kvm: guestmem: do not use a file system

Use a run-of-the-mill anonymous inode, there is nothing useful being
provided by kvm_gmem_fs.

Fixes: 1d46f95 ("KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory")
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Link: https://lore.kernel.org/r/20230928180651.1525674-1-pbonzini@redhat.com
[sean: add gmem to list using inode, not file]
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: bonzini

include/uapi/linux/magic.h

@@ -101,6 +101,5 @@
 #define DMA_BUF_MAGIC		0x444d4142	/* "DMAB" */
 #define DEVMEM_MAGIC		0x454d444d	/* "DMEM" */
 #define SECRETMEM_MAGIC		0x5345434d	/* "SECM" */
-#define KVM_GUEST_MEMORY_MAGIC	0x474d454d	/* "GMEM" */
 
 #endif /* __LINUX_MAGIC_H__ */

virt/kvm/guest_memfd.c

@@ -3,14 +3,10 @@
 #include <linux/falloc.h>
 #include <linux/kvm_host.h>
 #include <linux/pagemap.h>
-#include <linux/pseudo_fs.h>
-
-#include <uapi/linux/magic.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;
@@ -364,23 +360,35 @@ static const struct inode_operations kvm_gmem_iops = {
 	.setattr	= kvm_gmem_setattr,
 };
 
-static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags,
-			     struct vfsmount *mnt)
+static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags)
 {
 	const char *anon_name = "[kvm-gmem]";
-	const struct qstr qname = QSTR_INIT(anon_name, strlen(anon_name));
 	struct kvm_gmem *gmem;
 	struct inode *inode;
 	struct file *file;
 	int fd, err;
 
-	inode = alloc_anon_inode(mnt->mnt_sb);
-	if (IS_ERR(inode))
-		return PTR_ERR(inode);
+	fd = get_unused_fd_flags(0);
+	if (fd < 0)
+		return fd;
 
-	err = security_inode_init_security_anon(inode, &qname, NULL);
-	if (err)
-		goto err_inode;
+	gmem = kzalloc(sizeof(*gmem), GFP_KERNEL);
+	if (!gmem) {
+		err = -ENOMEM;
+		goto err_fd;
+	}
+
+	file = anon_inode_getfile(anon_name, &kvm_gmem_fops, gmem,
+				  O_RDWR);
+	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;
@@ -393,44 +401,18 @@ static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags,
 	/* Unmovable mappings are supposed to be marked unevictable as well. */
 	WARN_ON_ONCE(!mapping_unevictable(inode->i_mapping));
 
-	fd = get_unused_fd_flags(0);
-	if (fd < 0) {
-		err = fd;
-		goto err_inode;
-	}
-
-	file = alloc_file_pseudo(inode, mnt, "kvm-gmem", O_RDWR, &kvm_gmem_fops);
-	if (IS_ERR(file)) {
-		err = PTR_ERR(file);
-		goto err_fd;
-	}
-
-	file->f_flags |= O_LARGEFILE;
-	file->f_mapping = inode->i_mapping;
-
-	gmem = kzalloc(sizeof(*gmem), GFP_KERNEL);
-	if (!gmem) {
-		err = -ENOMEM;
-		goto err_file;
-	}
-
 	kvm_get_kvm(kvm);
 	gmem->kvm = kvm;
 	xa_init(&gmem->bindings);
-
-	file->private_data = gmem;
-
 	list_add(&gmem->entry, &inode->i_mapping->private_list);
 
 	fd_install(fd, file);
 	return fd;
 
-err_file:
-	fput(file);
+err_gmem:
+	kfree(gmem);
 err_fd:
 	put_unused_fd(fd);
-err_inode:
-	iput(inode);
 	return err;
 }
 
@@ -455,7 +437,7 @@ int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args)
 		return -EINVAL;
 #endif
 
-	return __kvm_gmem_create(kvm, size, flags, kvm_gmem_mnt);
+	return __kvm_gmem_create(kvm, size, flags);
 }
 
 int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot,
@@ -611,35 +593,3 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
 	return r;
 }
 EXPORT_SYMBOL_GPL(kvm_gmem_get_pfn);
-
-static int kvm_gmem_init_fs_context(struct fs_context *fc)
-{
-	if (!init_pseudo(fc, KVM_GUEST_MEMORY_MAGIC))
-		return -ENOMEM;
-
-	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,
-};
-
-int kvm_gmem_init(void)
-{
-	kvm_gmem_mnt = kern_mount(&kvm_gmem_fs);
-	if (IS_ERR(kvm_gmem_mnt))
-		return PTR_ERR(kvm_gmem_mnt);
-
-	/* For giggles.  Userspace can never map this anyways. */
-	kvm_gmem_mnt->mnt_flags |= MNT_NOEXEC;
-
-	return 0;
-}
-
-void kvm_gmem_exit(void)
-{
-	kern_unmount(kvm_gmem_mnt);
-	kvm_gmem_mnt = NULL;
-}

virt/kvm/kvm_main.c

@@ -6448,10 +6448,6 @@ int kvm_init(unsigned vcpu_size, unsigned vcpu_align, struct module *module)
 	if (r)
 		goto err_async_pf;
 
-	r = kvm_gmem_init();
-	if (r)
-		goto err_gmem;
-
 	kvm_chardev_ops.owner = module;
 
 	kvm_preempt_ops.sched_in = kvm_sched_in;
@@ -6478,8 +6474,6 @@ int kvm_init(unsigned vcpu_size, unsigned vcpu_align, struct module *module)
 err_register:
 	kvm_vfio_ops_exit();
 err_vfio:
-	kvm_gmem_exit();
-err_gmem:
 	kvm_async_pf_deinit();
 err_async_pf:
 	kvm_irqfd_exit();