Re: [PATCH v2] bpf: Fix KASAN use-after-free Read in compute_effective_progs

[Tadeusz Struk]

Hi Andrii,
On 4/20/22 10:07, Andrii Nakryiko wrote:
> > diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
> > index 128028efda64..5a64cece09f3 100644
> > --- a/kernel/bpf/cgroup.c
> > +++ b/kernel/bpf/cgroup.c
> > @@ -723,10 +723,8 @@ 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;
> >
> > -       /* now can actually delete it from this cgroup list */
> > +       /* now can delete it from this cgroup list */
> >          list_del(&pl->node);
> >          kfree(pl);
> >          if (list_empty(progs))
> > @@ -735,12 +733,55 @@ static int __cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog,
> >          if (old_prog)
> >                  bpf_prog_put(old_prog);
> >          static_branch_dec(&cgroup_bpf_enabled_key[atype]);
> > -       return 0;
> > +
> > +       if (!err)
> > +               return 0;
> >
> >   cleanup:
> > -       /* restore back prog or link */
> > -       pl->prog = old_prog;
> > -       pl->link = link;
> > +       /*
> > +        * If compute_effective_progs failed with -ENOMEM, i.e. alloc for
> > +        * cgrp->bpf.inactive table failed, we can recover by removing
> > +        * the detached prog from effective table and rearranging it.
> > +        */
> > +       if (err == -ENOMEM) {
> > +               struct bpf_prog_array_item *item;
> > +               struct bpf_prog *prog_tmp, *prog_detach, *prog_last;
> > +               struct bpf_prog_array *array;
> > +               int index = 0, index_detach = -1;
> > +
> > +               array = cgrp->bpf.effective[atype];
> > +               item = &array->items[0];
> > +
> > +               if (prog)
> > +                       prog_detach = prog;
> > +               else
> > +                       prog_detach = link->link.prog;
> > +
> > +               if (!prog_detach)
> > +                       return -EINVAL;
> > +
> > +               while ((prog_tmp = READ_ONCE(item->prog))) {
> > +                       if (prog_tmp == prog_detach)
> > +                               index_detach = index;
> > +                       item++;
> > +                       index++;
> > +                       prog_last = prog_tmp;
> > +               }
> > +
> > +               /* Check if we found what's needed for removing the prog */
> > +               if (index_detach == -1 || index_detach == index-1)
> > +                       return -EINVAL;
> > +
> > +               /* Remove the last program in the array */
> > +               if (bpf_prog_array_delete_safe_at(array, index-1))
> > +                       return -EINVAL;
> > +
> > +               /* and update the detached with the last just removed */
> > +               if (bpf_prog_array_update_at(array, index_detach, prog_last))
> > +                       return -EINVAL;
> > +
> > +               err = 0;
> > +       }

Thanks for feedback, and sorry for delay. I got pulled into something else.

> There are a bunch of problems with this implementation.
>
> 1. We should do this fallback right after update_effective_progs()
> returns error, before we get to list_del(&pl->node) and subsequent
> code that does some additional things (like clearing flags and stuff).
> This additional code needs to run even if update_effective_progs()
> fails. So I suggest to extract the logic of removing program from
> effective prog arrays into a helper function and doing
>
> err = update_effective_progs(...);
> if (err)
>      purge_effective_progs();
>
> where purge_effective_progs() will be the logic you are adding. And it
> will be void function because it can't fail.

I have implemented that in v3, will send that out soon.

> 2. We have to update not just cgrp->bpf.effective array, but all the
> descendants' lists as well. See what update_effective_progs() is
> doing, it has css_for_each_descendant_pre() iteration. You need to do
> it here as well. But instead of doing compute_effective_progs() which
> allocates a new copy of an array we'll need to update existing array
> in place.
>
> 3. Not clear why you need to do both bpf_prog_array_delete_safe_at()
> and bpf_prog_array_update_at(), isn't delete_safe_at() enought?

I thought that we need to reshuffle the table and move the progs around,
but your are right, delete_safe_at() is enough.

--
Thanks,
Tadeusz