Re: [PATCH] tpm: cap tpm_buf_append() at TPM_BUFSIZE, not PAGE_SIZE

[Breno Leitao <leitao@debian.org>]

Hello Jarkko,

On Mon, May 25, 2026 at 02:24:38AM +0000, Jarkko Sakkinen wrote:
> On Sun, May 24, 2026 at 10:01:17AM -0400, Breno Leitao wrote:
> > tpm_buf_append() guards against overflow of the underlying buffer by
> > comparing the running length against PAGE_SIZE. Every other site in the
> > TPM core uses TPM_BUFSIZE (4096) as the protocol-level cap on TPM
> > command and response sizes.
> > 
> > On 4K-page kernels PAGE_SIZE == TPM_BUFSIZE, so the two caps coincide
> > and the inconsistency is invisible. On kernels with a larger base page
> > size, e.g. CONFIG_ARM64_64K_PAGES=y or 16K pages, PAGE_SIZE exceeds
> > TPM_BUFSIZE.
> > 
> > This is a latent bug rather than user-visible bug, given most of the
> > cases PAGE_SIZE = 4096. The mismatch is still worth fixing because
> > future callers (e.g. the proposed TPM_BUFSIZE increase to 8 KiB, and the
> > Secure Launch tpm_buf rework) rely on the overflow flag being
> > authoritative.
> > 
> > Use TPM_BUFSIZE instead of PAGE_SIZE so the append-time check
> > matches the transmit-time cap on every page size.
> > 
> > Signed-off-by: Breno Leitao <leitao@debian.org>
> > Fixes: a74f8b36352e ("tpm: introduce tpm_buf")
> > ---
> 
> There is no bug w/o a sympton of some sort. Not sure what the problem is here.

Sorry, there is no current problem in here, but there is inconsistency.

There isn't a real problem today, just an inconsistency and what I
called a latent bug, let me justify myself.

Everywhere else in the TPM core uses TPM_BUFSIZE as the protocol cap,
but this particular site uses PAGE_SIZE instead. Since PAGE_SIZE >=
TPM_BUFSIZE, it doesn't cause any issue at the moment.

That said, I still think the change is worthwhile for two reasons:

1. Consistency with the rest of the TPM core.
2. Decoupling TPM_BUFSIZE from PAGE_SIZE, so that if TPM_BUFSIZE ever
   grows beyond PAGE_SIZE[1], this code won't silently break. That's what I
   was referring to as a "latent bug" — though admittedly that phrasing
   was probably too strong.

Link: https://lore.kernel.org/all/20260518151724.730443-6-armenon@redhat.com/ [1]

Thanks for the review,
--breno