On Mon, 2024-08-05 at 19:34 +0100, Elliot Berman wrote:
> Confidential/protected guest virtual machines want to share some memory
> back with the host Linux. For example, virtqueues allow host and
> protected guest to exchange data. In MMU-only isolation of protected
> guest virtual machines, the transition between "shared" and "private"
> can be done in-place without a trusted hypervisor copying pages.
>
> Add support for this feature and allow Linux to mmap host-accessible
> pages. When the owner provides an ->accessible() callback in the
> struct guest_memfd_operations, guest_memfd allows folios to be mapped
> when the ->accessible() callback returns 0.
Wouldn't the set of inaccessible folios always match exactly the set of
folios that have PG_private=1 set? At least guest_memfd instances that
have GUEST_MEMFD_FLAG_NO_DIRECT_MAP set, having folios without direct
map entries marked "accessible" sound like it may cause a lot of mayhem
(as those folios would essentially be secretmem folios, but this time
without the GUP checks). But even more generally, wouldn't tracking
accessibility via PG_private be enough?
> To safely make inaccessible:
>
> ```
> folio = guest_memfd_grab_folio(inode, index, flags);
> r = guest_memfd_make_inaccessible(inode, folio);
> if (r)
> goto err;
>
> hypervisor_does_guest_mapping(folio);
>
> folio_unlock(folio);
> ```
>
> hypervisor_does_s2_mapping(folio) should make it so
> ops->accessible(...) on those folios fails.
>
> The folio lock ensures atomicity.
>
> Signed-off-by: Elliot Berman <quic_eberman@xxxxxxxxxxx>
> ---
> include/linux/guest_memfd.h | 7 ++++
> mm/guest_memfd.c | 81 ++++++++++++++++++++++++++++++++++++++++++++-
> 2 files changed, 87 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/guest_memfd.h b/include/linux/guest_memfd.h
> index f9e4a27aed67..edcb4ba60cb0 100644
> --- a/include/linux/guest_memfd.h
> +++ b/include/linux/guest_memfd.h
> @@ -16,12 +16,18 @@
> * @invalidate_end: called after invalidate_begin returns success. Optional.
> * @prepare: called before a folio is mapped into the guest address space.
> * Optional.
> + * @accessible: called after prepare returns success and before it's mapped
> + * into the guest address space. Returns 0 if the folio can be
> + * accessed.
> + * Optional. If not present, assumes folios are never accessible.
> * @release: Called when releasing the guest_memfd file. Required.
> */
> struct guest_memfd_operations {
> int (*invalidate_begin)(struct inode *inode, pgoff_t offset, unsigned long nr);
> void (*invalidate_end)(struct inode *inode, pgoff_t offset, unsigned long nr);
> int (*prepare)(struct inode *inode, pgoff_t offset, struct folio *folio);
> + int (*accessible)(struct inode *inode, struct folio *folio,
> + pgoff_t offset, unsigned long nr);
> int (*release)(struct inode *inode);
> };
>
> @@ -48,5 +54,6 @@ struct file *guest_memfd_alloc(const char *name,
> const struct guest_memfd_operations *ops,
> loff_t size, unsigned long flags);
> bool is_guest_memfd(struct file *file, const struct guest_memfd_operations *ops);
> +int guest_memfd_make_inaccessible(struct file *file, struct folio *folio);
>
> #endif
> diff --git a/mm/guest_memfd.c b/mm/guest_memfd.c
> index e9d8cab72b28..6b5609932ca5 100644
> --- a/mm/guest_memfd.c
> +++ b/mm/guest_memfd.c
> @@ -9,6 +9,8 @@
> #include <linux/pagemap.h>
