Re: [PATCH 1/2] powerpc/tpm: Create linux,sml-base/size as big endian

[Jarkko Sakkinen <jarkko@kernel.org>]

On Tue, Feb 28, 2023 at 10:21:36PM +1100, Michael Ellerman wrote:
> Jarkko Sakkinen <jarkko@kernel.org> writes:
> > On Mon, Feb 27, 2023 at 06:08:31PM -0500, Stefan Berger wrote:
> >> On 2/23/23 22:25, Michael Ellerman wrote:
> >> > There's code in prom_instantiate_sml() to do a "SML handover" (Stored
> >> > Measurement Log) from OF to Linux, before Linux shuts down Open
> >> > Firmware.
> >> > 
> >> > This involves creating a buffer to hold the SML, and creating two device
> >> > tree properties to record its base address and size. The kernel then
> >> > later reads those properties from the device tree to find the SML.
> >> > 
> >> > When the code was initially added in commit 4a727429abec ("PPC64: Add
> >> > support for instantiating SML from Open Firmware") the powerpc kernel
> >> > was always built big endian, so the properties were created big endian
> >> > by default.
> >> > 
> >> > However since then little endian support was added to powerpc, and now
> >> > the code lacks conversions to big endian when creating the properties.
> >> > 
> >> > This means on little endian kernels the device tree properties are
> >> > little endian, which is contrary to the device tree spec, and in
> >> > contrast to all other device tree properties.
> >> > 
> >> > To cope with that a workaround was added in tpm_read_log_of() to skip
> >> > the endian conversion if the properties were created via the SML
> >> > handover.
> >> > 
> >> > A better solution is to encode the properties as big endian as they
> >> > should be, and remove the workaround.
> >> > 
> >> > Typically changing the encoding of a property like this would present
> >> > problems for kexec. However the SML is not propagated across kexec, so
> >> > changing the encoding of the properties is a non-issue.
> >> > 
> >> > Fixes: e46e22f12b19 ("tpm: enhance read_log_of() to support Physical TPM event log")
> >> > Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
> >> 
> >> Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
> >
> > 2/2 does not have a fixes tag.
> 
> True. Arguably the bug goes back to the introduction of
> kexec_file_load() support, although the patch won't backport that far
> due to code refactoring. So that would be:
> 
> Fixes: a0458284f062 ("powerpc: Add support code for kexec_file_load()")

Hmm... IMHO, it would still make sense to document this.

BR, Jarkko