On Tue, Oct 9, 2018 at 6:45 PM Laurent Vivier <laurent@xxxxxxxxx> 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.
Things don't work that way on alpha, but that's why READ_ONCE()
includes an smp_read_barrier_depends():
#define __READ_ONCE(x, check) \
({ \
union { typeof(x) __val; char __c[1]; } __u; \
if (check) \
__read_once_size(&(x), __u.__c, sizeof(x)); \
else \
__read_once_size_nocheck(&(x), __u.__c, sizeof(x)); \
smp_read_barrier_depends(); /* Enforce dependency ordering from x */ \
__u.__val; \
})
#define READ_ONCE(x) __READ_ONCE(x, 1)
