[PATCH net 2/3] wireguard: queueing: preserve tstamp_type when encapsulating packet

["Jason A. Donenfeld" <Jason@zx2c4.com>]

From: Ramses de Norre <ramses@well-founded.dev>

Sending traffic through a wireguard tunnel on a host using the fq
qdisc fills the log with:

  fq: likely mono tstamp with tstamp_type 0

An skb carries a timestamp in skb->tstamp and, separately, a
skb->tstamp_type field recording which clock that timestamp came from.
The two have to agree.

When wireguard encapsulates a packet it calls wg_reset_packet(), which
clears the fields that must not leak from the inner packet into the
tunnel packet. It does so in two steps:

  skb_scrub_packet(skb, true);
  memset(&skb->headers, 0, sizeof(skb->headers));

skb_scrub_packet() deliberately keeps skb->tstamp when it holds a
monotonic timestamp: that value is the time the packet is scheduled to
be sent, and the qdisc still needs it. The memset then zeroes
skb->tstamp_type, because that field sits inside the headers group
while skb->tstamp does not. The packet therefore leaves wireguard
carrying a monotonic timestamp labelled as a realtime one.

Nothing noticed until commit c4f796c4f16b ("net_sched: sch_fq: convert
skb->tstamp if not monotonic"): fq used to assume every timestamp was
monotonic. It now consults tstamp_type, spots the mismatch, warns, and
falls back to treating the value as monotonic. Pacing still ends up
correct, so the log spam is the actual problem.

Save tstamp_type before the memset and restore it when encapsulating,
next to the hash fields that are already carried over this way. When
decapsulating it stays zeroed, which is right: an incoming packet's
timestamp is a realtime receive timestamp.

Fixes: d98d58a00261 ("net: Set skb->mono_delivery_time and clear it after sch_handle_ingress()")
Signed-off-by: Ramses de Norre <ramses@well-founded.dev>
Reviewed-by: Toke Høiland-Jørgensen <toke@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 drivers/net/wireguard/queueing.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/net/wireguard/queueing.h b/drivers/net/wireguard/queueing.h
index 79b6d70de236..663b19b78395 100644
--- a/drivers/net/wireguard/queueing.h
+++ b/drivers/net/wireguard/queueing.h
@@ -78,12 +78,14 @@ static inline void wg_reset_packet(struct sk_buff *skb, bool encapsulating)
 	u8 l4_hash = skb->l4_hash;
 	u8 sw_hash = skb->sw_hash;
 	u32 hash = skb->hash;
+	u8 tstamp_type = skb->tstamp_type;
 	skb_scrub_packet(skb, true);
 	memset(&skb->headers, 0, sizeof(skb->headers));
 	if (encapsulating) {
 		skb->l4_hash = l4_hash;
 		skb->sw_hash = sw_hash;
 		skb->hash = hash;
+		skb->tstamp_type = tstamp_type;
 	}
 	skb->queue_mapping = 0;
 	skb->nohdr = 0;
-- 
2.56.0