On 11/1/22 5:43 PM, Martin KaFai Lau wrote:
On 11/1/22 4:39 PM, Stanislav Fomichev wrote:
On Tue, Nov 1, 2022 at 1:28 PM Martin KaFai Lau <martin.lau@xxxxxxxxx> wrote:
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.
For the previous issue we've added "skb->len != 0" check which works
for the cases that remove l2.
Yeah, I replied on the "bpf: Don't redirect packets with invalid pkt_len" thread
which is hitting the same syzbot report afaict. I don't think that patch is
actually fixing it.
For the ones that don't, I think you're right, and checking at the
time of bpf_prog_test_run_skb can probably be enough, lemme try
(require ETH_HLEN+1 vs ETH_HLEN).
For some reason I was under the impression that Lorenz changed the
size from 0 to 14 [0], but he went from 14 to 15, so we won't break at
least cilium again..
CC'd him just in case.
0: https://github.com/cilium/ebpf/pull/788
Thanks for the pointer.
The cilium's prog is SOCKET_FILTER (not l2). It is why the new "skb->len != 0"
test broke it.
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?
Yeah, idk, this seems like a last resort? I'd prefer to explicitly
fail and communicate it back to the user than slap some extra byte and
then fail in some other place unpredictably?
If fixing in the fast path in filter.c, is __bpf_redirect_no_mac the only place
that needs this check? bpf_redirect_neigh() looks ok to me since the neigh
should have filled the mac header.
I took a closer look. This seems to be the only place needed the check, so
applied. If it turns out there are other cases caused by test-run generated
skb, we will revisit a fix in test_run.c and the existing tests have to adjust.
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;
+ }
One question, if the "!skb->len" check is deleted from convert___skb_to_skb(),
this "unlikely(!skb->len)" block here has to be moved out of the "if (mlen)"?
