Re: [RFC PATCH] efi_loader: conditionally enable SetvariableRT

From: Ilias Apalodimas ilias.apalodimas@linaro.org Date: Fri, 29 Mar 2024 09:19:27 +0200

When EFI variables are stored on file we don't allow SetVariableRT, since the OS doesn't know how to access or write that file. At the same time keeping the U-Boot drivers alive in runtime sections and performing writes from the firmware is dangerous -- if at all possible.

For GetVariableRT we copy runtime variables in RAM and expose them to the OS. Add a Kconfig option and provide SetVariableRT using the same memory backend. The OS will be responsible for syncing the RAM contents to the file, otherwise any changes made during runtime won't persist reboots.

It's worth noting that the variable store format is defined in EBBR [0] and authenticated variables are explicitly prohibited, since they have to be stored on a medium that's tamper and rollback protected.

• pre-patch

$~ mount | grep efiva efivarfs on /sys/firmware/efi/efivars type efivarfs (ro,nosuid,nodev,noexec,relatime)

$~ efibootmgr -n 0001 Could not set BootNext: Read-only file system

• post-patch

$~ mount | grep efiva efivarfs on /sys/firmware/efi/efivars type efivarfs (rw,nosuid,nodev,noexec,relatime)

$~ efibootmgr -n 0001 BootNext: 0001 BootCurrent: 0000 BootOrder: 0000,0001 Boot0000* debian HD(1,GPT,bdae5610-3331-4e4d-9466-acb5caf0b4a6,0x800,0x100000)/File(EFI\debian\grubaa64.efi) Boot0001* virtio 0 VenHw(e61d73b9-a384-4acc-aeab-82e828f3628b,0000000000000000)/VenHw(e61d73b9-a384-4acc-aeab-82e828f3628b,850000001f000000)/VenHw(e61d73b9-a384-4acc-aeab-82e828f3628b,1600850000000000){auto_created_boot_option}

$~ efivar -p -n 8be4df61-93ca-11d2-aa0d-00e098032b8c-BootNext GUID: 8be4df61-93ca-11d2-aa0d-00e098032b8c Name: "BootNext" Attributes: Non-Volatile Boot Service Access Runtime Service Access Value: 00000000 01 00

[0] https://arm-software.github.io/ebbr/index.html#document-chapter5-variable-st...

Open questions: Looking at the EFI spec, I can't find a documented way of notifying the OS that the storage is volatile. I would like to send a hint to the OS about that and I was thinking of adding a configuration table with the filename, which U-Boot expects to be on the ESP. Alternatively we could add a device path which would be a bit harder to parse from the userspace application, but safer in case multiple ESPs are present.

Should there be a way for the OS to opt-in to this new behaviour?

Since I am not too familiar with how BSDs treat and expose config tables, we could instead add a RO efi variable with identical semantics, which would be easier to read from userspace.

There is an EFIIOC_GET_TABLE ioctl that allows retreiving a table by GUID.

Signed-off-by: Ilias Apalodimas ilias.apalodimas@linaro.org

lib/efi_loader/Kconfig | 14 +++ lib/efi_loader/efi_runtime.c | 4 + lib/efi_loader/efi_variable.c | 108 ++++++++++++++++-- .../efi_selftest_variables_runtime.c | 13 ++- 4 files changed, 126 insertions(+), 13 deletions(-)

diff --git a/lib/efi_loader/Kconfig b/lib/efi_loader/Kconfig index a7c3e05c13a0..c7f7383189e5 100644 --- a/lib/efi_loader/Kconfig +++ b/lib/efi_loader/Kconfig @@ -63,6 +63,20 @@ config EFI_VARIABLE_FILE_STORE Select this option if you want non-volatile UEFI variables to be stored as file /ubootefi.var on the EFI system partition.

