On 09.10.2018 19:45, Laurent Vivier wrote:
> Le 09/10/2018 à 18:15, Kirill Tkhai a écrit :
>> 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
