sysctl: require CAP_SYS_RAWIO to set mmap_min_addr

Currently the mmap_min_addr value can only be bypassed during mmap when
the task has CAP_SYS_RAWIO.  However, the mmap_min_addr sysctl value itself
can be adjusted to 0 if euid == 0, allowing a bypass without CAP_SYS_RAWIO.
This patch adds a check for the capability before allowing mmap_min_addr to
be changed.

Signed-off-by: Kees Cook <kees.cook@canonical.com>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Signed-off-by: James Morris <jmorris@namei.org>

security/min_addr.c

@@ -33,6 +33,9 @@ int mmap_min_addr_handler(struct ctl_table *table, int write,
 {
 	int ret;
 
+	if (!capable(CAP_SYS_RAWIO))
+		return -EPERM;
+
 	ret = proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
 
 	update_mmap_min_addr();