selinux: fix double free in selinux_parse_opts_str()

This patch is based on a discussion generated by an earlier patch
from Tetsuo Handa:

* https://marc.info/?t=149035659300001&r=1&w=2

The double free problem involves the mnt_opts field of the
security_mnt_opts struct, selinux_parse_opts_str() frees the memory
on error, but doesn't set the field to NULL so if the caller later
attempts to call security_free_mnt_opts() we trigger the problem.

In order to play it safe we change selinux_parse_opts_str() to call
security_free_mnt_opts() on error instead of free'ing the memory
directly.  This should ensure that everything is handled correctly,
regardless of what the caller may do.

Fixes: e0007529893c1c06 ("LSM/SELinux: Interfaces to allow FS to control mount options")
Cc: stable@vger.kernel.org
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>

security/selinux/hooks.c

@@ -1106,10 +1106,8 @@ static int selinux_parse_opts_str(char *options,
 
 	opts->mnt_opts_flags = kcalloc(NUM_SEL_MNT_OPTS, sizeof(int),
 				       GFP_KERNEL);
-	if (!opts->mnt_opts_flags) {
-		kfree(opts->mnt_opts);
+	if (!opts->mnt_opts_flags)
 		goto out_err;
-	}
 
 	if (fscontext) {
 		opts->mnt_opts[num_mnt_opts] = fscontext;
@@ -1132,6 +1130,7 @@ static int selinux_parse_opts_str(char *options,
 	return 0;
 
 out_err:
+	security_free_mnt_opts(opts);
 	kfree(context);
 	kfree(defcontext);
 	kfree(fscontext);