[PATCH AUTOSEL 5.15 11/46] bpf: make sure skb->len != 0 when redirecting to a tunneling device

Sasha Levin
2022-12-18T16:38:40Z

From: Stanislav Fomichev <sdf@google.com>

[ Upstream commit 07ec7b502800ba9f7b8b15cb01dd6556bb41aaca ]

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?

Cc: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot+f635e86ec3fa0a37e019@syzkaller.appspotmail.com
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/r/20221027225537.353077-1-sdf@google.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
net/core/filter.c | 4 ++++
1 file changed, 4 insertions(+)

diff --git a/net/core/filter.c b/net/core/filter.c
index fb5b9dbf3bc0..1a7903812a22 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -2125,6 +2125,10 @@ 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
--
2.35.1