[RFC PATCH 13/39] KVM: guest_memfd: Make guest mem use guest mem inodes instead of anonymous inodes

Ackerley Tng
2024-09-10T23:44:54Z

Using guest mem inodes allows us to store metadata for the backing
memory on the inode. Metadata will be added in a later patch to
support HugeTLB pages.

Metadata about backing memory should not be stored on the file, since
the file represents a guest_memfd's binding with a struct kvm, and
metadata about backing memory is not unique to a specific binding and
struct kvm.

Signed-off-by: Ackerley Tng <ackerleytng@google.com>
---
include/uapi/linux/magic.h | 1 +
virt/kvm/guest_memfd.c | 119 ++++++++++++++++++++++++++++++-------
2 files changed, 100 insertions(+), 20 deletions(-)

diff --git a/include/uapi/linux/magic.h b/include/uapi/linux/magic.h
index bb575f3ab45e..169dba2a6920 100644
--- a/include/uapi/linux/magic.h
+++ b/include/uapi/linux/magic.h
@@ -103,5 +103,6 @@
#define DEVMEM_MAGIC 0x454d444d /* "DMEM" */
#define SECRETMEM_MAGIC 0x5345434d /* "SECM" */
#define PID_FS_MAGIC 0x50494446 /* "PIDF" */
+#define GUEST_MEMORY_MAGIC 0x474d454d /* "GMEM" */

#endif /* __LINUX_MAGIC_H__ */
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 8f079a61a56d..5d7fd1f708a6 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -1,12 +1,17 @@
// SPDX-License-Identifier: GPL-2.0
+#include <linux/fs.h>
+#include <linux/mount.h>
#include <linux/backing-dev.h>
#include <linux/falloc.h>
#include <linux/kvm_host.h>
+#include <linux/pseudo_fs.h>
#include <linux/pagemap.h>
#include <linux/anon_inodes.h>

#include "kvm_mm.h"

+static struct vfsmount *kvm_gmem_mnt;
+
struct kvm_gmem {
struct kvm *kvm;
struct xarray bindings;
@@ -302,6 +307,38 @@ static inline struct file *kvm_gmem_get_file(struct kvm_memory_slot *slot)
return get_file_active(&slot->gmem.file);
}

+static const struct super_operations kvm_gmem_super_operations = {
+ .statfs = simple_statfs,
+};
+
+static int kvm_gmem_init_fs_context(struct fs_context *fc)
+{
+ struct pseudo_fs_context *ctx;
+
+ if (!init_pseudo(fc, GUEST_MEMORY_MAGIC))
+ return -ENOMEM;
+
+ ctx = fc->fs_private;
+ ctx->ops = &kvm_gmem_super_operations;
+
+ return 0;
+}
+
+static struct file_system_type kvm_gmem_fs = {
+ .name = "kvm_guest_memory",
+ .init_fs_context = kvm_gmem_init_fs_context,
+ .kill_sb = kill_anon_super,
+};
+
+static void kvm_gmem_init_mount(void)
+{
+ kvm_gmem_mnt = kern_mount(&kvm_gmem_fs);
+ BUG_ON(IS_ERR(kvm_gmem_mnt));
+
+ /* For giggles. Userspace can never map this anyways. */
+ kvm_gmem_mnt->mnt_flags |= MNT_NOEXEC;
+}
+
static struct file_operations kvm_gmem_fops = {
.open = generic_file_open,
.release = kvm_gmem_release,
@@ -311,6 +348,8 @@ static struct file_operations kvm_gmem_fops = {
void kvm_gmem_init(struct module *module)
{
kvm_gmem_fops.owner = module;
+
+ kvm_gmem_init_mount();
}

static int kvm_gmem_migrate_folio(struct address_space *mapping,
@@ -392,11 +431,67 @@ static const struct inode_operations kvm_gmem_iops = {
.setattr = kvm_gmem_setattr,
};

+static struct inode *kvm_gmem_inode_make_secure_inode(const char *name,
+ loff_t size, u64 flags)
+{
+ const struct qstr qname = QSTR_INIT(name, strlen(name));
+ struct inode *inode;
+ int err;
+
+ inode = alloc_anon_inode(kvm_gmem_mnt->mnt_sb);
+ if (IS_ERR(inode))
+ return inode;
+
+ err = security_inode_init_security_anon(inode, &qname, NULL);
+ if (err) {
+ iput(inode);
+ return ERR_PTR(err);
+ }
+
+ inode->i_private = (void *)(unsigned long)flags;
+ inode->i_op = &kvm_gmem_iops;
+ inode->i_mapping->a_ops = &kvm_gmem_aops;
+ inode->i_mode |= S_IFREG;
+ inode->i_size = size;
+ mapping_set_gfp_mask(inode->i_mapping, GFP_HIGHUSER);
+ mapping_set_inaccessible(inode->i_mapping);
+ /* Unmovable mappings are supposed to be marked unevictable as well. */
+ WARN_ON_ONCE(!mapping_unevictable(inode->i_mapping));
+
+ return inode;
+}
+
+static struct file *kvm_gmem_inode_create_getfile(void *priv, loff_t size,
+ u64 flags)
+{
+ static const char *name = "[kvm-gmem]";
+ struct inode *inode;
+ struct file *file;
+
+ if (kvm_gmem_fops.owner && !try_module_get(kvm_gmem_fops.owner))
+ return ERR_PTR(-ENOENT);
+
+ inode = kvm_gmem_inode_make_secure_inode(name, size, flags);
+ if (IS_ERR(inode))
+ return ERR_CAST(inode);
+
+ file = alloc_file_pseudo(inode, kvm_gmem_mnt, name, O_RDWR,
+ &kvm_gmem_fops);
+ if (IS_ERR(file)) {
+ iput(inode);
+ return file;
+ }
+
+ file->f_mapping = inode->i_mapping;
+ file->f_flags |= O_LARGEFILE;
+ file->private_data = priv;
+
+ return file;
+}
+
static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags)
{
- const char *anon_name = "[kvm-gmem]";
struct kvm_gmem *gmem;
- struct inode *inode;
struct file *file;
int fd, err;

@@ -410,32 +505,16 @@ static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags)
goto err_fd;
}

- file = anon_inode_create_getfile(anon_name, &kvm_gmem_fops, gmem,
- O_RDWR, NULL);
+ file = kvm_gmem_inode_create_getfile(gmem, size, flags);
if (IS_ERR(file)) {
err = PTR_ERR(file);
goto err_gmem;
}

- file->f_flags |= O_LARGEFILE;
-
- inode = file->f_inode;
- WARN_ON(file->f_mapping != inode->i_mapping);
-
- inode->i_private = (void *)(unsigned long)flags;
- inode->i_op = &kvm_gmem_iops;
- inode->i_mapping->a_ops = &kvm_gmem_aops;
- inode->i_mode |= S_IFREG;
- inode->i_size = size;
- mapping_set_gfp_mask(inode->i_mapping, GFP_HIGHUSER);
- mapping_set_inaccessible(inode->i_mapping);
- /* Unmovable mappings are supposed to be marked unevictable as well. */
- WARN_ON_ONCE(!mapping_unevictable(inode->i_mapping));
-
kvm_get_kvm(kvm);
gmem->kvm = kvm;
xa_init(&gmem->bindings);
- list_add(&gmem->entry, &inode->i_mapping->i_private_list);
+ list_add(&gmem->entry, &file_inode(file)->i_mapping->i_private_list);

fd_install(fd, file);
return fd;
--
2.46.0.598.g6f2099f65c-goog