[PATCH v1 0/4] Enable SetvariableRT

[Mark Kettenis]

> From: Ilias Apalodimas <ilias.apalodimas at linaro.org>
> Date: Sat,  6 Apr 2024 17:01:51 +0300
> 
> Hi all,

Hi Ilias,

> This is an updated version of [0].
> 
> 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 back end. 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 [1]
> and authenticated variables are explicitly prohibited, since they have
> to be stored on a medium that's tamper and rollback protected.
> 
> The original RFC was just enabling the memory back end. This is a more
> complete version and we should be able, with small adjustments of
> userspace tools, fix SetVariableRT. If enabled the firmware will add two
> new RO EFI variables after ExitBootServices.
> 
> RTStorageVolatile -- contains the filename, relative to the ESP
> VarToFile -- an EFI variable that holds all the BS,RT, NV variables and can
> be copied to the file defined by RTStorageVolatile.

How does the OS know what the right ESP is?  The system might have
multiple disks that each have an ESP and more than one of those ESPs
might contain an ubootefi.var file.  Maybe this should be a full
EFI device path that includes the partition?

Also, is RTStorageVolutile the right name for this?  This is about
storing Non-Volatile variables after all...

> 
> If any errors occur during the variable init, the firmware will delete them
> and adjust the RT_PROP table accordingly, disabling SetvarRT.
> 
> - 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)
> , disabling SetvarRT.
> $~ 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://lore.kernel.org/u-boot/20240329071929.2461441-1-ilias.apalodimas@linaro.org/
> [1] https://arm-software.github.io/ebbr/index.html#document-chapter5-variable-storage
> 
> Changes since the RFC:
> - Return EFI_INVALID_PARAM if attributes are not volatile
> - Add EFI_WRITE_PROTECTED checks for BS, RT *only* variables
> - Add 2 EFI variables and allow userspace to write the file
> - Add selftests
> 
> Ilias Apalodimas (4):
>   efi_loader: conditionally enable SetvariableRT
>   efi_loader: Add OS notifications for SetVariableRT in RAM
>   efi_loader: add an EFI variable with the variable file contents
>   efi_selftest: add tests for setvariableRT
> 
>  include/efi_loader.h                          |   4 +
>  include/efi_variable.h                        |  15 +-
>  lib/efi_loader/Kconfig                        |  16 ++
>  lib/efi_loader/efi_boottime.c                 |   2 +
>  lib/efi_loader/efi_runtime.c                  |   1 +
>  lib/efi_loader/efi_var_common.c               |  43 ++--
>  lib/efi_loader/efi_var_file.c                 |   1 -
>  lib/efi_loader/efi_var_mem.c                  |  90 +++-----
>  lib/efi_loader/efi_variable.c                 | 210 +++++++++++++++++-
>  lib/efi_loader/efi_variable_tee.c             |   1 -
>  .../efi_selftest_variables_runtime.c          | 116 +++++++++-
>  11 files changed, 401 insertions(+), 98 deletions(-)
> 
> --
> 2.37.2
> 
>