s390/cmm: fix information leak in cmm_timeout_handler()

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>

diff --git a/arch/s390/mm/cmm.c b/arch/s390/mm/cmm.c
index 510a18299196f3b797be51fd22b2bbfbe5cd003f..a51c892f14f3ee5bd393ed74003c96fd80268979 100644 (file)
--- 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;
 }