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

[Guo Zihao]

On Fri, Nov 06, 2026 at 03:24:39PM +0000, Jarkko Sakkinen wrote:
> Ditto.
>
> For all four address validation vs bug fix intersection. I did not
> check throughly enough to be exactly sure of all four.

For this one I'd like to argue the bug fix case, since the data being
validated is not something the TPM reports over a TCG-defined
protocol: both sizes come from EFI configuration tables written by
firmware, and nothing in the TCG spec constrains what firmware puts
there.

The firmware data is already treated as untrusted in the same
function: it bails out on an empty log (log_tbl->size == 0), on a
missing final log address, and on a wrong log format version. The
unchecked subtraction is the odd one out.

The consequence is also plain memory corruption rather than a
graceful rejection: final_events_log_size goes negative, the
following devm_krealloc() gets a wrapped size, and the memcpy()
appending the final events log runs with a negative length.

And the out-of-bounds write was introduced by 166a2809d65b ("tpm:
Don't duplicate events from the final event log in the TCG2 log"):
before that commit nothing subtracted the preboot size, so a
malformed table was harmless. The Fixes: tag refers to that
regression, not to the malformed table itself.

Ard also had a look at v2 and only asked for a pr_warn() on the
rejection path, which is in.

So I'd like to keep the Fixes: tag here, but I'm happy to drop it if
you disagree - your tree, your call.

Thanks.