> #include <linux/set_memory.h>
>
> +#include "internal.h"
> +
> static inline int guest_memfd_folio_private(struct folio *folio)
> {
> unsigned long nr_pages = folio_nr_pages(folio);
> @@ -89,7 +91,7 @@ struct folio *guest_memfd_grab_folio(struct file *file, pgoff_t index, u32 flags
> goto out_err;
> }
>
> - if (gmem_flags & GUEST_MEMFD_FLAG_NO_DIRECT_MAP) {
> + if (!ops->accessible && (gmem_flags & GUEST_MEMFD_FLAG_NO_DIRECT_MAP)) {
> r = guest_memfd_folio_private(folio);
> if (r)
> goto out_err;
> @@ -107,6 +109,82 @@ struct folio *guest_memfd_grab_folio(struct file *file, pgoff_t index, u32 flags
> }
> EXPORT_SYMBOL_GPL(guest_memfd_grab_folio);
>
> +int guest_memfd_make_inaccessible(struct file *file, struct folio *folio)
> +{
> + unsigned long gmem_flags = (unsigned long)file->private_data;
> + unsigned long i;
> + int r;
> +
> + unmap_mapping_folio(folio);
> +
> + /**
> + * We can't use the refcount. It might be elevated due to
> + * guest/vcpu trying to access same folio as another vcpu
> + * or because userspace is trying to access folio for same reason
> + *
> + * folio_lock serializes the transitions between (in)accessible
> + */
> + if (folio_maybe_dma_pinned(folio))
> + return -EBUSY;
> +
> + if (gmem_flags & GUEST_MEMFD_FLAG_NO_DIRECT_MAP) {
> + r = guest_memfd_folio_private(folio);
> + if (r)
> + return r;
> + }
> +
> + return 0;
> +}
> +
> +static vm_fault_t gmem_fault(struct vm_fault *vmf)
> +{
> + struct file *file = vmf->vma->vm_file;
> + struct inode *inode = file_inode(file);
> + const struct guest_memfd_operations *ops = inode->i_private;
> + struct folio *folio;
> + pgoff_t off;
> + int r;
> +
> + folio = guest_memfd_grab_folio(file, vmf->pgoff, GUEST_MEMFD_GRAB_UPTODATE);
> + if (!folio)
> + return VM_FAULT_SIGBUS;
> +
> + off = vmf->pgoff & (folio_nr_pages(folio) - 1);
> + r = ops->accessible(inode, folio, off, 1);
> + if (r) {
This made be stumble at first. I know you say ops->accessible returning
0 means "this is accessible", but if I only look at this if-statement it
reads as "if the folio is accessible, send a SIGBUS", which is not
what's actually happening.
> + folio_unlock(folio);
> + folio_put(folio);
> + return VM_FAULT_SIGBUS;
> + }
> +
> + guest_memfd_folio_clear_private(folio);
> +
> + vmf->page = folio_page(folio, off);
> +
> + return VM_FAULT_LOCKED;
> +}
> +
> +static const struct vm_operations_struct gmem_vm_ops = {
> + .fault = gmem_fault,
> +};
> +
> +static int gmem_mmap(struct file *file, struct vm_area_struct *vma)
> +{
> + const struct guest_memfd_operations *ops = file_inode(file)->i_private;
> +
> + if (!ops->accessible)
> + return -EPERM;
> +
> + /* No support for private mappings to avoid COW. */
> + if ((vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) !=
> + (VM_SHARED | VM_MAYSHARE))
> + return -EINVAL;
> +
> + file_accessed(file);
> + vma->vm_ops = &gmem_vm_ops;
> + return 0;
> +}
> +
> static long gmem_punch_hole(struct file *file, loff_t offset, loff_t len)
> {
> struct inode *inode = file_inode(file);
> @@ -220,6 +298,7 @@ static int gmem_release(struct inode *inode, struct file *file)
> static struct file_operations gmem_fops = {
> .open = generic_file_open,
> .llseek = generic_file_llseek,
> + .mmap = gmem_mmap,
> .release = gmem_release,
> .fallocate = gmem_fallocate,
> .owner = THIS_MODULE,
>
> --
> 2.34.1
>
