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efi_loader: add an EFI variable with the file contents

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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>
This commit is contained in:
Ilias Apalodimas 2024-04-18 15:54:52 +03:00 • committed by Heinrich Schuchardt
parent bc3dd2493e
commit 00da8d65a3
7 changed files with 146 additions and 65 deletions
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25
lib/efi_loader/efi_runtime.c
View file

@ -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;
}