Re: [PATCH v6 6/6] tpm/kexec: Duplicate TPM measurement log in of-tree for kexec

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From: Stefan Berger <stefanb@linux.ibm.com>
Sent: 07 July 2022 10:50 PM
To: kexec@lists.infradead.org; devicetree@vger.kernel.org; linux-integrity@vger.kernel.org; linux-kernel@vger.kernel.org; linuxppc-dev@lists.ozlabs.org
Cc: nayna@linux.ibm.com; Nageswara R Sastry; mpe@ellerman.id.au; Stefan Berger; Rob Herring; Frank Rowand; Eric Biederman
Subject: [PATCH v6 6/6] tpm/kexec: Duplicate TPM measurement log in of-tree for kexec

The memory area of the TPM measurement log is currently not properly
duplicated for carrying it across kexec when an Open Firmware
Devicetree is used. Therefore, the contents of the log get corrupted.
Fix this for the kexec_file_load() syscall by allocating a buffer and
copying the contents of the existing log into it. The new buffer is
preserved across the kexec and a pointer to it is available when the new
kernel is started. To achieve this, store the allocated buffer's address
in the flattened device tree (fdt) under the name linux,tpm-kexec-buffer
and search for this entry early in the kernel startup before the TPM
subsystem starts up. Adjust the pointer in the of-tree stored under
linux,sml-base to point to this buffer holding the preserved log. The TPM
driver can then read the base address from this entry when making the log
available. Invalidate the log by removing 'linux,sml-base' from the
devicetree if anything goes wrong with updating the buffer.

Use subsys_initcall() to call the function to restore the buffer even if
the TPM subsystem or driver are not used. This allows the buffer to be
carried across the next kexec without involvement of the TPM subsystem
and ensures a valid buffer pointed to by the of-tree.

Use the subsys_initcall(), rather than an ealier initcall, since
page_is_ram() in get_kexec_buffer() only starts working at this stage.

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Frank Rowand <redacted>
Cc: Eric Biederman <redacted>

---
v6:
- Define prototype for tpm_add_kexec_buffer under same config options
 as drivers/of/kexec.c is compiled, provide inline function otherwise.
  (kernel test robot)

Tested-by: Nageswara R Sastry <redacted>