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.
Thanks,
Laurent
