[PATCH net 3/3] ip_gre, ip6_gre: Fix race condition on o_seqno in collect_md mode

Peilin Ye
2022-04-21T22:09:18Z

As pointed out by Jakub Kicinski, currently using TUNNEL_SEQ in
collect_md mode is racy for [IP6]GRE[TAP] devices. Consider the
following sequence of events:

1. An [IP6]GRE[TAP] device is created in collect_md mode using "ip link
add ... external". "ip" ignores "[o]seq" if "external" is specified,
so TUNNEL_SEQ is off, and the device is marked as NETIF_F_LLTX (i.e.
it uses lockless TX);
2. Someone sets TUNNEL_SEQ on outgoing skb's, using e.g.
bpf_skb_set_tunnel_key() in an eBPF program attached to this device;
3. gre_fb_xmit() or __gre6_xmit() processes these skb's:

gre_build_header(skb, tun_hlen,
flags, protocol,
tunnel_id_to_key32(tun_info->key.tun_id),
(flags & TUNNEL_SEQ) ? htonl(tunnel->o_seqno++)
: 0); ^^^^^^^^^^^^^^^^^

Since we are not using the TX lock (&txq->_xmit_lock), multiple CPUs may
try to do this tunnel->o_seqno++ in parallel, which is racy. Fix it by
making o_seqno atomic_t.

As mentioned by Eric Dumazet in commit b790e01aee74 ("ip_gre: lockless
xmit"), making o_seqno atomic_t increases "chance for packets being out
of order at receiver" when NETIF_F_LLTX is on.

Maybe a better fix would be:

1. Do not ignore "oseq" in external mode. Users MUST specify "oseq" if
they want the kernel to allow sequencing of outgoing packets;
2. Reject all outgoing TUNNEL_SEQ packets if the device was not created
with "oseq".

Unfortunately, that would break userspace.

We could now make [IP6]GRE[TAP] devices always NETIF_F_LLTX, but let us
do it in separate patches to keep this fix minimal.

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Fixes: 77a5196a804e ("gre: add sequence number for collect md mode.")
Signed-off-by: Peilin Ye <peilin.ye@bytedance.com>
---
include/net/ip6_tunnel.h | 2 +-
include/net/ip_tunnels.h | 2 +-
net/ipv4/ip_gre.c | 6 +++---
net/ipv6/ip6_gre.c | 7 ++++---
4 files changed, 9 insertions(+), 8 deletions(-)

diff --git a/include/net/ip6_tunnel.h b/include/net/ip6_tunnel.h
index a38c4f1e4e5c..74b369bddf49 100644
--- a/include/net/ip6_tunnel.h
+++ b/include/net/ip6_tunnel.h
@@ -58,7 +58,7 @@ struct ip6_tnl {

/* These fields used only by GRE */
__u32 i_seqno; /* The last seen seqno */
- __u32 o_seqno; /* The last output seqno */
+ atomic_t o_seqno; /* The last output seqno */
int hlen; /* tun_hlen + encap_hlen */
int tun_hlen; /* Precalculated header length */
int encap_hlen; /* Encap header length (FOU,GUE) */
diff --git a/include/net/ip_tunnels.h b/include/net/ip_tunnels.h
index 0219fe907b26..3ec6146f8734 100644
--- a/include/net/ip_tunnels.h
+++ b/include/net/ip_tunnels.h
@@ -116,7 +116,7 @@ struct ip_tunnel {

/* These four fields used only by GRE */
u32 i_seqno; /* The last seen seqno */
- u32 o_seqno; /* The last output seqno */
+ atomic_t o_seqno; /* The last output seqno */
int tun_hlen; /* Precalculated header length */

/* These four fields used only by ERSPAN */
diff --git a/net/ipv4/ip_gre.c b/net/ipv4/ip_gre.c
index ca70b92e80d9..8cf86e42c1d1 100644
--- a/net/ipv4/ip_gre.c
+++ b/net/ipv4/ip_gre.c
@@ -464,7 +464,7 @@ static void __gre_xmit(struct sk_buff *skb, struct net_device *dev,
/* Push GRE header. */
gre_build_header(skb, tunnel->tun_hlen,
flags, proto, tunnel->parms.o_key,
- (flags & TUNNEL_SEQ) ? htonl(tunnel->o_seqno++) : 0);
+ (flags & TUNNEL_SEQ) ? htonl(atomic_fetch_inc(&tunnel->o_seqno)) : 0);

ip_tunnel_xmit(skb, dev, tnl_params, tnl_params->protocol);
}
@@ -502,7 +502,7 @@ static void gre_fb_xmit(struct sk_buff *skb, struct net_device *dev,
(TUNNEL_CSUM | TUNNEL_KEY | TUNNEL_SEQ);
gre_build_header(skb, tunnel_hlen, flags, proto,
tunnel_id_to_key32(tun_info->key.tun_id),
- (flags & TUNNEL_SEQ) ? htonl(tunnel->o_seqno++) : 0);
+ (flags & TUNNEL_SEQ) ? htonl(atomic_fetch_inc(&tunnel->o_seqno)) : 0);

ip_md_tunnel_xmit(skb, dev, IPPROTO_GRE, tunnel_hlen);

@@ -579,7 +579,7 @@ static void erspan_fb_xmit(struct sk_buff *skb, struct net_device *dev)
}

gre_build_header(skb, 8, TUNNEL_SEQ,
- proto, 0, htonl(tunnel->o_seqno++));
+ proto, 0, htonl(atomic_fetch_inc(&tunnel->o_seqno)));

ip_md_tunnel_xmit(skb, dev, IPPROTO_GRE, tunnel_hlen);

diff --git a/net/ipv6/ip6_gre.c b/net/ipv6/ip6_gre.c
index d9e4ac94eab4..5136959b3dc5 100644
--- a/net/ipv6/ip6_gre.c
+++ b/net/ipv6/ip6_gre.c
@@ -766,7 +766,7 @@ static netdev_tx_t __gre6_xmit(struct sk_buff *skb,
gre_build_header(skb, tun_hlen,
flags, protocol,
tunnel_id_to_key32(tun_info->key.tun_id),
- (flags & TUNNEL_SEQ) ? htonl(tunnel->o_seqno++)
+ (flags & TUNNEL_SEQ) ? htonl(atomic_fetch_inc(&tunnel->o_seqno))
: 0);

} else {
@@ -777,7 +777,8 @@ static netdev_tx_t __gre6_xmit(struct sk_buff *skb,

gre_build_header(skb, tunnel->tun_hlen, flags,
protocol, tunnel->parms.o_key,
- (flags & TUNNEL_SEQ) ? htonl(tunnel->o_seqno++) : 0);
+ (flags & TUNNEL_SEQ) ? htonl(atomic_fetch_inc(&tunnel->o_seqno))
+ : 0);
}

return ip6_tnl_xmit(skb, dev, dsfield, fl6, encap_limit, pmtu,
@@ -1055,7 +1056,7 @@ static netdev_tx_t ip6erspan_tunnel_xmit(struct sk_buff *skb,
/* Push GRE header. */
proto = (t->parms.erspan_ver == 1) ? htons(ETH_P_ERSPAN)
: htons(ETH_P_ERSPAN2);
- gre_build_header(skb, 8, TUNNEL_SEQ, proto, 0, htonl(t->o_seqno++));
+ gre_build_header(skb, 8, TUNNEL_SEQ, proto, 0, htonl(atomic_fetch_inc(&t->o_seqno)));

/* TooBig packet may have updated dst->dev's mtu */
if (!t->parms.collect_md && dst && dst_mtu(dst) > dst->dev->mtu)
--
2.20.1