Messages in this thread | | | Date | Thu, 3 Nov 2022 15:42:23 -0700 | Subject | Re: [PATCH v4,bpf-next] bpf: Don't redirect packets with invalid pkt_len | From | Martin KaFai Lau <> |
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On 11/3/22 2:36 PM, Stanislav Fomichev wrote: > On Thu, Nov 3, 2022 at 2:07 PM Martin KaFai Lau <martin.lau@linux.dev> wrote: >> >> On 7/15/22 4:55 AM, Zhengchao Shao wrote: >>> Syzbot found an issue [1]: fq_codel_drop() try to drop a flow whitout any >>> skbs, that is, the flow->head is null. >>> The root cause, as the [2] says, is because that bpf_prog_test_run_skb() >>> run a bpf prog which redirects empty skbs. >>> So we should determine whether the length of the packet modified by bpf >>> prog or others like bpf_prog_test is valid before forwarding it directly. >>> >>> LINK: [1] https://syzkaller.appspot.com/bug?id=0b84da80c2917757915afa89f7738a9d16ec96c5 >>> LINK: [2] https://www.spinics.net/lists/netdev/msg777503.html >>> >>> Reported-by: syzbot+7a12909485b94426aceb@syzkaller.appspotmail.com >>> Signed-off-by: Zhengchao Shao <shaozhengchao@huawei.com> >>> --- >>> v3: modify debug print >>> v2: need move checking to convert___skb_to_skb and add debug info >>> v1: should not check len in fast path >>> >>> include/linux/skbuff.h | 8 ++++++++ >>> net/bpf/test_run.c | 3 +++ >>> net/core/dev.c | 1 + >>> 3 files changed, 12 insertions(+) >>> >>> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h >>> index f6a27ab19202..82e8368ba6e6 100644 >>> --- a/include/linux/skbuff.h >>> +++ b/include/linux/skbuff.h >>> @@ -2459,6 +2459,14 @@ static inline void skb_set_tail_pointer(struct sk_buff *skb, const int offset) >>> >>> #endif /* NET_SKBUFF_DATA_USES_OFFSET */ >>> >>> +static inline void skb_assert_len(struct sk_buff *skb) >>> +{ >>> +#ifdef CONFIG_DEBUG_NET >>> + if (WARN_ONCE(!skb->len, "%s\n", __func__)) >>> + DO_ONCE_LITE(skb_dump, KERN_ERR, skb, false); >>> +#endif /* CONFIG_DEBUG_NET */ >>> +} >>> + >>> /* >>> * Add data to an sk_buff >>> */ >>> diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c >>> index 2ca96acbc50a..dc9dc0bedca0 100644 >>> --- a/net/bpf/test_run.c >>> +++ b/net/bpf/test_run.c >>> @@ -955,6 +955,9 @@ static int convert___skb_to_skb(struct sk_buff *skb, struct __sk_buff *__skb) >>> { >>> struct qdisc_skb_cb *cb = (struct qdisc_skb_cb *)skb->cb; >>> >>> + if (!skb->len) >>> + return -EINVAL; >> >> From another recent report [0], I don't think this change is fixing the report >> from syzbot. It probably makes sense to revert this patch. >> >> afaict, This '!skb->len' test is done after >> if (is_l2) >> __skb_push(skb, hh_len); >> >> Hence, skb->len is not zero in convert___skb_to_skb(). The proper place to test >> skb->len is before __skb_push() to ensure there is some network header after the >> mac or may as well ensure "data_size_in > ETH_HLEN" at the beginning. > > When is_l2==true, __skb_push will result in non-zero skb->len, so we > should be good, right? > The only issue is when we do bpf_redirect into a tunneling device and > do __skb_pull, but that's now fixed by [0]. > > When is_l2==false, the existing check in convert___skb_to_skb will > make sure there is something in the l3 headers. > > So it seems like this patch is still needed. Or am I missing something?
Replied in [0]. I think a small change in [0] will make this patch obsolete.
My thinking is the !skb->len test in this patch does not address all cases, at least not the most common one (the sch_cls prog where is_l2 == true) and then it needs another change in __bpf_redirect_no_mac [0]. Then, instead of breaking the existing test cases, may as well solely depend on the change in __bpf_redirect_no_mac which seems to be the only redirect function that does not have the len check now.
> >> The fix in [0] is applied. If it turns out there are other cases caused by the >> skb generated by test_run that needs extra fixes in bpf_redirect_*, it needs to >> revisit an earlier !skb->len check mentioned above and the existing test cases >> outside of test_progs would have to adjust accordingly. >> >> [0]: https://lore.kernel.org/bpf/20221027225537.353077-1-sdf@google.com/ >> >>> + >>> if (!__skb) >>> return 0; >>> >>> diff --git a/net/core/dev.c b/net/core/dev.c >>> index d588fd0a54ce..716df64fcfa5 100644 >>> --- a/net/core/dev.c >>> +++ b/net/core/dev.c >>> @@ -4168,6 +4168,7 @@ int __dev_queue_xmit(struct sk_buff *skb, struct net_device *sb_dev) >>> bool again = false; >>> >>> skb_reset_mac_header(skb); >>> + skb_assert_len(skb); >>> >>> if (unlikely(skb_shinfo(skb)->tx_flags & SKBTX_SCHED_TSTAMP)) >>> __skb_tstamp_tx(skb, NULL, NULL, skb->sk, SCM_TSTAMP_SCHED); >>
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