Re: [PATCH v3] bpf: Fix KASAN use-after-free Read in compute_effective_progs

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Mon, May 16, 2022 at 4:35 PM Tadeusz Struk <tadeusz.struk@linaro.org> wrote:
>
> On 5/16/22 16:16, Andrii Nakryiko wrote:
> > On Fri, May 13, 2022 at 12:08 PM Tadeusz Struk <tadeusz.struk@linaro.org> wrote:
> >>   kernel/bpf/cgroup.c | 64 +++++++++++++++++++++++++++++++++++++++------
> >>   1 file changed, 56 insertions(+), 8 deletions(-)
> >>
> >> diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
> >> index 128028efda64..9d3af4d6c055 100644
> >> --- a/kernel/bpf/cgroup.c
> >> +++ b/kernel/bpf/cgroup.c
> >> @@ -681,6 +681,57 @@ static struct bpf_prog_list *find_detach_entry(struct list_head *progs,
> >>          return ERR_PTR(-ENOENT);
> >>   }
> >>
> >> +/**
> >> + * purge_effective_progs() - After compute_effective_progs fails to alloc new
> >> + *                           cgrp->bpf.inactive table we can recover by
> >> + *                           recomputing the array in place.
> >> + *
> >> + * @cgrp: The cgroup which descendants to traverse
> >> + * @link: A link to detach
> >> + * @atype: Type of detach operation
> >> + */
> >> +static void purge_effective_progs(struct cgroup *cgrp, struct bpf_prog *prog,
> >> +                                 enum cgroup_bpf_attach_type atype)
> >> +{
> >> +       struct cgroup_subsys_state *css;
> >> +       struct bpf_prog_array_item *item;
> >> +       struct bpf_prog *tmp;
> >> +       struct bpf_prog_array *array;
> >> +       int index = 0, index_purge = -1;
> >> +
> >> +       if (!prog)
> >> +               return;
> >> +
> >> +       /* recompute effective prog array in place */
> >> +       css_for_each_descendant_pre(css, &cgrp->self) {
> >> +               struct cgroup *desc = container_of(css, struct cgroup, self);
> >> +
> >> +               array = desc->bpf.effective[atype];
> >
> > ../kernel/bpf/cgroup.c:748:23: warning: incorrect type in assignment
> > (different address spaces)
> > ../kernel/bpf/cgroup.c:748:23:    expected struct bpf_prog_array *array
> > ../kernel/bpf/cgroup.c:748:23:    got struct bpf_prog_array [noderef] __rcu *
> >
> >
> > you need rcu_dereference here? but also see suggestions below to avoid
> > iterating effective directly (it's ambiguous to search by prog only)
>
> I didn't check it with sparse so I didn't see this warning.
> Will fix in the next version.
>
> >
> >> +               item = &array->items[0];
> >> +
> >> +               /* Find the index of the prog to purge */
> >> +               while ((tmp = READ_ONCE(item->prog))) {
> >> +                       if (tmp == prog) {
> >
> > I think comparing just prog isn't always correct, as the same program
> > can be in effective array multiple times if attached through bpf_link.
> >
> > Looking at replace_effective_prog() I think we can do a very similar
> > (and tested) approach:
> >
> > 1. restore original pl state in __cgroup_bpf_detach (so we can find it
> > by comparing pl->prog == prog && pl->link == link)
> > 2. use replace_effective_prog's approach to find position of pl in
> > effective array (using this nested for loop over cgroup parents and
> > list_for_each_entry inside)
> > 3. then instead of replacing one prog with another do
> > bpf_prog_array_delete_safe_at ?
> >
> > I'd feel more comfortable using the same tested overall approach of
> > replace_effective_prog.
>
> Ok, I can try that.
>
> >
> >> +                               index_purge = index;
> >> +                               break;
> >> +                       }
> >> +                       item++;
> >> +                       index++;
> >> +               }
> >> +
> >> +               /* Check if we found what's needed for removing the prog */
> >> +               if (index_purge == -1 || index_purge == index - 1)
> >> +                       continue;
> >
> > the search shouldn't fail, should it?
>
> I wasn't if the prog will be present in all parents so I decided to add this
> check to make sure it is found.

Looking at replace_effective_prog (it's been a while since I touched
this code) it has to be present, otherwise it's a bug

>
> >
> >> +
> >> +               /* Remove the program from the array */
> >> +               WARN_ONCE(bpf_prog_array_delete_safe_at(array, index_purge),
> >> +                         "Failed to purge a prog from array at index %d", index_purge);
> >> +
> >> +               index = 0;
> >> +               index_purge = -1;
> >> +       }
> >> +}
> >> +
> >>   /**
> >>    * __cgroup_bpf_detach() - Detach the program or link from a cgroup, and
> >>    *                         propagate the change to descendants
> >> @@ -723,8 +774,11 @@ static int __cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog,
> >>          pl->link = NULL;
> >>
> >>          err = update_effective_progs(cgrp, atype);
> >> -       if (err)
> >> -               goto cleanup;
> >> +       if (err) {
> >> +               struct bpf_prog *prog_purge = prog ? prog : link->link.prog;
> >> +
> >
> > so here we shouldn't forget link, instead pass both link and prog (one
> > of them will have to be NULL) into purge_effective_progs
>
> ok, I will pass in both.
>
> --
> Thanks,
> Tadeusz