+config EFI_RT_VOLATILE_STORE

• bool "Allow variable runtime services in volatile storage (e.g RAM)"
• depends on EFI_VARIABLE_FILE_STORE
• help

•  When EFI variables are stored on file we don't allow SetVariableRT,
  

•  since the OS doesn't know how to write that file. At he same time
  

•  we copy runtime variables in DRAM and support GetVariableRT
  

•  Enable this option to allow SetVariableRT on the RAM backend of
  

•  the EFI variable storate. The OS will be responsible for syncing
  

•  the RAM contents to the file, otherwise any changes made during
  

•  runtime won't persist reboots.
  

•  Authenticated variables are not supported.
  

config EFI_MM_COMM_TEE bool "UEFI variables storage service via the trusted world" depends on OPTEE diff --git a/lib/efi_loader/efi_runtime.c b/lib/efi_loader/efi_runtime.c index 18da6892e796..b38ce239e2d2 100644 --- a/lib/efi_loader/efi_runtime.c +++ b/lib/efi_loader/efi_runtime.c @@ -126,6 +126,10 @@ efi_status_t efi_init_runtime_supported(void) EFI_RT_SUPPORTED_SET_VIRTUAL_ADDRESS_MAP | EFI_RT_SUPPORTED_CONVERT_POINTER;

• if (IS_ENABLED(CONFIG_EFI_RT_VOLATILE_STORE))

• rt_table->runtime_services_supported |=
  

• 	EFI_RT_SUPPORTED_SET_VARIABLE;
  

• /*
  • This value must be synced with efi_runtime_detach_list
  • as well as efi_runtime_services.

diff --git a/lib/efi_loader/efi_variable.c b/lib/efi_loader/efi_variable.c index 40f7a0fb10d5..3b770ff16971 100644 --- a/lib/efi_loader/efi_variable.c +++ b/lib/efi_loader/efi_variable.c @@ -218,17 +218,21 @@ efi_get_next_variable_name_int(efi_uintn_t *variable_name_size, return efi_get_next_variable_name_mem(variable_name_size, variable_name, vendor); }

-efi_status_t efi_set_variable_int(const u16 *variable_name,

• 		  const efi_guid_t *vendor,
  

• 		  u32 attributes, efi_uintn_t data_size,
  

• 		  const void *data, bool ro_check)
  

+/**

• • efi_check_setvariable() - checks defined by the UEFI spec for setvariable
• • @variable_name: name of the variable
• • @vendor: vendor GUID
• • @attributes: attributes of the variable
• • @data_size: size of the buffer with the variable value
• • @data: buffer with the variable value
• • Return: status code
• */

+static efi_status_t __efi_runtime EFIAPI +efi_check_setvariable(const u16 *variable_name, const efi_guid_t *vendor,

•       u32 attributes, efi_uintn_t data_size,
  

•       const void *data)
  

{

• struct efi_var_entry *var;
• efi_uintn_t ret;
• bool append, delete;
• u64 time = 0;
• enum efi_auth_var_type var_type;
• if (!variable_name || !*variable_name || !vendor) return EFI_INVALID_PARAMETER;

@@ -256,6 +260,25 @@ efi_status_t efi_set_variable_int(const u16 *variable_name, !(attributes & EFI_VARIABLE_BOOTSERVICE_ACCESS))) return EFI_INVALID_PARAMETER;

• return EFI_SUCCESS;

+}

+efi_status_t efi_set_variable_int(const u16 *variable_name,

• 		  const efi_guid_t *vendor,
  

• 		  u32 attributes, efi_uintn_t data_size,
  

• 		  const void *data, bool ro_check)
  

+{

• struct efi_var_entry *var;
• efi_uintn_t ret;
• bool append, delete;
• u64 time = 0;
• enum efi_auth_var_type var_type;
• ret = efi_check_setvariable(variable_name, vendor, attributes, data_size,

• 		    data);
  

• if (ret != EFI_SUCCESS)

• return ret;
  

• /* check if a variable exists */ var = efi_var_mem_find(vendor, variable_name, NULL); append = !!(attributes & EFI_VARIABLE_APPEND_WRITE);

