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.
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@vger.kernel.org Cc: Peter Jones pjones@redhat.com Cc: Jarkko Sakkinen jarkko@kernel.org Cc: Matthew Garrett mjg59@srcf.ucam.org Reported-by: Nathan Chancellor nathan@kernel.org Link: https://github.com/ClangBuiltLinux/linux/issues/1782 Signed-off-by: Ard Biesheuvel ardb@kernel.org
Based on the thread, I tested this patch without barrier() and my machine boots up just fine now with an LTO kernel. Thanks a lot for the analysis and fix!
Tested-by: Nathan Chancellor nathan@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();
/* Verify that it's the log header */ if (event_header->pcr_idx != 0 || -- 2.39.0