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

[Jann Horn <jannh@google.com>]

On Tue, Oct 9, 2018 at 6:57 PM Laurent Vivier <laurent@vivier.eu> wrote:
> Le 09/10/2018 à 18:53, Jann Horn a écrit :
> > On Tue, Oct 9, 2018 at 6:45 PM Laurent Vivier <laurent@vivier.eu> 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@vivier.eu>
> >>>> ---
> >>>>  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)
> >
>
> So my questions are:
>
> - do we need a smp_wmb() barrier if we use READ_ONCE() and WRITE_ONCE()?

Yes.

> - if we need an smp_wmb() barrier, do we need an smp_rmb() barrier as
> the data we want to "protect" are behind an access to the pointer?

No. You only need an smp_read_barrier_depends() to access things
behind the pointer you're reading (documented in a big comment block
in arch/alpha/include/asm/barrier.h); and that barrier is implied by
READ_ONCE(), so you don't have to write it yourself.