Re: [PATCH] tpm: check event log version before reading final events

From: Javier Martinez Canillas <javierm@redhat.com>
To: Ard Biesheuvel <ardb@kernel.org>
Cc: "Loïc Yhuel" <loic.yhuel@gmail.com>,
	"Jarkko Sakkinen" <jarkko.sakkinen@linux.intel.com>,
	"Matthew Garrett" <mjg59@google.com>,
	linux-integrity <linux-integrity@vger.kernel.org>
Subject: Re: [PATCH] tpm: check event log version before reading final events
Date: Thu, 14 May 2020 15:51:42 +0200	[thread overview]
Message-ID: <9ebd9e60-12dd-a6df-923c-dc9c6dbd709b@redhat.com> (raw)
In-Reply-To: <CAMj1kXFs6Z8VwqRMHf_OOZct4xNnNqiVOGBO0TtLfirg7WX8Eg@mail.gmail.com>

On 5/14/20 3:04 PM, Ard Biesheuvel wrote:
> On Thu, 14 May 2020 at 14:56, Javier Martinez Canillas

[snip]

>>
>> I still don't know if something like that would be acceptable or if we
>> should just consider a bug if a firmware doesn't conform with the spec.
>>
> 
> I'd like Matt's and Jarkko's take on this - for me, considering it a
> bug is just fine. I just want to understand what exactly to warn
> about, since we currently silently ignore the lack of a final events
> table, while it apparently defeats the purpose of having a log in the
> first.
>
> So given that the ordinary event log will contain everything *except*
> the events that were logged during EBS(), I agree that the log may
> still be useful if those final events only affected PCRs that you
> don't care about.
>

Agreed.

> Something like the below perhaps? Should we suppress the warning for
> tables of size 0x0?
>

That's what is not clear to me. For example, if a firmware follows the
spec what happens during EBS() when a TPM2 only supports the SHA-1 format?

Does the firmware always log events during EBS() even when it won't provide
a Final Events Table? If so, are the SHA-1 logs always incomplete?

What I wonder is if the firmware bug error should always be printed if the
crypto agile log format isn't used or only if isn't used and the firmware
provides a Final Events Table, which is a behavior that's out of the spec.

> diff --git a/drivers/char/tpm/eventlog/efi.c b/drivers/char/tpm/eventlog/efi.c
> index 6bb023de17f1..65bfee8e636d 100644
> --- a/drivers/char/tpm/eventlog/efi.c
> +++ b/drivers/char/tpm/eventlog/efi.c
> @@ -62,8 +62,10 @@ int tpm_read_log_efi(struct tpm_chip *chip)
> 
>         if (efi.tpm_final_log == EFI_INVALID_TABLE_ADDR ||
>             efi_tpm_final_log_size == 0 ||
> -           tpm_log_version != EFI_TCG2_EVENT_LOG_FORMAT_TCG_2)
> +           tpm_log_version != EFI_TCG2_EVENT_LOG_FORMAT_TCG_2) {
> +               pr_warn(FW_BUG "TPM Final Event table not found - some
> events may be missing\n");

Maybe mentioning that the missing logs are for events that happen during EBS()?

>                 goto out;
> +       }
> 
>         final_tbl = memremap(efi.tpm_final_log,
>                              sizeof(*final_tbl) + efi_tpm_final_log_size,
> 

Best regards,
-- 
Javier Martinez Canillas
Software Engineer - Desktop Hardware Enablement
Red Hat