Re: [RFC v5 1/1] ns: add binfmt_misc to the user namespace

[Kirill Tkhai]

On 09.10.2018 19:45, Laurent Vivier wrote:
> Le 09/10/2018 Ã 18:15, Kirill Tkhai a ÃcritÂ:
>> On 09.10.2018 13:37, Laurent Vivier 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.
>>>
>>> Signed-off-by: Laurent Vivier <laurent@xxxxxxxxx>
>>> ---
>>>  fs/binfmt_misc.c               | 106 ++++++++++++++++++++++++---------
>>>  include/linux/user_namespace.h |  13 ++++
>>>  kernel/user.c                  |  13 ++++
>>>  kernel/user_namespace.c        |   3 +
>>>  4 files changed, 107 insertions(+), 28 deletions(-)
>>>
>>> diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
>>> index aa4a7a23ff99..1e0029d097d9 100644
>>> --- a/fs/binfmt_misc.c
>>> +++ b/fs/binfmt_misc.c
> ...
>>> @@ -80,18 +74,32 @@ static int entry_count;
>>>   */
>>>  #define MAX_REGISTER_LENGTH 1920
>>>  
>>> +static struct binfmt_namespace *binfmt_ns(struct user_namespace *ns)
>>> +{
>>> +	struct binfmt_namespace *b_ns;
>>> +
>>> +	while (ns) {
>>> +		b_ns = READ_ONCE(ns->binfmt_ns);
>>> +		if (b_ns)
>>> +			return b_ns;
>>> +		ns = ns->parent;
>>> +	}
>>> +	WARN_ON_ONCE(1);
>>> +	return NULL;
>>> +}
>>> +
> ...
>>> @@ -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();
>>
>> (I haven't dived into patch logic, here just small barrier remark from quick sight).
>> smp_wmb() has no sense without paired smp_rmb() on the read side. Possible,
>> you want something like below in read hunk:
>>
>> +		b_ns = READ_ONCE(ns->binfmt_ns);
>> +		if (b_ns) {
>> +			smp_rmb();
>> +			return b_ns;
>> +		}
>>
>>
> 
> The write barrier is here to ensure the structure is fully written
> before we set the pointer.
> 
> I don't understand how read barrier can change something at this level,
> IMHO the couple WRITE_ONCE()/READ_ONCE() should be enough to ensure we
> have correctly initialized the pointer and the structure when we read
> the pointer back.
> 
> I think the pointer itself is the "barrier" to access the memory
> modified before.

smp_rmb() guarantees you see stores in the order you want. If you have:

[cpu0]					[cpu1]
new_ns->entry_count = 0; 
smp_wmb();
WRITE_ONCE(ns->binfmt_ns, new_ns); 	b_ns = READ_ONCE(ns->binfmt_ns);
					smp_rmb();
					<access b_ns->entry_count>

smp_rmb() guarantees you see true entry_count on the cpu1. Without
smp_rmb() you may see old value of new_ns->entry_count.
					
See Documentation/memory-barriers.txt