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

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

On Tue, Apr 5, 2022 at 10:04 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 don't preserve the pointer to the link on the cgroup list
> in __cgroup_bpf_detach(), but proceed with the cleanup and retry calling
> update_effective_progs() again afterwards.

I think it's still problematic. BPF link might have been the last one
that holds bpf_prog's refcnt, so when link is put, its prog can stay
there in effective_progs array(s) and will cause use-after-free
anyways.

It would be best to make sure that detach never fails. On detach
effective prog array can only shrink, so even if
update_effective_progs() fails to allocate memory, we should be able
to iterate and just replace that prog with NULL, as a fallback
strategy.

>
>
> 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>
> ---
>  kernel/bpf/cgroup.c | 25 ++++++++++++++-----------
>  1 file changed, 14 insertions(+), 11 deletions(-)
>
> diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
> index 128028efda64..b6307337a3c7 100644
> --- a/kernel/bpf/cgroup.c
> +++ b/kernel/bpf/cgroup.c
> @@ -723,10 +723,11 @@ 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;
> -
> -       /* now can actually delete it from this cgroup list */
> +       /*
> +        * Proceed regardless of error. The link and/or prog will be freed
> +        * just after this function returns so just delete it from this
> +        * cgroup list and retry calling update_effective_progs again later.
> +        */
>         list_del(&pl->node);
>         kfree(pl);
>         if (list_empty(progs))
> @@ -735,12 +736,11 @@ 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;
>
> -cleanup:
> -       /* restore back prog or link */
> -       pl->prog = old_prog;
> -       pl->link = link;
> +       /* In case of error call update_effective_progs again */
> +       if (err)
> +               err = update_effective_progs(cgrp, atype);

there is no guarantee that this will now succeed, right? so it's more
like "let's try just in case we are lucky this time"?

> +
>         return err;
>  }
>
> @@ -881,6 +881,7 @@ static void bpf_cgroup_link_release(struct bpf_link *link)
>         struct bpf_cgroup_link *cg_link =
>                 container_of(link, struct bpf_cgroup_link, link);
>         struct cgroup *cg;
> +       int err;
>
>         /* link might have been auto-detached by dying cgroup already,
>          * in that case our work is done here
> @@ -896,8 +897,10 @@ static void bpf_cgroup_link_release(struct bpf_link *link)
>                 return;
>         }
>
> -       WARN_ON(__cgroup_bpf_detach(cg_link->cgroup, NULL, cg_link,
> -                                   cg_link->type));
> +       err = __cgroup_bpf_detach(cg_link->cgroup, NULL, cg_link,
> +                                 cg_link->type);
> +       if (err)
> +               pr_warn("cgroup_bpf_detach() failed, err %d\n", err);
>
>         cg = cg_link->cgroup;
>         cg_link->cgroup = NULL;
> --
> 2.35.1