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Date: Fri, 13 May 2022 11:38:28 -0700 From: Tadeusz Struk <tadeusz.struk@...aro.org> To: Andrii Nakryiko <andrii.nakryiko@...il.com> 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 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
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