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

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

On Fri, May 13, 2022 at 12:08 PM 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 replace it with a dummy prog without shrinking
> the table. 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().
>
> v3: Implement the fallback in a separate function purge_effective_progs
> ---
>  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)

> +               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.

> +                               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?

> +
> +               /* 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

> +               purge_effective_progs(cgrp, prog_purge, atype);
> +       }
>
>         /* now can actually delete it from this cgroup list */
>         list_del(&pl->node);
> @@ -736,12 +790,6 @@ static int __cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog,
>                 bpf_prog_put(old_prog);
>         static_branch_dec(&cgroup_bpf_enabled_key[atype]);
>         return 0;
> -
> -cleanup:
> -       /* restore back prog or link */
> -       pl->prog = old_prog;
> -       pl->link = link;
> -       return err;
>  }
>
>  static int cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog,
> --
> 2.36.1
>