linux-kernel - [PATCH] s390/cmm: Account for NUL when calculating 'len' in cmm_timeout

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Message-ID: <20251215122214.381098-2-thorsten.blum@linux.dev>
Date: Mon, 15 Dec 2025 13:22:14 +0100
From: Thorsten Blum <thorsten.blum@...ux.dev>
To: Alexander Gordeev <agordeev@...ux.ibm.com>,
	Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
	Heiko Carstens <hca@...ux.ibm.com>,
	Vasily Gorbik <gor@...ux.ibm.com>,
	Christian Borntraeger <borntraeger@...ux.ibm.com>,
	Sven Schnelle <svens@...ux.ibm.com>
Cc: Thorsten Blum <thorsten.blum@...ux.dev>,
	linux-s390@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] s390/cmm: Account for NUL when calculating 'len' in cmm_timeout_handler

When the input length 'lenp' equals sizeof(buf), the current code copies
all 64 bytes, but then immediately overwrites the last byte with a NUL
terminator. Limit the number of bytes to copy to 'sizeof(buf) - 1' to
reserve space for the NUL terminator.

Signed-off-by: Thorsten Blum <thorsten.blum@...ux.dev>
---
 arch/s390/mm/cmm.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/s390/mm/cmm.c b/arch/s390/mm/cmm.c
index eb7ef63fab1e..06512bc178a5 100644
--- a/arch/s390/mm/cmm.c
+++ b/arch/s390/mm/cmm.c
@@ -311,9 +311,9 @@ static int cmm_timeout_handler(const struct ctl_table *ctl, int write,
 	}
 
 	if (write) {
-		len = min(*lenp, sizeof(buf));
+		len = min(*lenp, sizeof(buf) - 1);
 		memcpy(buf, buffer, len);
-		buf[len - 1] = '\0';
+		buf[len] = '\0';
 		cmm_skip_blanks(buf, &p);
 		nr = simple_strtoul(p, &p, 0);
 		cmm_skip_blanks(p, &p);
-- 
Thorsten Blum <thorsten.blum@...ux.dev>
GPG: 1D60 735E 8AEF 3BE4 73B6  9D84 7336 78FD 8DFE EAD4