Le 09/10/2018 à 14:43, Jann Horn a écrit :
> On Tue, Oct 9, 2018 at 12:38 PM Laurent Vivier <laurent@xxxxxxxxx> wrote:
>> This patch allows to have a different binfmt_misc configuration
>> for each new user namespace. By default, the binfmt_misc configuration
>> is the one of the previous level, but if the binfmt_misc filesystem is
>> mounted in the new namespace a new empty binfmt instance is created and
>> used in this namespace.
>>
>> For instance, using "unshare" we can start a chroot of an another
>> architecture and configure the binfmt_misc interpreter without being root
>> to run the binaries in this chroot.
> [...]
>> @@ -823,12 +847,34 @@ static const struct super_operations s_ops = {
>> static int bm_fill_super(struct super_block *sb, void *data, int silent)
>> {
>> int err;
>> + struct user_namespace *ns = sb->s_user_ns;
>> static const struct tree_descr bm_files[] = {
>> [2] = {"status", &bm_status_operations, S_IWUSR|S_IRUGO},
>> [3] = {"register", &bm_register_operations, S_IWUSR},
>> /* last one */ {""}
>> };
>>
>> + /* create a new binfmt namespace
>> + * if we are not in the first user namespace
>> + * but the binfmt namespace is the first one
>> + */
>> + if (READ_ONCE(ns->binfmt_ns) == NULL) {
>> + struct binfmt_namespace *new_ns;
>> +
>> + new_ns = kmalloc(sizeof(struct binfmt_namespace),
>> + GFP_KERNEL);
>> + if (new_ns == NULL)
>> + return -ENOMEM;
>> + INIT_LIST_HEAD(&new_ns->entries);
>> + new_ns->enabled = 1;
>> + rwlock_init(&new_ns->entries_lock);
>> + new_ns->bm_mnt = NULL;
>> + new_ns->entry_count = 0;
>> + /* ensure new_ns is completely initialized before sharing it */
>> + smp_wmb();
>> + WRITE_ONCE(ns->binfmt_ns, new_ns);
>> + }
>
> You're still not preventing a concurrent race of two mount() calls,
> right? What prevents two instances of this code block from running
> concurrently in two different namespaces? I think you want to take
> some sort of global lock around this.
>
My guess was we have only one binfmt superblock by user namespace, so as
we can't have duplicate superblock, we will not have duplicate binfmt_ns
structure. This function is only called once in the namespace and I
think the superblock creation is already protected by some kind of lock.
But I'm not a VFS expert, if someone wants to clarify the situation,
please go ahead.
Thanks,
Laurent
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