[PATCH 5.10 23/86] ip6_gre: Fix skb_under_panic in __gre6_xmit()

From: Greg Kroah-Hartman
Date: Tue Apr 26 2022 - 04:49:13 EST

From: Peilin Ye <peilin.ye@xxxxxxxxxxxxx>

[ Upstream commit ab198e1d0dd8dc4bc7575fb50758e2cbd51e14e1 ]

Feng reported an skb_under_panic BUG triggered by running
test_ip6gretap() in tools/testing/selftests/bpf/test_tunnel.sh:

[ 82.492551] skbuff: skb_under_panic: text:ffffffffb268bb8e len:403 put:12 head:ffff9997c5480000 data:ffff9997c547fff8 tail:0x18b end:0x2c0 dev:ip6gretap11
[ 82.607380] Call Trace:
[ 82.609389] <TASK>
[ 82.611136] skb_push.cold.109+0x10/0x10
[ 82.614289] __gre6_xmit+0x41e/0x590
[ 82.617169] ip6gre_tunnel_xmit+0x344/0x3f0
[ 82.620526] dev_hard_start_xmit+0xf1/0x330
[ 82.623882] sch_direct_xmit+0xe4/0x250
[ 82.626961] __dev_queue_xmit+0x720/0xfe0
[ 82.633431] packet_sendmsg+0x96a/0x1cb0
[ 82.636568] sock_sendmsg+0x30/0x40

The following sequence of events caused the BUG:

1. During ip6gretap device initialization, tunnel->tun_hlen (e.g. 4) is
calculated based on old flags (see ip6gre_calc_hlen());
2. packet_snd() reserves header room for skb A, assuming
tunnel->tun_hlen is 4;
3. Later (in clsact Qdisc), the eBPF program sets a new tunnel key for
skb A using bpf_skb_set_tunnel_key() (see _ip6gretap_set_tunnel());
4. __gre6_xmit() detects the new tunnel key, and recalculates
"tun_hlen" (e.g. 12) based on new flags (e.g. TUNNEL_KEY and
5. gre_build_header() calls skb_push() with insufficient reserved header
room, triggering the BUG.

As sugguested by Cong, fix it by moving the call to skb_cow_head() after
the recalculation of tun_hlen.



ip netns add at_ns0
ip link add veth0 type veth peer name veth1
ip link set veth0 netns at_ns0
ip netns exec at_ns0 ip addr add dev veth0
ip netns exec at_ns0 ip link set dev veth0 up
ip link set dev veth1 up mtu 1500
ip addr add dev veth1

ip netns exec at_ns0 ip addr add ::11/96 dev veth0
ip netns exec at_ns0 ip link set dev veth0 up
ip addr add dev veth1 ::22/96
ip link set dev veth1 up

ip netns exec at_ns0 \
ip link add dev ip6gretap00 type ip6gretap seq flowlabel 0xbcdef key 2 \
local ::11 remote ::22

ip netns exec at_ns0 ip addr add dev ip6gretap00
ip netns exec at_ns0 ip addr add dev ip6gretap00 fc80::100/96
ip netns exec at_ns0 ip link set dev ip6gretap00 up

ip link add dev ip6gretap11 type ip6gretap external
ip addr add dev ip6gretap11
ip addr add dev ip6gretap11 fc80::200/24
ip link set dev ip6gretap11 up

tc qdisc add dev ip6gretap11 clsact
tc filter add dev ip6gretap11 egress bpf da obj $OBJ sec ip6gretap_set_tunnel
tc filter add dev ip6gretap11 ingress bpf da obj $OBJ sec ip6gretap_get_tunnel

ping6 -c 3 -w 10 -q ::11

Fixes: 6712abc168eb ("ip6_gre: add ip6 gre and gretap collect_md mode")
Reported-by: Feng Zhou <zhoufeng.zf@xxxxxxxxxxxxx>
Co-developed-by: Cong Wang <cong.wang@xxxxxxxxxxxxx>
Signed-off-by: Cong Wang <cong.wang@xxxxxxxxxxxxx>
Signed-off-by: Peilin Ye <peilin.ye@xxxxxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
net/ipv6/ip6_gre.c | 9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/net/ipv6/ip6_gre.c b/net/ipv6/ip6_gre.c
index 949d6fbc1ca0..1f6c752f13b4 100644
--- a/net/ipv6/ip6_gre.c
+++ b/net/ipv6/ip6_gre.c
@@ -733,9 +733,6 @@ static netdev_tx_t __gre6_xmit(struct sk_buff *skb,
fl6->daddr = tunnel->parms.raddr;

- if (skb_cow_head(skb, dev->needed_headroom ?: tunnel->hlen))
- return -ENOMEM;
/* Push GRE header. */
protocol = (dev->type == ARPHRD_ETHER) ? htons(ETH_P_TEB) : proto;

@@ -763,6 +760,9 @@ static netdev_tx_t __gre6_xmit(struct sk_buff *skb,
tun_hlen = gre_calc_hlen(flags);

+ if (skb_cow_head(skb, dev->needed_headroom ?: tun_hlen + tunnel->encap_hlen))
+ return -ENOMEM;
gre_build_header(skb, tun_hlen,
flags, protocol,
@@ -773,6 +773,9 @@ static netdev_tx_t __gre6_xmit(struct sk_buff *skb,
if (tunnel->parms.o_flags & TUNNEL_SEQ)

+ if (skb_cow_head(skb, dev->needed_headroom ?: tunnel->hlen))
+ return -ENOMEM;
gre_build_header(skb, tunnel->tun_hlen, tunnel->parms.o_flags,
protocol, tunnel->parms.o_key,