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

[Tom Rini <trini@konsulko.com>]

On Wed, Oct 07, 2020 at 12:17:56AM +0200, Rasmus Villemoes wrote:
> On 07/10/2020 00.02, Simon Glass wrote:
> > Hi Rasmus,
> > 
> > On Tue, 6 Oct 2020 at 04:10, Rasmus Villemoes
> > <rasmus.villemoes@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@prevas.dk>
> >> ---
> >>  lib/rsa/rsa-mod-exp.c | 11 +++++++++--
> >>  1 file changed, 9 insertions(+), 2 deletions(-)
> >>
> > 
> > Reviewed-by: Simon Glass <sjg@chromium.org>
> > 
> > Is there a way to add a test for this?
> 
> Not that I can think of, other than finding some BE board and hooking it
> up in some CI. Apparently not very many people use verified boot on BE
> platforms :( or at least they don't follow upstream U-Boot closely.

We have tests for verified boot for sandbox.  Can we not expand them to
run on qemu* including ppce500?

-- 
Tom
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