Re: [PATCH 1/2] efi+tpm: Don't access event->count when it isn't mapped.

[Ard Biesheuvel]

On Mon, 26 Aug 2019 at 18:30, Peter Jones <pjones@xxxxxxxxxx> wrote:
>
> Some machines generate a lot of event log entries.  When we're
> iterating over them, the code removes the old mapping and adds a
> new one, so once we cross the page boundary we're unmapping the page
> with the count on it.  Hilarity ensues.
>
> This patch keeps the info from the header in local variables so we don't
> need to access that page again or keep track of if it's mapped.
>
> Signed-off-by: Peter Jones <pjones@xxxxxxxxxx>
> Tested-by: Lyude Paul <lyude@xxxxxxxxxx>
> ---
>  include/linux/tpm_eventlog.h | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/tpm_eventlog.h b/include/linux/tpm_eventlog.h
> index 63238c84dc0..549dab0b56b 100644
> --- a/include/linux/tpm_eventlog.h
> +++ b/include/linux/tpm_eventlog.h
> @@ -170,6 +170,7 @@ static inline int __calc_tpm2_event_size(struct tcg_pcr_event2_head *event,
>         u16 halg;
>         int i;
>         int j;
> +       u32 count, event_type;
>
>         marker = event;
>         marker_start = marker;
> @@ -190,16 +191,22 @@ static inline int __calc_tpm2_event_size(struct tcg_pcr_event2_head *event,
>         }
>
>         event = (struct tcg_pcr_event2_head *)mapping;
> +       /*
> +        * the loop below will unmap these fields if the log is larger than
> +        * one page, so save them here for reference.
> +        */
> +       count = event->count;
> +       event_type = event->event_type;
>

These assignments should be using READ_ONCE(), since otherwise, the
compiler may reload these quantities from memory anyway.

With that fixed,

Acked-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>

>         efispecid = (struct tcg_efi_specid_event_head *)event_header->event;
>
>         /* Check if event is malformed. */
> -       if (event->count > efispecid->num_algs) {
> +       if (count > efispecid->num_algs) {
>                 size = 0;
>                 goto out;
>         }
>
> -       for (i = 0; i < event->count; i++) {
> +       for (i = 0; i < count; i++) {
>                 halg_size = sizeof(event->digests[i].alg_id);
>
>                 /* Map the digest's algorithm identifier */
> @@ -256,8 +263,9 @@ static inline int __calc_tpm2_event_size(struct tcg_pcr_event2_head *event,
>                 + event_field->event_size;
>         size = marker - marker_start;
>
> -       if ((event->event_type == 0) && (event_field->event_size == 0))
> +       if (event_type == 0 && event_field->event_size == 0)
>                 size = 0;
> +
>  out:
>         if (do_mapping)
>                 TPM_MEMUNMAP(mapping, mapping_size);
> --
> 2.23.0.rc2
>