From: Yihui ZENG <yzeng56@asu.edu>
Date: Fri, 25 Oct 2019 12:31:48 +0300
Subject: s390/cmm: fix information leak in cmm_timeout_handler()
Git-commit: b8e51a6a9db94bc1fb18ae831b3dab106b5a4b5f
Patch-mainline: 5.4-rc6
References: CVE-2020-10773 bsc#1172999 bnc#1151927 5.3.9

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@asu.edu>
Cc: stable@vger.kernel.org
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
[heiko.carstens@de.ibm.com: keep semantics of how *lenp and *ppos are handled]
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
---
 arch/s390/mm/cmm.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/arch/s390/mm/cmm.c b/arch/s390/mm/cmm.c
index 510a18299196..a51c892f14f3 100644
--- a/arch/s390/mm/cmm.c
+++ b/arch/s390/mm/cmm.c
@@ -298,16 +298,16 @@ static int cmm_timeout_handler(struct ctl_table *ctl, int write,
 	}
 
 	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 ctl_table *ctl, int write,
 			len = *lenp;
 		if (copy_to_user(buffer, buf, len))
 			return -EFAULT;
+		*lenp = len;
+		*ppos += len;
 	}
-	*lenp = len;
-	*ppos += len;
 	return 0;
 }
 
-- 
2.24.0