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

[Jann Horn]

On Tue, Oct 9, 2018 at 6:45 PM Laurent Vivier <laurent@xxxxxxxxx> wrote:
> Le 09/10/2018 Ã 18:15, Kirill Tkhai a Ãcrit :
> > 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)