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Message-ID: <CAEf4Bzah9K7dEa_7sXE4TnkuMTRHypMU9DxiLezgRvLjcqE_YA@mail.gmail.com>
Date: Wed, 20 Apr 2022 10:07:27 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Tadeusz Struk <tadeusz.struk@...aro.org>
Cc: Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Andrii Nakryiko <andrii@...nel.org>,
Martin KaFai Lau <kafai@...com>,
Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
John Fastabend <john.fastabend@...il.com>,
KP Singh <kpsingh@...nel.org>,
Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
linux- stable <stable@...r.kernel.org>,
open list <linux-kernel@...r.kernel.org>,
syzbot+f264bffdfbd5614f3bb2@...kaller.appspotmail.com
Subject: Re: [PATCH v2] bpf: Fix KASAN use-after-free Read in compute_effective_progs
On Fri, Apr 15, 2022 at 7:14 AM Tadeusz Struk <tadeusz.struk@...aro.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@...nel.org>
> Cc: "Daniel Borkmann" <daniel@...earbox.net>
> Cc: "Andrii Nakryiko" <andrii@...nel.org>
> Cc: "Martin KaFai Lau" <kafai@...com>
> Cc: "Song Liu" <songliubraving@...com>
> Cc: "Yonghong Song" <yhs@...com>
> Cc: "John Fastabend" <john.fastabend@...il.com>
> Cc: "KP Singh" <kpsingh@...nel.org>
> Cc: <netdev@...r.kernel.org>
> Cc: <bpf@...r.kernel.org>
> Cc: <stable@...r.kernel.org>
> Cc: <linux-kernel@...r.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@...kaller.appspotmail.com>
> Signed-off-by: Tadeusz Struk <tadeusz.struk@...aro.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
>
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