Re: [PATCH] tpm: verify that it's actually an event log header before parsing

[Ard Biesheuvel <ardb@kernel.org>]

On Fri, 5 Jun 2020 at 16:22, Fabian Vogt <fvogt@suse.de> wrote:
>
> It's possible that the first event in the log is not actually a log
> header at all, but rather a normal event. This leads to the cast in
> __calc_tpm2_event_size being an invalid conversion, which means that
> the values read are effectively garbage. Depending on the first event's
> contents, this leads either to apparently normal behaviour, a crash or
> a freeze.
>
> While this behaviour of the firmware is not in accordance with the
> TCG Client EFI Specification, this happens on a Dell Precision 5510
> with the TPM enabled but hidden from the OS ("TPM On" disabled, state
> otherwise untouched). The EFI claims that the TPM is present and active
>  and supports the TCG 2.0 event log format.
>
> Fortunately, this can be worked around by simply checking the header
> of the first event and the event log header signature itself.
>
> Commit b4f1874c6216 ("tpm: check event log version before reading final
> events") addressed a similar issue also found on Dell models.
>
> Fixes: 6b0326190205 ("efi: Attempt to get the TCG2 event log in the boot stub")
> Bugzilla: https://bugzilla.suse.com/show_bug.cgi?id=1165773
> Signed-off-by: Fabian Vogt <fvogt@suse.de>
> ---
>  include/linux/tpm_eventlog.h | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/tpm_eventlog.h b/include/linux/tpm_eventlog.h
> index 4f8c90c93c29..b913faeedcb5 100644
> --- a/include/linux/tpm_eventlog.h
> +++ b/include/linux/tpm_eventlog.h
> @@ -81,6 +81,8 @@ struct tcg_efi_specid_event_algs {
>         u16 digest_size;
>  } __packed;
>
> +#define TCG_SPECID_SIG "Spec ID Event03"
> +
>  struct tcg_efi_specid_event_head {
>         u8 signature[16];
>         u32 platform_class;
> @@ -171,6 +173,7 @@ static inline int __calc_tpm2_event_size(struct tcg_pcr_event2_head *event,
>         int i;
>         int j;
>         u32 count, event_type;
> +       const u8 zero_digest[sizeof(event_header->digest)] = {0};
>
>         marker = event;
>         marker_start = marker;
> @@ -198,10 +201,19 @@ static inline int __calc_tpm2_event_size(struct tcg_pcr_event2_head *event,
>         count = READ_ONCE(event->count);
>         event_type = READ_ONCE(event->event_type);
>
> +       /* Verify that it's the log header */
> +       if (READ_ONCE(event_header->pcr_idx) != 0 ||
> +           READ_ONCE(event_header->event_type) != NO_ACTION ||

Is the READ_ONCE() necessary here?

> +           memcmp(event_header->digest, zero_digest, sizeof(zero_digest))) {
> +               size = 0;
> +               goto out;
> +       }
> +
>         efispecid = (struct tcg_efi_specid_event_head *)event_header->event;
>
>         /* Check if event is malformed. */
> -       if (count > efispecid->num_algs) {
> +       if (memcmp(efispecid->signature, TCG_SPECID_SIG,
> +                  sizeof(TCG_SPECID_SIG)) || count > efispecid->num_algs) {

So is it guaranteed that every well formed event starts with TCG_SPECID_SIG?


>                 size = 0;
>                 goto out;
>         }
> --
> 2.25.1
>
>
>
>