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08 September 2024, 02:26:05 UTC
Revision d3f450533bbcb6dd4d7d59cadc9b61b7321e4ac1 authored by Ard Biesheuvel on 09 January 2023, 09:44:31 UTC, committed by Ard Biesheuvel on 13 January 2023, 16:15:17 UTC 
efi: tpm: Avoid READ_ONCE() for accessing the event log
 
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, but this is already taken
care of by the implicit barrier() semantics of the early_memunmap()
call.

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>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Link: https://github.com/ClangBuiltLinux/linux/issues/1782
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
1 parent 18bba18
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Tip revision: d3f450533bbcb6dd4d7d59cadc9b61b7321e4ac1 authored by Ard Biesheuvel on 09 January 2023, 09:44:31 UTC
efi: tpm: Avoid READ_ONCE() for accessing the event log
Tip revision: d3f4505
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