efi_loader: add an EFI variable with the file contents

Previous patches enabled SetVariableRT using a RAM backend.
Although EBBR [0] defines a variable format we can teach userspace tools
and write the altered variables, it's better if we skip the ABI
requirements completely.

So let's add a new variable, in its own namespace called "VarToFile"
which contains a binary dump of the updated RT, BS and, NV variables
and will be updated when GetVariable is called.

Some adjustments are needed to do that.
Currently we discard BS-only variables in EBS(). We need to preserve
those on the RAM backend that exposes the variables. Since BS-only
variables can't appear at runtime we need to move the memory masking
checks from efi_var_collect() to efi_get_next_variable_name_mem()/
efi_get_variable_mem() and do the filtering at runtime.

We also need an efi_var_collect() variant available at runtime, in order
to construct the "VarToFile" buffer on the fly.

All users and applications (for linux) have to do when updating a variable
is dd that variable in the file described by "RTStorageVolatile".

Linux efivarfs uses a first 4 bytes of the output to represent attributes
in little-endian format. So, storing variables works like this:

$~ efibootmgr -n 0001
$~ dd if=/sys/firmware/efi/efivars/VarToFile-b2ac5fc9-92b7-4acd-aeac-11e818c3130c of=/boot/efi/ubootefi.var skip=4 bs=1

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

Suggested-by: Ard Biesheuvel <ardb@kernel.org> # dumping all variables to a variable
Co-developed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com> # contributed on efi_var_collect_mem()
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>

1 files changed, 25 insertions, 0 deletions

diff --git a/lib/efi_loader/efi_runtime.c b/lib/efi_loader/efi_runtime.c
index c8f7a88ba8d..73831c527e0 100644
--- a/lib/efi_loader/efi_runtime.c
+++ b/lib/efi_loader/efi_runtime.c
@@ -130,6 +130,8 @@ efi_status_t efi_init_runtime_supported(void)
 				EFI_RT_SUPPORTED_CONVERT_POINTER;
 
 	if (IS_ENABLED(CONFIG_EFI_RT_VOLATILE_STORE)) {
+		u8 s = 0;
+
 		ret = efi_set_variable_int(u"RTStorageVolatile",
 					   &efi_guid_efi_rt_var_file,
 					   EFI_VARIABLE_BOOTSERVICE_ACCESS |
@@ -141,6 +143,29 @@ efi_status_t efi_init_runtime_supported(void)
 			log_err("Failed to set RTStorageVolatile\n");
 			return ret;
 		}
+		/*
+		 * This variable needs to be visible so users can read it,
+		 * but the real contents are going to be filled during
+		 * GetVariable
+		 */
+		ret = efi_set_variable_int(u"VarToFile",
+					   &efi_guid_efi_rt_var_file,
+					   EFI_VARIABLE_BOOTSERVICE_ACCESS |
+					   EFI_VARIABLE_RUNTIME_ACCESS |
+					   EFI_VARIABLE_READ_ONLY,
+					   sizeof(s),
+					   &s, false);
+		if (ret != EFI_SUCCESS) {
+			log_err("Failed to set VarToFile\n");
+			efi_set_variable_int(u"RTStorageVolatile",
+					     &efi_guid_efi_rt_var_file,
+					     EFI_VARIABLE_BOOTSERVICE_ACCESS |
+					     EFI_VARIABLE_RUNTIME_ACCESS |
+					     EFI_VARIABLE_READ_ONLY,
+					     0, NULL, false);
+
+			return ret;
+		}
 		rt_table->runtime_services_supported |= EFI_RT_SUPPORTED_SET_VARIABLE;
 	}