" On Thu, Nov 3, 2022 at 3:58 PM Stanislav Fomichev <sdf@xxxxxxxxxx> wrote: > > On Thu, Nov 3, 2022 at 3:42 PM Martin KaFai Lau <martin.lau@xxxxxxxxx> wrote: > > > > On 11/3/22 2:36 PM, Stanislav Fomichev wrote: > > > On Thu, Nov 3, 2022 at 2:07 PM Martin KaFai Lau <martin.lau@xxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxxxxxx > > >>> Signed-off-by: Zhengchao Shao <shaozhengchao@xxxxxxxxxx> > > >>> --- > > >>> 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. > > Removing this check in convert___skb_to_skb and moving the new one in > __bpf_redirect_no_mac out of (mlen) SGTM. > Can follow up unless you or Zhengchao prefer to do it. > There were some concerns about doing this len check at runtime per > packet, but not sure whether it really affects anything.. I've implemented a simple selftest for both mac/no-mac cases, and I feel like this explicit len==0 has to happen in both cases. That "skb->mac_header >= skb->network_header" check is not enough :-( I'll send the series early next week after another round of xdp metadata.. > > >> 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@xxxxxxxxxx/ > > >> > > >>> + > > >>> 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); > > >> > >
