[PATCH 5.4 85/86] tcp: do not mess with cloned skbs in tcp_add_backlog()

Greg Kroah-Hartman
2021-01-26T09:05:51Z

From: Eric Dumazet <edumazet@google.com>

commit b160c28548bc0a87cbd16d5af6d3edcfd70b8c9a upstream.

Heiner Kallweit reported that some skbs were sent with
the following invalid GSO properties :
- gso_size > 0
- gso_type == 0

This was triggerring a WARN_ON_ONCE() in rtl8169_tso_csum_v2.

Juerg Haefliger was able to reproduce a similar issue using
a lan78xx NIC and a workload mixing TCP incoming traffic
and forwarded packets.

The problem is that tcp_add_backlog() is writing
over gso_segs and gso_size even if the incoming packet will not
be coalesced to the backlog tail packet.

While skb_try_coalesce() would bail out if tail packet is cloned,
this overwriting would lead to corruptions of other packets
cooked by lan78xx, sharing a common super-packet.

The strategy used by lan78xx is to use a big skb, and split
it into all received packets using skb_clone() to avoid copies.
The drawback of this strategy is that all the small skb share a common
struct skb_shared_info.

This patch rewrites TCP gso_size/gso_segs handling to only
happen on the tail skb, since skb_try_coalesce() made sure
it was not cloned.

Fixes: 4f693b55c3d2 ("tcp: implement coalescing on backlog queue")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Bisected-by: Juerg Haefliger <juergh@canonical.com>
Tested-by: Juerg Haefliger <juergh@canonical.com>
Reported-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=209423
Link: https://lore.kernel.org/r/20210119164900.766957-1-eric.dumazet@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
net/ipv4/tcp_ipv4.c | 25 +++++++++++++------------
1 file changed, 13 insertions(+), 12 deletions(-)

--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1657,6 +1657,7 @@ int tcp_v4_early_demux(struct sk_buff *s
bool tcp_add_backlog(struct sock *sk, struct sk_buff *skb)
{
u32 limit = READ_ONCE(sk->sk_rcvbuf) + READ_ONCE(sk->sk_sndbuf);
+ u32 tail_gso_size, tail_gso_segs;
struct skb_shared_info *shinfo;
const struct tcphdr *th;
struct tcphdr *thtail;
@@ -1664,6 +1665,7 @@ bool tcp_add_backlog(struct sock *sk, st
unsigned int hdrlen;
bool fragstolen;
u32 gso_segs;
+ u32 gso_size;
int delta;

/* In case all data was pulled from skb frags (in __pskb_pull_tail()),
@@ -1689,13 +1691,6 @@ bool tcp_add_backlog(struct sock *sk, st
*/
th = (const struct tcphdr *)skb->data;
hdrlen = th->doff * 4;
- shinfo = skb_shinfo(skb);
-
- if (!shinfo->gso_size)
- shinfo->gso_size = skb->len - hdrlen;
-
- if (!shinfo->gso_segs)
- shinfo->gso_segs = 1;

tail = sk->sk_backlog.tail;
if (!tail)
@@ -1718,6 +1713,15 @@ bool tcp_add_backlog(struct sock *sk, st
goto no_coalesce;

__skb_pull(skb, hdrlen);
+
+ shinfo = skb_shinfo(skb);
+ gso_size = shinfo->gso_size ?: skb->len;
+ gso_segs = shinfo->gso_segs ?: 1;
+
+ shinfo = skb_shinfo(tail);
+ tail_gso_size = shinfo->gso_size ?: (tail->len - hdrlen);
+ tail_gso_segs = shinfo->gso_segs ?: 1;
+
if (skb_try_coalesce(tail, skb, &fragstolen, &delta)) {
TCP_SKB_CB(tail)->end_seq = TCP_SKB_CB(skb)->end_seq;

@@ -1744,11 +1748,8 @@ bool tcp_add_backlog(struct sock *sk, st
}

/* Not as strict as GRO. We only need to carry mss max value */
- skb_shinfo(tail)->gso_size = max(shinfo->gso_size,
- skb_shinfo(tail)->gso_size);
-
- gso_segs = skb_shinfo(tail)->gso_segs + shinfo->gso_segs;
- skb_shinfo(tail)->gso_segs = min_t(u32, gso_segs, 0xFFFF);
+ shinfo->gso_size = max(gso_size, tail_gso_size);
+ shinfo->gso_segs = min_t(u32, gso_segs + tail_gso_segs, 0xFFFF);

sk->sk_backlog.len += delta;
__NET_INC_STATS(sock_net(sk),