Re: [PATCH bpf-next] bpf: make sure skb->len != 0 when redirecting to a tunneling device

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On 10/27/22 3:55 PM, Stanislav Fomichev wrote:
syzkaller managed to trigger another case where skb->len == 0
when we enter __dev_queue_xmit:

WARNING: CPU: 0 PID: 2470 at include/linux/skbuff.h:2576 skb_assert_len include/linux/skbuff.h:2576 [inline]
WARNING: CPU: 0 PID: 2470 at include/linux/skbuff.h:2576 __dev_queue_xmit+0x2069/0x35e0 net/core/dev.c:4295

Call Trace:
  dev_queue_xmit+0x17/0x20 net/core/dev.c:4406
  __bpf_tx_skb net/core/filter.c:2115 [inline]
  __bpf_redirect_no_mac net/core/filter.c:2140 [inline]
  __bpf_redirect+0x5fb/0xda0 net/core/filter.c:2163
  ____bpf_clone_redirect net/core/filter.c:2447 [inline]
  bpf_clone_redirect+0x247/0x390 net/core/filter.c:2419
  bpf_prog_48159a89cb4a9a16+0x59/0x5e
  bpf_dispatcher_nop_func include/linux/bpf.h:897 [inline]
  __bpf_prog_run include/linux/filter.h:596 [inline]
  bpf_prog_run include/linux/filter.h:603 [inline]
  bpf_test_run+0x46c/0x890 net/bpf/test_run.c:402
  bpf_prog_test_run_skb+0xbdc/0x14c0 net/bpf/test_run.c:1170
  bpf_prog_test_run+0x345/0x3c0 kernel/bpf/syscall.c:3648
  __sys_bpf+0x43a/0x6c0 kernel/bpf/syscall.c:5005
  __do_sys_bpf kernel/bpf/syscall.c:5091 [inline]
  __se_sys_bpf kernel/bpf/syscall.c:5089 [inline]
  __x64_sys_bpf+0x7c/0x90 kernel/bpf/syscall.c:5089
  do_syscall_64+0x54/0x70 arch/x86/entry/common.c:48
  entry_SYSCALL_64_after_hwframe+0x61/0xc6

The reproducer doesn't really reproduce outside of syzkaller
environment, so I'm taking a guess here. It looks like we
do generate correct ETH_HLEN-sized packet, but we redirect
the packet to the tunneling device. Before we do so, we
__skb_pull l2 header and arrive again at skb->len == 0.
Doesn't seem like we can do anything better than having
an explicit check after __skb_pull?
hmm... I recall there was similar report but I didn't follow those earlier fixes and discussion. Not sure if this has been considered:
If this skb can only happen in the bpf_prog_test_run (?),
how about ensure that the skb will at least have some header after l2 header in bpf_prog_test_run_skb(). Adding some headers/bytes if the data_size_in does not have it. This may break some external test cases that somehow has no l3/4? test_progs should be mostly fine considering they are using the pkt_v[46] in network_helpers.h.


Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
Reported-by: syzbot+f635e86ec3fa0a37e019@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Stanislav Fomichev <sdf@xxxxxxxxxx>
---
  net/core/filter.c | 4 ++++
  1 file changed, 4 insertions(+)

diff --git a/net/core/filter.c b/net/core/filter.c
index bb0136e7a8e4..cb3b635e35be 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -2126,6 +2126,10 @@ static int __bpf_redirect_no_mac(struct sk_buff *skb, struct net_device *dev,
if (mlen) {
  		__skb_pull(skb, mlen);
+		if (unlikely(!skb->len)) {
+			kfree_skb(skb);
+			return -ERANGE;
+		}
/* At ingress, the mac header has already been pulled once.
  		 * At egress, skb_pospull_rcsum has to be done in case that




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