[PATCH] efi: tpm: Avoid READ_ONCE() for accessing the event log

[Will Deacon]

On Mon, Jan 09, 2023 at 10:59:48AM +0100, Ard Biesheuvel wrote:
> Nathan reports that recent kernels built with LTO will crash when doing
> EFI boot using Fedora's GRUB and SHIM. The culprit turns out to be a
> misaligned load from the TPM event log, which is annotated with
> READ_ONCE(), and under LTO, this gets translated into a LDAR instruction
> which does not tolerate misaligned accesses.

Interesting, that's a funny change in behaviour. READ_ONCE() of an unaligned
address is pretty sketchy, but if this ends up tripping lots of folks up
then I suppose we could use a plain load and a DMB LD as an alternative.
It's likely to be more expensive in the LDAPR case, though.

> Interestingly, this does not happen when booting the same kernel
> straight from the UEFI shell, and so the fact that the event log may
> appear misaligned in memory may be caused by a bug in GRUB or SHIM.
> 
> However, using READ_ONCE() to access firmware tables is slightly unusual
> in any case, and here, we only need to ensure that 'event' is not
> dereferenced again after it gets unmapped, so a compiler barrier should
> be sufficient, and works around the reported issue.
> 
> Cc: <stable at vger.kernel.org>
> Cc: Peter Jones <pjones at redhat.com>
> Cc: Jarkko Sakkinen <jarkko at kernel.org>
> Cc: Matthew Garrett <mjg59 at srcf.ucam.org>
> Reported-by: Nathan Chancellor <nathan at kernel.org>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1782
> Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
> ---
>  include/linux/tpm_eventlog.h | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/tpm_eventlog.h b/include/linux/tpm_eventlog.h
> index 20c0ff54b7a0d313..0abcc85904cba874 100644
> --- a/include/linux/tpm_eventlog.h
> +++ b/include/linux/tpm_eventlog.h
> @@ -198,8 +198,10 @@ static __always_inline int __calc_tpm2_event_size(struct tcg_pcr_event2_head *ev
>  	 * The loop below will unmap these fields if the log is larger than
>  	 * one page, so save them here for reference:
>  	 */
> -	count = READ_ONCE(event->count);
> -	event_type = READ_ONCE(event->event_type);
> +	count = event->count;
> +	event_type = event->event_type;
> +
> +	barrier();

It would be handy to have a comment here, but when I started thinking about
what that would say, it occurred to me that the unmap operation should
already have a barrier inside it due to the TLB invalidation, so I'm not
sure why this is needed at all.

Will