Re: [PATCH] tpm: efi: reject a final events table with an oversized preboot log

[Jarkko Sakkinen <jarkko@kernel.org>]

On Tue, Nov 10, 2026 at 11:08:29AM +0800, Guo Zihao wrote:
> Hi Jarkko,
> 
> No bus interposer is involved: the threat here is firmware shipping a
> self-contradictory final events table (preboot size larger than the
> table itself), not tampering on the wire. So the quantitative
> question - does this actually happen - does have data.
> 
> This area of efi.c has already needed fixes for real firmware tables
> four times:
> 
> - 05c8c1ff81ed ("efi/tpm: Don't traverse an event log with no
>   events"): some machines return an unpopulated template table whose
>   contents are just garbage.
> 
> - b4f1874c6216 ("tpm: check event log version before reading final
>   events"): several Dell models froze at boot because the firmware
>   event log and final events table did not match each other.
>   Fixes: includes 166a2809d65b.
> 
> - 80a6c216b16d ("tpm/eventlog: Don't abort tpm_read_log on faulty
>   ACPI address"): a Framework laptop with an INSYDE BIOS shipped an
>   un-mappable TPM2 ACPI table.
> 
> - 48cff270b037 ("tpm: efi: Use local variable for calculating final
>   log size"): this one is the closest. The same subtraction went
>   negative and hit __memcpy with a wrapped size, with a real oops in
>   the commit message. It carries Fixes: 166a2809d65b and went to
>   stable.
> 
> So the corrupted-table input and the negative-size memcpy path have
> both been observed in the wild, just not through this exact
> combination yet.
> 
> I'm fine either way on the tag. If you still consider this one
> hardening, I'll send a v3 without Fixes - the change itself is not
> affected by that.
> 
> Thanks.

OK, I think you are making a fair argument.

This is not just pure hardening given the various possibilities for
pure software (firmware) mishaps.

Maybe v3 with a reminder of why this crossed the border?

That said, I think I agree with you now.

Br, Jarkko