selinux: free contexts previously transferred in selinux_add_opt()

`selinux_add_opt()` stopped taking ownership of the passed context since
commit 70f4169a ("selinux: parse contexts for mount options early").

    unreferenced object 0xffff888114dfd140 (size 64):
      comm "mount", pid 15182, jiffies 4295687028 (age 796.340s)
      hex dump (first 32 bytes):
        73 79 73 74 65 6d 5f 75 3a 6f 62 6a 65 63 74 5f  system_u:object_
        72 3a 74 65 73 74 5f 66 69 6c 65 73 79 73 74 65  r:test_filesyste
      backtrace:
        [<ffffffffa07dbef4>] kmemdup_nul+0x24/0x80
        [<ffffffffa0d34253>] selinux_sb_eat_lsm_opts+0x293/0x560
        [<ffffffffa0d13f08>] security_sb_eat_lsm_opts+0x58/0x80
        [<ffffffffa0af1eb2>] generic_parse_monolithic+0x82/0x180
        [<ffffffffa0a9c1a5>] do_new_mount+0x1f5/0x550
        [<ffffffffa0a9eccb>] path_mount+0x2ab/0x1570
        [<ffffffffa0aa019e>] __x64_sys_mount+0x20e/0x280
        [<ffffffffa1f47124>] do_syscall_64+0x34/0x80
        [<ffffffffa200007e>] entry_SYSCALL_64_after_hwframe+0x46/0xb0

    unreferenced object 0xffff888108e71640 (size 64):
      comm "fsmount", pid 7607, jiffies 4295044974 (age 1601.016s)
      hex dump (first 32 bytes):
        73 79 73 74 65 6d 5f 75 3a 6f 62 6a 65 63 74 5f  system_u:object_
        72 3a 74 65 73 74 5f 66 69 6c 65 73 79 73 74 65  r:test_filesyste
      backtrace:
        [<ffffffff861dc2b1>] memdup_user+0x21/0x90
        [<ffffffff861dc367>] strndup_user+0x47/0xa0
        [<ffffffff864f6965>] __do_sys_fsconfig+0x485/0x9f0
        [<ffffffff87940124>] do_syscall_64+0x34/0x80
        [<ffffffff87a0007e>] entry_SYSCALL_64_after_hwframe+0x46/0xb0

Cc: stable@vger.kernel.org
Fixes: 70f4169a ("selinux: parse contexts for mount options early")
Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

security/selinux/hooks.c

@@ -2600,8 +2600,9 @@ static int selinux_sb_eat_lsm_opts(char *options, void **mnt_opts)
 				}
 			}
 			rc = selinux_add_opt(token, arg, mnt_opts);
-			if (unlikely(rc)) {
 			kfree(arg);
+			arg = NULL;
+			if (unlikely(rc)) {
 				goto free_opt;
 			}
 		} else {
@@ -2792,17 +2793,13 @@ static int selinux_fs_context_parse_param(struct fs_context *fc,
 					  struct fs_parameter *param)
 {
 	struct fs_parse_result result;
-	int opt, rc;
+	int opt;
 
 	opt = fs_parse(fc, selinux_fs_parameters, param, &result);
 	if (opt < 0)
 		return opt;
 
-	rc = selinux_add_opt(opt, param->string, &fc->security);
-	if (!rc)
-		param->string = NULL;
-
-	return rc;
+	return selinux_add_opt(opt, param->string, &fc->security);
 }
 
 /* inode security operations */