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

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 <[email protected]>
Fixes: a74f8b36352e ("tpm: introduce tpm_buf")
---
 drivers/char/tpm/tpm-buf.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/char/tpm/tpm-buf.c b/drivers/char/tpm/tpm-buf.c
index dc882fc9fa9e..8da5de0f7159 100644
--- a/drivers/char/tpm/tpm-buf.c
+++ b/drivers/char/tpm/tpm-buf.c
@@ -7,6 +7,8 @@
 #include <linux/module.h>
 #include <linux/tpm.h>
 
+#include "tpm.h"
+
 /**
  * tpm_buf_init() - Allocate and initialize a TPM command
  * @buf:	A &tpm_buf
@@ -108,7 +110,7 @@ void tpm_buf_append(struct tpm_buf *buf, const u8 *new_data, u16 new_length)
 	if (buf->flags & TPM_BUF_OVERFLOW)
 		return;
 
-	if ((buf->length + new_length) > PAGE_SIZE) {
+	if ((buf->length + new_length) > TPM_BUFSIZE) {
 		WARN(1, "tpm_buf: write overflow\n");
 		buf->flags |= TPM_BUF_OVERFLOW;
 		return;

---
base-commit: c1ecb239fa3456529a32255359fc78b69eb9d847
change-id: 20260524-tpm-402b8478fec9

Best regards,
--  
Breno Leitao <[email protected]>