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.
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);
