[PATCH 4.19 127/149] s390/cmm: fix information leak in cmm_timeout_handler()

[Greg Kroah-Hartman]

From: Yihui ZENG <yzeng56@xxxxxxx>

commit b8e51a6a9db94bc1fb18ae831b3dab106b5a4b5f upstream.

The problem is that we were putting the NUL terminator too far:

	buf[sizeof(buf) - 1] = '\0';

If the user input isn't NUL terminated and they haven't initialized the
whole buffer then it leads to an info leak.  The NUL terminator should
be:

	buf[len - 1] = '\0';

Signed-off-by: Yihui Zeng <yzeng56@xxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
[heiko.carstens@xxxxxxxxxx: keep semantics of how *lenp and *ppos are handled]
Signed-off-by: Heiko Carstens <heiko.carstens@xxxxxxxxxx>
Signed-off-by: Vasily Gorbik <gor@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 arch/s390/mm/cmm.c |   12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

--- a/arch/s390/mm/cmm.c
+++ b/arch/s390/mm/cmm.c
@@ -298,16 +298,16 @@ static int cmm_timeout_handler(struct ct
 	}
 
 	if (write) {
-		len = *lenp;
-		if (copy_from_user(buf, buffer,
-				   len > sizeof(buf) ? sizeof(buf) : len))
+		len = min(*lenp, sizeof(buf));
+		if (copy_from_user(buf, buffer, len))
 			return -EFAULT;
-		buf[sizeof(buf) - 1] = '\0';
+		buf[len - 1] = '\0';
 		cmm_skip_blanks(buf, &p);
 		nr = simple_strtoul(p, &p, 0);
 		cmm_skip_blanks(p, &p);
 		seconds = simple_strtoul(p, &p, 0);
 		cmm_set_timeout(nr, seconds);
+		*ppos += *lenp;
 	} else {
 		len = sprintf(buf, "%ld %ld\n",
 			      cmm_timeout_pages, cmm_timeout_seconds);
@@ -315,9 +315,9 @@ static int cmm_timeout_handler(struct ct
 			len = *lenp;
 		if (copy_to_user(buffer, buf, len))
 			return -EFAULT;
+		*lenp = len;
+		*ppos += len;
 	}
-	*lenp = len;
-	*ppos += len;
 	return 0;
 }