Re: [RFC PATCH bpf-next 0/6] bpf: Handle reuse in bpf memory alloc

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

On Wed, Jan 04, 2023 at 07:26:12PM CET, Alexei Starovoitov wrote:
> On Tue, Jan 3, 2023 at 11:14 PM Yonghong Song <yhs@meta.com> wrote:
> >
> >
> >
> > On 1/3/23 10:30 PM, Hou Tao wrote:
> > > Hi,
> > >
> > > On 1/4/2023 2:10 PM, Yonghong Song wrote:
> > >>
> > >>
> > >> On 1/3/23 5:47 AM, Hou Tao wrote:
> > >>> Hi,
> > >>>
> > >>> On 1/2/2023 2:48 AM, Yonghong Song wrote:
> > >>>>
> > >>>>
> > >>>> On 12/31/22 5:26 PM, Alexei Starovoitov wrote:
> > >>>>> On Fri, Dec 30, 2022 at 12:11:45PM +0800, Hou Tao wrote:
> > >>>>>> From: Hou Tao <houtao1@huawei.com>
> > >>>>>>
> > >>>>>> Hi,
> > >>>>>>
> > >>>>>> The patchset tries to fix the problems found when checking how htab map
> > >>>>>> handles element reuse in bpf memory allocator. The immediate reuse of
> > >>>>>> freed elements may lead to two problems in htab map:
> > >>>>>>
> > >>>>>> (1) reuse will reinitialize special fields (e.g., bpf_spin_lock) in
> > >>>>>>        htab map value and it may corrupt lookup procedure with BFP_F_LOCK
> > >>>>>>        flag which acquires bpf-spin-lock during value copying. The
> > >>>>>>        corruption of bpf-spin-lock may result in hard lock-up.
> > >>>>>> (2) lookup procedure may get incorrect map value if the found element is
> > >>>>>>        freed and then reused.
> > >>>>>>
> > >>>>>> Because the type of htab map elements are the same, so problem #1 can be
> > >>>>>> fixed by supporting ctor in bpf memory allocator. The ctor initializes
> > >>>>>> these special fields in map element only when the map element is newly
> > >>>>>> allocated. If it is just a reused element, there will be no
> > >>>>>> reinitialization.
> > >>>>>
> > >>>>> Instead of adding the overhead of ctor callback let's just
> > >>>>> add __GFP_ZERO to flags in __alloc().
> > >>>>> That will address the issue 1 and will make bpf_mem_alloc behave just
> > >>>>> like percpu_freelist, so hashmap with BPF_F_NO_PREALLOC and default
> > >>>>> will behave the same way.
> > >>>>
> > >>>> Patch https://lore.kernel.org/all/20220809213033.24147-3-memxor@gmail.com/
> > >>>> tried to address a similar issue for lru hash table.
> > >>>> Maybe we need to do similar things after bpf_mem_cache_alloc() for
> > >>>> hash table?
> > >>> IMO ctor or __GFP_ZERO will fix the issue. Did I miss something here ?
> > >>
> > >> The following is my understanding:
> > >> in function alloc_htab_elem() (hashtab.c), we have
> > >>
> > >>                  if (is_map_full(htab))
> > >>                          if (!old_elem)
> > >>                                  /* when map is full and update() is replacing
> > >>                                   * old element, it's ok to allocate, since
> > >>                                   * old element will be freed immediately.
> > >>                                   * Otherwise return an error
> > >>                                   */
> > >>                                  return ERR_PTR(-E2BIG);
> > >>                  inc_elem_count(htab);
> > >>                  l_new = bpf_mem_cache_alloc(&htab->ma);
> > >>                  if (!l_new) {
> > >>                          l_new = ERR_PTR(-ENOMEM);
> > >>                          goto dec_count;
> > >>                  }
> > >>                  check_and_init_map_value(&htab->map,
> > >>                                           l_new->key + round_up(key_size, 8));
> > >>
> > >> In the above check_and_init_map_value() intends to do initializing
> > >> for an element from bpf_mem_cache_alloc (could be reused from the free list).
> > >>
> > >> The check_and_init_map_value() looks like below (in include/linux/bpf.h)
> > >>
> > >> static inline void bpf_obj_init(const struct btf_field_offs *foffs, void *obj)
> > >> {
> > >>          int i;
> > >>
> > >>          if (!foffs)
> > >>                  return;
> > >>          for (i = 0; i < foffs->cnt; i++)
> > >>                  memset(obj + foffs->field_off[i], 0, foffs->field_sz[i]);
> > >> }
> > >>
> > >> static inline void check_and_init_map_value(struct bpf_map *map, void *dst)
> > >> {
> > >>          bpf_obj_init(map->field_offs, dst);
> > >> }
> > >>
> > >> IIUC, bpf_obj_init() will bzero those fields like spin_lock, timer,
> > >> list_head, list_node, etc.
> > >>
> > >> This is the problem for above problem #1.
> > >> Maybe I missed something?
> > > Yes. It is the problem patch #1 tries to fix exactly. Patch #1 tries to fix the
> > > problem by only calling check_and_init_map_value() once for the newly-allocated
> > > element, so if a freed element is reused, its special fields will not be zeroed
> > > again. Is there any other cases which are not covered by the solution or any
> > > other similar problems in hash-tab ?
> >
> > No, I checked all cases of check_and_init_map_value() and didn't find
> > any other instances.
>
> check_and_init_map_value() is called in two other cases:
> lookup_and_delete[_batch].
> There the zeroing of the fields is necessary because the 'value'
> is a temp buffer that is going to be copied to user space.
> I think the way forward is to add GFP_ZERO to mem_alloc
> (to make it equivalent to prealloc), remove one case
> of check_and_init_map_value from hashmap, add short comments
> to two other cases and add a big comment to check_and_init_map_value()
> that should say that 'dst' must be a temp buffer and should not
> point to memory that could be used in parallel by a bpf prog.
> It feels like we've dealt with this issue a couple times already
> and keep repeating this mistake, so the more comments the better.

Hou, are you plannning to resubmit this change? I also hit this while testing my
changes on bpf-next.