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

[Tim Harvey]

On Wed, May 22, 2024 at 12:47 AM Michal Simek <michal.simek at amd.com> wrote:
>
>
>
> 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;
>        |                ^
>

Hi Michal,

Thanks... missed those. What defconfig are you building that gets to
this codepath?

I'll send a v4 after another day or so for others to respond.

With these changes (and the previous response) do you have a rb or
tested-by tag for me?

Best Regards,

Tim