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

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

On Fri, Apr 15, 2022 at 7:14 AM Tadeusz Struk <tadeusz.struk@linaro.org> wrote:
>
> Syzbot found a Use After Free bug in compute_effective_progs().
> The reproducer creates a number of BPF links, and causes a fault
> injected alloc to fail, while calling bpf_link_detach on them.
> Link detach triggers the link to be freed by bpf_link_free(),
> which calls __cgroup_bpf_detach() and update_effective_progs().
> If the memory allocation in this function fails, the function restores
> the pointer to the bpf_cgroup_link on the cgroup list, but the memory
> gets freed just after it returns. After this, every subsequent call to
> update_effective_progs() causes this already deallocated pointer to be
> dereferenced in prog_list_length(), and triggers KASAN UAF error.
>
> To fix this issue don't preserve the pointer to the prog or link in the
> list, but remove it and rearrange the effective table without
> shrinking it. The subsequent call to __cgroup_bpf_detach() or
> __cgroup_bpf_detach() will correct it.
>
> Cc: "Alexei Starovoitov" <ast@kernel.org>
> Cc: "Daniel Borkmann" <daniel@iogearbox.net>
> Cc: "Andrii Nakryiko" <andrii@kernel.org>
> Cc: "Martin KaFai Lau" <kafai@fb.com>
> Cc: "Song Liu" <songliubraving@fb.com>
> Cc: "Yonghong Song" <yhs@fb.com>
> Cc: "John Fastabend" <john.fastabend@gmail.com>
> Cc: "KP Singh" <kpsingh@kernel.org>
> Cc: <netdev@vger.kernel.org>
> Cc: <bpf@vger.kernel.org>
> Cc: <stable@vger.kernel.org>
> Cc: <linux-kernel@vger.kernel.org>
>
> Link: https://syzkaller.appspot.com/bug?id=8ebf179a95c2a2670f7cf1ba62429ec044369db4
> Fixes: af6eea57437a ("bpf: Implement bpf_link-based cgroup BPF program attachment")
> Reported-by: <syzbot+f264bffdfbd5614f3bb2@syzkaller.appspotmail.com>
> Signed-off-by: Tadeusz Struk <tadeusz.struk@linaro.org>
> ---
> v2: Add a fall back path that removes a prog from the effective progs
>     table in case detach fails to allocate memory in compute_effective_progs().
> ---
>  kernel/bpf/cgroup.c | 55 +++++++++++++++++++++++++++++++++++++++------
>  1 file changed, 48 insertions(+), 7 deletions(-)
>
> 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;
> +       }

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.

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?


>         return err;
>  }
>
> --
> 2.35.1
>