@@ -434,7 +457,72 @@ efi_set_variable_runtime(u16 *variable_name, const efi_guid_t *vendor, u32 attributes, efi_uintn_t data_size, const void *data) { +#if IS_ENABLED(CONFIG_EFI_RT_VOLATILE_STORE)

• struct efi_var_entry *var;
• efi_uintn_t ret;
• bool append, delete;
• u64 time = 0;
• /* Authenticated variables are not supported */
• if (attributes & EFI_VARIABLE_TIME_BASED_AUTHENTICATED_WRITE_ACCESS)

• 	return EFI_INVALID_PARAMETER;
  

• ret = efi_check_setvariable(variable_name, vendor, attributes, data_size,

• 		    data);
  

• if (ret != EFI_SUCCESS)

• return ret;
  

• /* check if a variable exists */
• var = efi_var_mem_find(vendor, variable_name, NULL);
• append = !!(attributes & EFI_VARIABLE_APPEND_WRITE);
• attributes &= ~(u32)EFI_VARIABLE_APPEND_WRITE;
• delete = !append && (!data_size || !attributes);
• if (var) {

• if (var->attr & EFI_VARIABLE_READ_ONLY)
  

• 	return EFI_WRITE_PROTECTED;
  

• /* attributes won't be changed */
  

• if (!delete && (((var->attr & ~(u32)EFI_VARIABLE_READ_ONLY) !=
  

•     (attributes & ~(u32)EFI_VARIABLE_READ_ONLY))))
  

• 	return EFI_INVALID_PARAMETER;
  

• time = var->time;
  

• } else {

• if (delete || append)
  

• 	/*
  

• 	 * Trying to delete or to update a non-existent
  

• 	 * variable.
  

• 	 */
  

• 	return EFI_NOT_FOUND;
  

• }
• if (delete) {

• /* EFI_NOT_FOUND has been handled before */
  

• attributes = var->attr;
  

• ret = EFI_SUCCESS;
  

• } else if (append) {

• u16 *old_data = var->name;
  

• for (; *old_data; ++old_data)
  

• 	;
  

• ++old_data;
  

• ret = efi_var_mem_ins(variable_name, vendor, attributes,
  

• 		      var->length, old_data, data_size, data,
  

• 		      time);
  

• } else {

• ret = efi_var_mem_ins(variable_name, vendor, attributes,
  

• 		      data_size, data, 0, NULL, time);
  

• }
• if (ret != EFI_SUCCESS)

• return ret;
  

• /* We are always inserting new variables, get rid of the old copy */
• efi_var_mem_del(var);
• return EFI_SUCCESS;

+#else return EFI_UNSUPPORTED; +#endif }

/** diff --git a/lib/efi_selftest/efi_selftest_variables_runtime.c b/lib/efi_selftest/efi_selftest_variables_runtime.c index 4700d9424105..6001b44989c8 100644 --- a/lib/efi_selftest/efi_selftest_variables_runtime.c +++ b/lib/efi_selftest/efi_selftest_variables_runtime.c @@ -62,9 +62,16 @@ static int execute(void) EFI_VARIABLE_BOOTSERVICE_ACCESS | EFI_VARIABLE_RUNTIME_ACCESS, 3, v + 4);

• if (ret != EFI_UNSUPPORTED) {

• efi_st_error("SetVariable failed\n");
  

• return EFI_ST_FAILURE;
  

• if (IS_ENABLED(CONFIG_EFI_RT_VOLATILE_STORE)) {

• if (ret != EFI_SUCCESS) {
  

• 	efi_st_error("SetVariable failed\n");
  

• 	return EFI_ST_FAILURE;
  

• }
  

• } else {

• if (ret != EFI_UNSUPPORTED) {
  

• 	efi_st_error("SetVariable failed\n");
  

• 	return EFI_ST_FAILURE;
  

• }
  

  } len = EFI_ST_MAX_DATA_SIZE; ret = runtime->get_variable(u"PlatformLangCodes", &guid_vendor0,

-- 2.37.2