efivarfs: Move efivarfs list into superblock s_fs_info

syzbot reports issues with concurrent fsopen()/fsconfig() invocations on
efivarfs, which are the result of the fact that the efivarfs list (which
caches the names and GUIDs of existing EFI variables) is a global
structure. In normal use, these issues are unlikely to trigger, even in
the presence of multiple mounts of efivarfs, but the execution pattern
used by the syzkaller reproducer may result in multiple instances of the
superblock that share the global efivarfs list, and this causes list
corruption when the list is reinitialized by one user while another is
traversing it.

So let's move the list head into the superblock s_fs_info field, so that
it will never be shared between distinct instances of the superblock. In
the common case, there will still be a single instance of this list, but
in the artificial syzkaller case, no list corruption can occur any
longer.

Reported-by: syzbot+1902c359bfcaf39c46f2@syzkaller.appspotmail.com
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

fs/efivarfs/inode.c

@@ -77,6 +77,7 @@ bool efivarfs_valid_name(const char *str, int len)
 static int efivarfs_create(struct mnt_idmap *idmap, struct inode *dir,
 			   struct dentry *dentry, umode_t mode, bool excl)
 {
+	struct efivarfs_fs_info *info = dir->i_sb->s_fs_info;
 	struct inode *inode = NULL;
 	struct efivar_entry *var;
 	int namelen, i = 0, err = 0;
@@ -118,7 +119,7 @@ static int efivarfs_create(struct mnt_idmap *idmap, struct inode *dir,
 	inode->i_private = var;
 	kmemleak_ignore(var);
 
-	err = efivar_entry_add(var, &efivarfs_list);
+	err = efivar_entry_add(var, &info->efivarfs_list);
 	if (err)
 		goto out;
 

fs/efivarfs/internal.h

@@ -16,6 +16,7 @@ struct efivarfs_mount_opts {
 
 struct efivarfs_fs_info {
 	struct efivarfs_mount_opts mount_opts;
+	struct list_head efivarfs_list;
 };
 
 struct efi_variable {
@@ -33,7 +34,8 @@ struct efivar_entry {
 	struct kobject kobj;
 };
 
-int efivar_init(int (*func)(efi_char16_t *, efi_guid_t, unsigned long, void *),
+int efivar_init(int (*func)(efi_char16_t *, efi_guid_t, unsigned long, void *,
+			    struct list_head *),
 		void *data, bool duplicates, struct list_head *head);
 
 int efivar_entry_add(struct efivar_entry *entry, struct list_head *head);
@@ -64,6 +66,4 @@ extern struct inode *efivarfs_get_inode(struct super_block *sb,
 			const struct inode *dir, int mode, dev_t dev,
 			bool is_removable);
 
-extern struct list_head efivarfs_list;
-
 #endif /* EFIVAR_FS_INTERNAL_H */

fs/efivarfs/super.c

@@ -19,8 +19,6 @@
 
 #include "internal.h"
 
-LIST_HEAD(efivarfs_list);
-
 static void efivarfs_evict_inode(struct inode *inode)
 {
 	clear_inode(inode);
@@ -167,7 +165,8 @@ static struct dentry *efivarfs_alloc_dentry(struct dentry *parent, char *name)
 }
 
 static int efivarfs_callback(efi_char16_t *name16, efi_guid_t vendor,
-			     unsigned long name_size, void *data)
+			     unsigned long name_size, void *data,
+			     struct list_head *list)
 {
 	struct super_block *sb = (struct super_block *)data;
 	struct efivar_entry *entry;
@@ -222,7 +221,7 @@ static int efivarfs_callback(efi_char16_t *name16, efi_guid_t vendor,
 	}
 
 	__efivar_entry_get(entry, NULL, &size, NULL);
-	__efivar_entry_add(entry, &efivarfs_list);
+	__efivar_entry_add(entry, list);
 
 	/* copied by the above to local storage in the dentry. */
 	kfree(name);
@@ -292,6 +291,7 @@ static int efivarfs_parse_param(struct fs_context *fc, struct fs_parameter *para
 
 static int efivarfs_fill_super(struct super_block *sb, struct fs_context *fc)
 {
+	struct efivarfs_fs_info *sfi = sb->s_fs_info;
 	struct inode *inode = NULL;
 	struct dentry *root;
 	int err;
@@ -317,11 +317,10 @@ static int efivarfs_fill_super(struct super_block *sb, struct fs_context *fc)
 	if (!root)
 		return -ENOMEM;
 
-	INIT_LIST_HEAD(&efivarfs_list);
-
-	err = efivar_init(efivarfs_callback, (void *)sb, true, &efivarfs_list);
+	err = efivar_init(efivarfs_callback, (void *)sb, true,
+			  &sfi->efivarfs_list);
 	if (err)
-		efivar_entry_iter(efivarfs_destroy, &efivarfs_list, NULL);
+		efivar_entry_iter(efivarfs_destroy, &sfi->efivarfs_list, NULL);
 
 	return err;
 }
@@ -358,6 +357,8 @@ static int efivarfs_init_fs_context(struct fs_context *fc)
 	if (!sfi)
 		return -ENOMEM;
 
+	INIT_LIST_HEAD(&sfi->efivarfs_list);
+
 	sfi->mount_opts.uid = GLOBAL_ROOT_UID;
 	sfi->mount_opts.gid = GLOBAL_ROOT_GID;
 
@@ -373,7 +374,7 @@ static void efivarfs_kill_sb(struct super_block *sb)
 	kill_litter_super(sb);
 
 	/* Remove all entries and destroy */
-	efivar_entry_iter(efivarfs_destroy, &efivarfs_list, NULL);
+	efivar_entry_iter(efivarfs_destroy, &sfi->efivarfs_list, NULL);
 	kfree(sfi);
 }
 

fs/efivarfs/vars.c

@@ -369,7 +369,8 @@ static void dup_variable_bug(efi_char16_t *str16, efi_guid_t *vendor_guid,
  *
  * Returns 0 on success, or a kernel error code on failure.
  */
-int efivar_init(int (*func)(efi_char16_t *, efi_guid_t, unsigned long, void *),
+int efivar_init(int (*func)(efi_char16_t *, efi_guid_t, unsigned long, void *,
+			    struct list_head *),
 		void *data, bool duplicates, struct list_head *head)
 {
 	unsigned long variable_name_size = 1024;
@@ -420,7 +421,7 @@ int efivar_init(int (*func)(efi_char16_t *, efi_guid_t, unsigned long, void *),
 				status = EFI_NOT_FOUND;
 			} else {
 				err = func(variable_name, vendor_guid,
-					   variable_name_size, data);
+					   variable_name_size, data, head);
 				if (err)
 					status = EFI_NOT_FOUND;
 			}