[PATCH 1/1] rsa: fix retrieving public exponent on big-endian systems

[Simon Glass]

Hi Rasmus,

On Tue, 6 Oct 2020 at 04:10, Rasmus Villemoes
<rasmus.villemoes at prevas.dk> wrote:
>
> Commit fdf0819afb (rsa: fix alignment issue when getting public
> exponent) changed the logic to avoid doing an 8-byte access to a
> possibly-not-8-byte-aligned address.
>
> However, using rsa_convert_big_endian is wrong: That function converts
> an array of big-endian (32-bit) words with the most significant word
> first (aka a BE byte array) to an array of cpu-endian words with the
> least significant word first. While the exponent is indeed _stored_ as
> a big-endian 64-bit word (two BE words with MSW first), we want to
> extract it as a cpu-endian 64 bit word. On a little-endian host,
> swapping the words and byte-swapping each 32-bit word works, because
> that's the same as byte-swapping the whole 64 bit word. But on a
> big-endian host, the fdt32_to_cpu are no-ops, but
> rsa_convert_big_endian() still does the word-swapping, breaking
> verified boot.
>
> To fix that, while still ensuring we don't do unaligned accesses, add
> a little helper that first memcpy's the bytes to a local fdt64_t, then
> applies fdt64_to_cpu(). [The name is chosen based on the
> [bl]eXX_to_cpup in linux/byteorder/generic.h].
>
> Fixes: fdf0819afb ("rsa: fix alignment issue when getting public exponent")
> Signed-off-by: Rasmus Villemoes <rasmus.villemoes at prevas.dk>
> ---
>  lib/rsa/rsa-mod-exp.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
>

Reviewed-by: Simon Glass <sjg at chromium.org>

Is there a way to add a test for this?

Regards,
Simon