[PATCH v7 1/4] Add fdt_kaslrseed function to add kaslr-seed to chosen node

[Caleb Connolly]

Hi Tim,

On 18/06/2024 23:06, 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.
> 
> Add a fdt_kaslrseed function to accommodate this allowing an existing
> node to be overwritten if present. For now use the first rng device
> but it would be good to enhance this in the future to allow some sort
> of selection or policy in choosing the rng device used.
> 
> Signed-off-by: Tim Harvey <tharvey at gateworks.com>
> Reviewed-by: Simon Glass <sjg at chromium.org>
> 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>
> ---
> v6:
>   - collected tags
> v5:
>   - move function to boot/fdt_support.c
>   - remove ability to select rng index and note in the commit log
>     something like this as a future enhancement.
>   - fixed typo in commit message s/it's/its/
>   - use cmd_process_error per Michal's suggestion
> v4:
>   - add missing /n to notice in kaslrseed cmd
>   - combine ints in declaration
>   - remove unused vars from board/xilinx/common/board.c ft_board_setup
> 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
> ---
>   boot/fdt_support.c    | 44 +++++++++++++++++++++++++++++++++++++++++++
>   include/fdt_support.h | 10 ++++++++++
>   2 files changed, 54 insertions(+)
> 
> diff --git a/boot/fdt_support.c b/boot/fdt_support.c
> index 2bd80a9dfb18..b1b2679dea0c 100644
> --- a/boot/fdt_support.c
> +++ b/boot/fdt_support.c
> @@ -7,12 +7,15 @@
>    */
>   
>   #include <common.h>
> +#include <dm.h>
>   #include <abuf.h>
>   #include <env.h>
>   #include <log.h>
>   #include <mapmem.h>
>   #include <net.h>
> +#include <rng.h>
>   #include <stdio_dev.h>
> +#include <dm/device_compat.h>
>   #include <dm/ofnode.h>
>   #include <linux/ctype.h>
>   #include <linux/types.h>
> @@ -274,6 +277,47 @@ int fdt_initrd(void *fdt, ulong initrd_start, ulong initrd_end)
>   	return 0;
>   }
>   
> +int fdt_kaslrseed(void *fdt, bool overwrite)
> +{
> +	int len, err, nodeoffset;
> +	struct udevice *dev;
> +	const u64 *orig;
> +	u64 data = 0;
> +
> +	err = fdt_check_header(fdt);
> +	if (err < 0)
> +		return err;

All the paths that call fdt_kaslrseed() call fdt_check_header() 
beforehand, I think it's safe to drop this.
> +
> +	/* find or create "/chosen" node. */
> +	nodeoffset = fdt_find_or_add_subnode(fdt, 0, "chosen");
> +	if (nodeoffset < 0)
> +		return nodeoffset;

Arguably, this shouldn't be the responsibility of this function, maybe 
it would be better to error our here?
> +
> +	/* return without error if we are not overwriting and existing non-zero node */
> +	orig = fdt_getprop(fdt, nodeoffset, "kaslr-seed", &len);
> +	if (orig && len == sizeof(*orig))
> +		data = fdt64_to_cpu(*orig);
> +	if (data && !overwrite) {
> +		debug("not overwriting existing kaslr-seed\n");
> +		return 0;
> +	}
> +	err = uclass_get_device(UCLASS_RNG, 0, &dev);
> +	if (err) {
> +		printf("No RNG device\n");
> +		return err;
> +	}
> +	err = dm_rng_read(dev, &data, sizeof(data));
> +	if (err) {
> +		dev_err(dev, "dm_rng_read failed: %d\n", err);
> +		return err;
> +	}
> +	err = fdt_setprop(fdt, nodeoffset, "kaslr-seed", &data, sizeof(data));
> +	if (err < 0)
> +		printf("WARNING: could not set kaslr-seed %s.\n", fdt_strerror(err));
> +
> +	return err;
> +}
> +
>   /**
>    * board_fdt_chosen_bootargs - boards may override this function to use
>    *                             alternative kernel command line arguments
> diff --git a/include/fdt_support.h b/include/fdt_support.h
> index 4b71b8948d99..741e2360c224 100644
> --- a/include/fdt_support.h
> +++ b/include/fdt_support.h
> @@ -463,4 +463,14 @@ void fdt_fixup_board_enet(void *blob);
>   #ifdef CONFIG_CMD_PSTORE
>   void fdt_fixup_pstore(void *blob);
>   #endif
> +
> +/**
> + * fdt_kaslrseed() - create a 'kaslr-seed' node in chosen
> + *
> + * @blob:	fdt blob
> + * @overwrite:	do not overwrite existing non-zero node unless true
> + * Return:	0 if OK, -ve on error
> + */
> +int fdt_kaslrseed(void *blob, bool overwrite);
> +
>   #endif /* ifndef __FDT_SUPPORT_H */

-- 
// Caleb (they/them)