Subject: Re: [RFC][PATCH 0/2] Add support for using reserved memory for ima buffer pass

From: Rob Herring <[email-masked]>
Date: Tue, 12 May 2020 18:05:09 -0500

On Wed, May 06, 2020 at 10:50:04PM -0700, Prakhar Srivastava wrote:
> Hi Mark,

Please don't top post.

> This patch set currently only address the Pure DT implementation.
> EFI and ACPI implementations will be posted in subsequent patchsets.
> 
> The logs are intended to be carried over the kexec and once read the
> logs are no longer needed and in prior conversation with James(
> https://lore.kernel.org/linux-arm-kernel/[email-masked]/)
> the apporach of using a chosen node doesn't
> support the case.
> 
> The DT entries make the reservation permanent and thus doesnt need kernel
> segments to be used for this, however using a chosen-node with
> reserved memory only changes the node information but memory still is
> reserved via reserved-memory section.

I think Mark's point was whether it needs to be permanent. We don't 
hardcode the initrd address for example.

> On 5/5/20 2:59 AM, Mark Rutland wrote:
> > Hi Prakhar,
> > 
> > On Mon, May 04, 2020 at 01:38:27PM -0700, Prakhar Srivastava wrote:
> > > IMA during kexec(kexec file load) verifies the kernel signature and measures

What's IMA?

> > > the signature of the kernel. The signature in the logs can be used to verfiy the
> > > authenticity of the kernel. The logs don not get carried over kexec and thus
> > > remote attesation cannot verify the signature of the running kernel.
> > > 
> > > Introduce an ABI to carry forward the ima logs over kexec.
> > > Memory reserved via device tree reservation can be used to store and read
> > > via the of_* functions.
> > 
> > This flow needs to work for:
> > 
> > 1) Pure DT
> > 2) DT + EFI memory map
> > 3) ACPI + EFI memory map
> > 
> > ... and if this is just for transiently passing the log, I don't think
> > that a reserved memory region is the right thing to use, since they're
> > supposed to be more permanent.
> > 
> > This sounds analogous to passing the initrd, and should probably use
> > properties under the chosen node (which can be used for all three boot
> > flows above).
> > 
> > For reference, how big is the IMA log likely to be? Does it need
> > physically contiguous space?
> 
> It purely depends on the policy used and the modules/files that are accessed
> for my local testing over a kexec session the log in
> about 30KB.
> 
> Current implementation expects enough contiguous memory to allocated to
> carry forward the logs. If the log size exceeds the reserved memory the
> call will fail.
> 
> Thanks,
> Prakhar Srivastava
> > 
> > Thanks,
> > Mark.
> > 
> > > 
> > > Reserved memory stores the size(sizeof(size_t)) of the buffer in the starting
> > > address, followed by the IMA log contents.
> > > 
> > > Tested on:
> > >    arm64 with Uboot
> > > 
> > > Prakhar Srivastava (2):
> > >    Add a layer of abstraction to use the memory reserved by device tree
> > >      for ima buffer pass.
> > >    Add support for ima buffer pass using reserved memory for arm64 kexec.
> > >      Update the arch sepcific code path in kexec file load to store the
> > >      ima buffer in the reserved memory. The same reserved memory is read
> > >      on kexec or cold boot.
> > > 
> > >   arch/arm64/Kconfig                     |   1 +
> > >   arch/arm64/include/asm/ima.h           |  22 ++++
> > >   arch/arm64/include/asm/kexec.h         |   5 +
> > >   arch/arm64/kernel/Makefile             |   1 +
> > >   arch/arm64/kernel/ima_kexec.c          |  64 ++++++++++
> > >   arch/arm64/kernel/machine_kexec_file.c |   1 +
> > >   arch/powerpc/include/asm/ima.h         |   3 +-
> > >   arch/powerpc/kexec/ima.c               |  14 ++-
> > >   drivers/of/Kconfig                     |   6 +
> > >   drivers/of/Makefile                    |   1 +
> > >   drivers/of/of_ima.c                    | 165 +++++++++++++++++++++++++
> > >   include/linux/of.h                     |  34 +++++
> > >   security/integrity/ima/ima_kexec.c     |  15 ++-
> > >   13 files changed, 325 insertions(+), 7 deletions(-)
> > >   create mode 100644 arch/arm64/include/asm/ima.h
> > >   create mode 100644 arch/arm64/kernel/ima_kexec.c
> > >   create mode 100644 drivers/of/of_ima.c
> > > 
> > > -- 
> > > 2.25.1
> > >