SELinux: fix array out of bounds when mounting with selinux options

Given an illegal selinux option it was possible for match_token to work in
random memory at the end of the match_table_t array.

Note that privilege is required to perform a context mount, so this issue is
effectively limited to root only.
Signed-off-by: Eric Paris <eparis@redhat.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: James Morris <jmorris@namei.org>

security/selinux/hooks.c

@@ -316,6 +316,7 @@ static inline int inode_doinit(struct inode *inode)
 }
 
 enum {
+	Opt_error = -1,
 	Opt_context = 1,
 	Opt_fscontext = 2,
 	Opt_defcontext = 4,
@@ -327,6 +328,7 @@ static match_table_t tokens = {
 	{Opt_fscontext, "fscontext=%s"},
 	{Opt_defcontext, "defcontext=%s"},
 	{Opt_rootcontext, "rootcontext=%s"},
+	{Opt_error, NULL},
 };
 
 #define SEL_MOUNT_FAIL_MSG "SELinux:  duplicate or incompatible mount options\n"