selinux: Allow context mounts for unpriviliged overlayfs

Now overlayfs allow unpriviliged mounts. That is root inside a non-init
user namespace can mount overlayfs. This is being added in 5.11 kernel.

Giuseppe tried to mount overlayfs with option "context" and it failed
with error -EACCESS.

$ su test
$ unshare -rm
$ mkdir -p lower upper work merged
$ mount -t overlay -o lowerdir=lower,workdir=work,upperdir=upper,userxattr,context='system_u:object_r:container_file_t:s0' none merged

This fails with -EACCESS. It works if option "-o context" is not specified.

Little debugging showed that selinux_set_mnt_opts() returns -EACCESS.

So this patch adds "overlay" to the list, where it is fine to specific
context from non init_user_ns.

Reported-by: Giuseppe Scrivano <gscrivan@redhat.com>
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
[PM: trimmed the changelog from the description]
Signed-off-by: Paul Moore <paul@paul-moore.com>

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index ddd0977..eca9fc0 100644 (file)
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -760,7 +760,8 @@ static int selinux_set_mnt_opts(struct super_block *sb,
        if (sb->s_user_ns != &init_user_ns &&
            strcmp(sb->s_type->name, "tmpfs") &&
            strcmp(sb->s_type->name, "ramfs") &&
-           strcmp(sb->s_type->name, "devpts")) {
+           strcmp(sb->s_type->name, "devpts") &&
+           strcmp(sb->s_type->name, "overlay")) {
                if (context_sid || fscontext_sid || rootcontext_sid ||
                    defcontext_sid) {
                        rc = -EACCES;