[Skiboot] [PATCH v2 1/6] secvar/secboot_tpm: correctly reset the control index on secboot format

[Daniel Axtens]

Eric Richter <erichte at linux.ibm.com> writes:

> When the SECBOOT partition is formatted, the bank hash stored in the
> control TPM NV index must be updated to match, or else we will immediately
> fail to load the freshly formatted data at the .load_bank() step.
>
> However, while the secboot_format() function does calculate and update the
> bank hash, it only writes the new hash for bank 0. It does not update the
> value for bank 1, or set the current active bank. This works as expected if
> the active bank bit happens to be set to 0. On the other hand, if the active
> bit is set to 1, the freshly formatted bank 1 will be compared against the
> unchanged bank hash in bank 1 at the load step, therefore causing an error.
>
> This patch fixes this issue by also setting the active bit to 0 to match
> the freshly calculated hash.
>
> Signed-off-by: Eric Richter <erichte at linux.ibm.com>
> Tested-by: Nick Child <nick.child at ibm.com>
> ---
>  libstb/secvar/storage/secboot_tpm.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
>
> diff --git a/libstb/secvar/storage/secboot_tpm.c b/libstb/secvar/storage/secboot_tpm.c
> index 129f674a..5907ff07 100644
> --- a/libstb/secvar/storage/secboot_tpm.c
> +++ b/libstb/secvar/storage/secboot_tpm.c
> @@ -127,12 +127,15 @@ static int secboot_format(void)
>  		prlog(PR_ERR, "Bank hash failed to calculate somehow\n");
>  		return rc;
>  	}
> +	/* Clear bank_hash[1] anyway, to match state of PNOR */
> +	memset(tpmnv_control_image->bank_hash[1], 0x00, sizeof(tpmnv_control_image->bank_hash[1]));

I am a bit confused by this line. The next thing you do is to set the
active bit to 0, which I understand and which matches what the commit
message says, but I don't understand why setting the bank hash of the
first bank to 0 is the right thing to do. The sha256 of a run of zeros
isn't going to be 0, so this is still - afaict - wrong, just differently
wrong?

> +
> +	tpmnv_control_image->active_bit = 0;

This bit I get.

>  
>  	rc = tpmnv_ops.write(SECBOOT_TPMNV_CONTROL_INDEX,
> -			     tpmnv_control_image->bank_hash[0],
> -			     SHA256_DIGEST_SIZE,
> -			     offsetof(struct tpmnv_control,
> -			     bank_hash[0]));
> +			     tpmnv_control_image,
> +			     sizeof(struct tpmnv_control),
> +			     0);

I understand that this writes out both bank hashes and the active
bit. It'll also write out the header again but hard to see that being an
issue. This looks good to me.

Kind regards,
Daniel