x86, efivars: firmware bug workarounds should be in platform code

Let's not burden ia64 with checks in the common efivars code that we're not
writing too much data to the variable store. That kind of thing is an x86
firmware bug, plain and simple.

efi_query_variable_store() provides platforms with a wrapper in which they can
perform checks and workarounds for EFI variable storage bugs.

Cc: H. Peter Anvin <[e-mail]>
Cc: Matthew Garrett <[e-mail]>
Signed-off-by: Matt Fleming <[e-mail]>

arch/x86/platform/efi/efi.c

···
 	}
 	return 0;
 }
+
+/*
+ * Some firmware has serious problems when using more than 50% of the EFI
+ * variable store, i.e. it triggers bugs that can brick machines. Ensure that
+ * we never use more than this safe limit.
+ *
+ * Return EFI_SUCCESS if it is safe to write 'size' bytes to the variable
+ * store.
+ */
+efi_status_t efi_query_variable_store(u32 attributes, unsigned long size)
+{
+	efi_status_t status;
+	u64 storage_size, remaining_size, max_size;
+
+	status = efi.query_variable_info(attributes, &storage_size,
+					 &remaining_size, &max_size);
+	if (status != EFI_SUCCESS)
+		return status;
+
+	if (!storage_size || size > remaining_size || size > max_size ||
+	    (remaining_size - size) < (storage_size / 2))
+		return EFI_OUT_OF_RESOURCES;
+
+	return EFI_SUCCESS;
+}

drivers/firmware/efivars.c

···
 check_var_size_locked(struct efivars *efivars, u32 attributes,
 			unsigned long size)
 {
-	u64 storage_size, remaining_size, max_size;
-	efi_status_t status;
 	const struct efivar_operations *fops = efivars->ops;
 
-	if (!efivars->ops->query_variable_info)
+	if (!efivars->ops->query_variable_store)
 		return EFI_UNSUPPORTED;
 
-	status = fops->query_variable_info(attributes, &storage_size,
-					   &remaining_size, &max_size);
-
-	if (status != EFI_SUCCESS)
-		return status;
-
-	if (!storage_size || size > remaining_size || size > max_size ||
-	    (remaining_size - size) < (storage_size / 2))
-		return EFI_OUT_OF_RESOURCES;
-
-	return status;
+	return fops->query_variable_store(attributes, size);
 }
 
 
···
 	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;
+	ops.query_variable_store = efi_query_variable_store;
 
 	error = register_efivars(&__efivars, &ops, efi_kobj);
 	if (error)

include/linux/efi.h

···
 					      unsigned long count,
 					      u64 *max_size,
 					      int *reset_type);
+typedef efi_status_t efi_query_variable_store_t(u32 attributes, unsigned long size);
 
 /*
  *  EFI Configuration Table and GUID definitions
···
 #ifdef CONFIG_X86
 extern void efi_late_init(void);
 extern void efi_free_boot_services(void);
+extern efi_status_t efi_query_variable_store(u32 attributes, unsigned long size);
 #else
 static inline void efi_late_init(void) {}
 static inline void efi_free_boot_services(void) {}
+
+static inline efi_status_t efi_query_variable_store(u32 attributes, unsigned long size)
+{
+	return EFI_SUCCESS;
+}
 #endif
 extern void __iomem *efi_lookup_mapped_addr(u64 phys_addr);
 extern u64 efi_get_iobase (void);
···
 	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;
+	efi_query_variable_store_t *query_variable_store;
 };
 
 struct efivars {