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@xxxxxxxxxxxxxxx> > Cc: Peter Jones <pjones@xxxxxxxxxx> > Cc: Jarkko Sakkinen <jarkko@xxxxxxxxxx> > Cc: Matthew Garrett <mjg59@xxxxxxxxxxxxx> > Reported-by: Nathan Chancellor <nathan@xxxxxxxxxx> > Link: https://github.com/ClangBuiltLinux/linux/issues/1782 > Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx> > --- > 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
