfig
> @@ -109,6 +109,7 @@ config KVM_VM_MEMORY_ATTRIBUTES
>
> config KVM_GUEST_MEMFD
> select XARRAY_MULTI
> + select KVM_MEMORY_ATTRIBUTES
> bool
>
> config HAVE_KVM_ARCH_GMEM_PREPARE
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index 506219e2359eb..9a26eca717047 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -552,11 +552,125 @@ unsigned long kvm_gmem_get_memory_attributes(struct kvm *kvm, gfn_t gfn)
> }
> EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_gmem_get_memory_attributes);
>
> +/*
> + * Preallocate memory for attributes to be stored on a maple tree, pointed to
> + * by mas. Adjacent ranges with attributes identical to the new attributes
> + * will be merged. Also sets mas's bounds up for storing attributes.
> + *
> + * This maintains the invariant that ranges with the same attributes will
> + * always be merged.
> + */
> +static int kvm_gmem_mas_preallocate(struct ma_state *mas, u64 attributes,
> + pgoff_t start, size_t nr_pages)
> +{
> + pgoff_t end = start + nr_pages;
> + pgoff_t last = end - 1;
> + void *entry;
> +
> + /* Try extending range. entry is NULL on overflow/wrap-around. */
> + mas_set_range(mas, end, end);
> + entry = mas_find(mas, end);

Please read the documentation as I believe you have a bug here. What
happens if there is another range stored higher than end + 1?

Do you have testing of these functions somewhere?

> + if (entry && xa_to_value(entry) == attributes)
> + last = mas->last;
> +
> + if (start > 0) {
> + mas_set_range(mas, start - 1, start - 1);
> + entry = mas_find(mas, start - 1);
> + if (entry && xa_to_value(entry) == attributes)
> + start = mas->index;
> + }
> +
> + mas_set_range(mas, start, last);
> + return mas_preallocate(mas, xa_mk_value(attributes), GFP_KERNEL);
> +}
> +
> +static int __kvm_gmem_set_attributes(struct inode *inode, pgoff_t start,
> + size_t nr_pages, uint64_t attrs)
> +{
> + struct address_space *mapping = inode->i_mapping;
> + struct gmem_inode *gi = GMEM_I(inode);
> + pgoff_t end = start + nr_pages;
> + struct maple_tree *mt;
> + struct ma_state mas;
> + int r;
> +
> + mt = &gi->attributes;
> +
> + filemap_invalidate_lock(mapping);
> +
> + mas_init(&mas, mt, start);
> + r = kvm_gmem_mas_preallocate(&mas, attrs, start, nr_pages);
> + if (r)
> + goto out;
> +
> + /*
> + * From this point on guest_memfd has performed necessary
> + * checks and can proceed to do guest-breaking changes.
> + */
> +
> + kvm_gmem_invalidate_begin(inode, start, end);
> + mas_store_prealloc(&mas, xa_mk_value(attrs));
> + kvm_gmem_invalidate_end(inode, start, end);
> +out:
> + filemap_invalidate_unlock(mapping);
> + return r;
> +}
> +
> +static long kvm_gmem_set_attributes(struct file *file, void __user *argp)
> +{
> + struct gmem_file *f = file->private_data;
> + struct inode *inode = file_inode(file);
> + struct kvm_memory_attributes2 attrs;
> + size_t nr_pages;
> + pgoff_t index;
> + int i;
> +
> + if (copy_from_user(&attrs, argp, sizeof(attrs)))
> + return -EFAULT;
> +
> + if (attrs.flags)
> + return -EINVAL;
> + for (i = 0; i < ARRAY_SIZE(attrs.reserved); i++) {
> + if (attrs.reserved[i])
> + return -EINVAL;
> + }
> + if (attrs.attributes & ~kvm_supported_mem_attributes(f->kvm))
> + return -EINVAL;
> + if (attrs.size == 0 || attrs.offset + attrs.size < attrs.offset)
> + return -EINVAL;
> + if (!PAGE_ALIGNED(attrs.offset) || !PAGE_ALIGNED(attrs.size))
> + return -EINVAL;
> +
> + if (attrs.offset >= i_size_read(inode) ||
> + attrs.offset + attrs.size > i_size_read(inode))
> + return -EINVAL;
> +
> + nr_pages = attrs.size >> PAGE_SHIFT;
> + index = attrs.offset >> PAGE_SHIFT;
> + return __kvm_gmem_set_attributes(inode, index, nr_pages,
> + attrs.attributes);
> +}
> +
> +static long kvm_gmem_ioctl(struct file *file, unsigned int ioctl,
> + unsigned long arg)
> +{
> + switch (ioctl) {
> + case KVM_SET_MEMORY_ATTRIBUTES2:
> + if (vm_memory_attributes)
> + return -ENOTTY;
> +
> + return kvm_gmem_set_attributes(file, (void __user *)arg);
> + default:
> + return -ENOTTY;
> + }
> +}
> +
> static struct file_operations kvm_gmem_fops = {
> .mmap = kvm_gmem_mmap,
> .open = generic_file_open,
> .release = kvm_gmem_release,
> .fallocate = kvm_gmem_fallocate,
> + .unlocked_ioctl = kvm_gmem_ioctl,
> };
>
> static int kvm_gmem_migrate_folio(struct address_space *mapping,
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index ff20e63143642..4d7bf52b7b717 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -110,6 +110,18 @@ EXPORT_SYMBOL_FOR_KVM_INTERNAL(STATIC_CALL_KEY(__kvm_get_memory_attributes));
> EXPORT_SYMBOL_FOR_KVM_INTERNAL(STATIC_CALL_TRAMP(__kvm_get_memory_attributes));
> #endif
>
> +#define MEMORY_ATTRIBUTES_MATCH(one, two) \
> + static_assert(offsetof(struct kvm_memory_attributes, one) == \
> + offsetof(struct kvm_memory_attributes2, two)); \
> + static_assert(sizeof_field(struct kvm_memory_attributes, one) ==\
> + sizeof_field(struct kvm_memory_attributes2, two))
> +
> +/* Ensure the common parts of the two structs are identical. */
> +MEMORY_ATTRIBUTES_MATCH(address, address);
> +MEMORY_ATTRIBUTES_MATCH(size, size);
> +MEMORY_ATTRIBUTES_MATCH(attributes, attributes);
> +MEMORY_ATTRIBUTES_MATCH(flags, flags);
> +
> /*
> * Ordering of locks:
> *
>
> --
> 2.54.0.545.g6539524ca2-goog
>
>
>