[PATCH v3] fdt: automatically add /chosen/kaslr-seed if DM_RNG is enabled

[Michal Simek]

On 5/21/24 22:59, Tim Harvey wrote:
> If RANDOMIZE_BASE is enabled in the Linux kernel instructing it to
> randomize the virtual address at which the kernel image is loaded, it
> expects entropy to be provided by the bootloader by populating
> /chosen/kaslr-seed with a 64-bit value from source of entropy at boot.
> 
> If we have DM_RNG enabled populate this value automatically when
> fdt_chosen is called. We skip this if ARMV8_SEC_FIRMWARE_SUPPORT
> is enabled as it's implementation uses a different source of entropy
> that is not yet implemented as DM_RNG. We also skip this if
> MEASURED_BOOT is enabled as in that case any modifications to the
> dt will cause measured boot to fail (although there are many other
> places the dt is altered).
> 
> As this fdt node is added elsewhere create a library function and
> use it to deduplicate code. We will provide a parameter to specify the
> index of the rng device as well as a boolean to overwrite if present.
> 
> For our automatic injection, we will use the first rng device and
> not overwrite if already present with a non-zero value (which may
> have been populated by an earlier boot stage). This way if a board
> specific ft_board_setup() function wants to customize this behavior
> it can call fdt_kaslrseed with a rng device index of its choosing and
> set overwrite true.
> 
> Note that the kalsrseed command (CMD_KASLRSEED) is likely pointless now
> but left in place in case boot scripts exist that rely on this command
> existing and returning success. An informational message is printed to
> alert users of this command that it is likely no longer needed.
> 
> Note that the Kernel's EFI STUB only relies on EFI_RNG_PROTOCOL for
> randomization and completely ignores the kaslr-seed for its own
> randomness needs (i.e the randomization of the physical placement of
> the kernel). It gets weeded out from the DTB that gets handed over via
> efi_install_fdt() as it would also mess up the measured boot DTB TPM
> measurements as well.
> 
> Signed-off-by: Tim Harvey <tharvey at gateworks.com>
> Cc: Michal Simek <michal.simek at amd.com>
> Cc: Andy Yan <andy.yan at rock-chips.com>
> Cc: Akash Gajjar <gajjar04akash at gmail.com>
> Cc: Ilias Apalodimas <ilias.apalodimas at linaro.org>
> Cc: Simon Glass <sjg at chromium.org>
> Cc: Patrick Delaunay <patrick.delaunay at foss.st.com>
> Cc: Patrice Chotard <patrice.chotard at foss.st.com>
> Cc: Devarsh Thakkar <devarsht at ti.com>
> Cc: Heinrich Schuchardt <xypron.glpk at gmx.de>
> Cc: Hugo Villeneuve <hvilleneuve at dimonoff.com>
> Cc: Marek Vasut <marex at denx.de>
> Cc: Tom Rini <trini at konsulko.com>
> Cc: Chris Morgan <macromorgan at hotmail.com>
> ---
> v3:
>   - skip if CONFIG_MEASURED_BOOT
>   - fix skip for CONFIG_ARMV8_SEC_FIRMWARE_SUPPORT
>   - pass in rng index and bool to specify overwrite
>   - remove duplicate error strings printed outside of fdt_kaslrseed
>   - added note to commit log about how EFI STUB weeds out kalsr-seed
> 
> v2:
>   - fix typo in commit msg
>   - use stack for seed to avoid unecessary malloc/free
>   - move to a library function and deduplicate code by using it
>     elsewhere
> ---
>   board/xilinx/common/board.c | 35 -------------------------
>   boot/fdt_support.c          |  6 +++++
>   boot/pxe_utils.c            | 35 ++-----------------------
>   cmd/kaslrseed.c             | 45 +++++---------------------------
>   include/kaslrseed.h         | 19 ++++++++++++++
>   lib/Makefile                |  1 +
>   lib/kaslrseed.c             | 51 +++++++++++++++++++++++++++++++++++++
>   7 files changed, 85 insertions(+), 107 deletions(-)
>   create mode 100644 include/kaslrseed.h
>   create mode 100644 lib/kaslrseed.c
> 
> diff --git a/board/xilinx/common/board.c b/board/xilinx/common/board.c
> index 30a81376ac41..f741e8957818 100644
> --- a/board/xilinx/common/board.c
> +++ b/board/xilinx/common/board.c
> @@ -713,41 +713,6 @@ int ft_board_setup(void *blob, struct bd_info *bd)
>   	if (IS_ENABLED(CONFIG_FDT_FIXUP_PARTITIONS) && IS_ENABLED(CONFIG_NAND_ZYNQ))
>   		fdt_fixup_mtdparts(blob, nodes, ARRAY_SIZE(nodes));
>   

one more thing here. Please also removed unused variables.

board/xilinx/common/board.c: In function 'ft_board_setup':
board/xilinx/common/board.c:707:25: warning: unused variable 'ret' 
[-Wunused-variable]
   707 |         int nodeoffset, ret;
       |                         ^~~
board/xilinx/common/board.c:707:13: warning: unused variable 'nodeoffset' 
[-Wunused-variable]
   707 |         int nodeoffset, ret;
       |             ^~~~~~~~~~
   AS      arch/arm/cpu/armv8/cache.o
board/xilinx/common/board.c:706:12: warning: unused variable 'buf' 
[-Wunused-variable]
   706 |         u8 buf[MAX_RAND_SIZE];
       |            ^~~
board/xilinx/common/board.c:705:25: warning: unused variable 'dev' 
[-Wunused-variable]
   705 |         struct udevice *dev;
       |                         ^~~
board/xilinx/common/board.c:704:16: warning: unused variable 'n' [-Wunused-variable]
   704 |         size_t n = MAX_RAND_SIZE;
       |                ^


Thanks,
Michal