efivarfs: Use query_variable_info() to limit kmalloc()

We don't want someone who can write EFI variables to be able to
allocate arbitrarily large amounts of memory, so cap it to something
sensible like the amount of free space for EFI variables.
Acked-by: Jeremy Kerr <jeremy.kerr@canonical.com>
Cc: Matthew Garrett <mjg@redhat.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>

drivers/firmware/efivars.c

@@ -694,28 +694,51 @@ static ssize_t efivarfs_file_write(struct file *file,
 	struct inode *inode = file->f_mapping->host;
 	unsigned long datasize = count - sizeof(attributes);
 	unsigned long newdatasize;
+	u64 storage_size, remaining_size, max_size;
 	ssize_t bytes = 0;
 
 	if (count < sizeof(attributes))
 		return -EINVAL;
 
-	data = kmalloc(datasize, GFP_KERNEL);
+	if (copy_from_user(&attributes, userbuf, sizeof(attributes)))
+		return -EFAULT;
 
-	if (!data)
-		return -ENOMEM;
+	if (attributes & ~(EFI_VARIABLE_MASK))
+		return -EINVAL;
 
 	efivars = var->efivars;
 
-	if (copy_from_user(&attributes, userbuf, sizeof(attributes))) {
-		bytes = -EFAULT;
-		goto out;
+	/*
+	 * Ensure that the user can't allocate arbitrarily large
+	 * amounts of memory. Pick a default size of 64K if
+	 * QueryVariableInfo() isn't supported by the firmware.
+	 */
+	spin_lock(&efivars->lock);
+
+	if (!efivars->ops->query_variable_info)
+		status = EFI_UNSUPPORTED;
+	else {
+		const struct efivar_operations *fops = efivars->ops;
+		status = fops->query_variable_info(attributes, &storage_size,
+						   &remaining_size, &max_size);
 	}
 
-	if (attributes & ~(EFI_VARIABLE_MASK)) {
-		bytes = -EINVAL;
-		goto out;
+	spin_unlock(&efivars->lock);
+
+	if (status != EFI_SUCCESS) {
+		if (status != EFI_UNSUPPORTED)
+			return efi_status_to_err(status);
+
+		remaining_size = 65536;
 	}
 
+	if (datasize > remaining_size)
+		return -ENOSPC;
+
+	data = kmalloc(datasize, GFP_KERNEL);
+	if (!data)
+		return -ENOMEM;
+
 	if (copy_from_user(data, userbuf + sizeof(attributes), datasize)) {
 		bytes = -EFAULT;
 		goto out;
@@ -1709,6 +1732,8 @@ efivars_init(void)
 	ops.get_variable = efi.get_variable;
 	ops.set_variable = efi.set_variable;
 	ops.get_next_variable = efi.get_next_variable;
+	ops.query_variable_info = efi.query_variable_info;
+
 	error = register_efivars(&__efivars, &ops, efi_kobj);
 	if (error)
 		goto err_put;

include/linux/efi.h

@@ -646,6 +646,7 @@ struct efivar_operations {
 	efi_get_variable_t *get_variable;
 	efi_get_next_variable_t *get_next_variable;
 	efi_set_variable_t *set_variable;
+	efi_query_variable_info_t *query_variable_info;
 };
 
 struct efivars {