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Date:   Fri, 16 Oct 2020 09:35:27 +0200
From:   Christian Eggers <>
To:     Woojung Huh <>,
        Andrew Lunn <>,
        Vivien Didelot <>,
        Florian Fainelli <>,
        Vladimir Oltean <>
CC:     Microchip Linux Driver Support <>,
        "David S . Miller" <>,
        Jakub Kicinski <>, <>,
        <>, Christian Eggers <>
Subject: [PATCH net] net: dsa: ksz: don't pad a cloned sk_buff

If the supplied sk_buff is cloned (e.g. in dsa_skb_tx_timestamp()),
__skb_put_padto() will allocate a new sk_buff with size = skb->len +
padlen. So the condition just tested for (skb_tailroom(skb) >= padlen +
len) is not fulfilled anymore. Although the real size will usually be
larger than skb->len + padlen (due to alignment), there is no guarantee
that the required memory for the tail tag will be available

Instead of letting __skb_put_padto allocate a new (too small) sk_buff,
lets take the already existing path and allocate a new sk_buff ourself
(with sufficient size).

Fixes: 8b8010fb7876 ("dsa: add support for Microchip KSZ tail tagging")
Signed-off-by: Christian Eggers <>
I am not sure whether this is a problem for current kernels (it depends
whether cloned sk_buffs can happen on any paths). But when adding time
stamping (will be submitted soon), this will become an issue.

This patch supersedes "net: dsa: ksz: fix padding size of skb" from

 net/dsa/tag_ksz.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/dsa/tag_ksz.c b/net/dsa/tag_ksz.c
index 945a9bd5ba35..cb1f27e15201 100644
--- a/net/dsa/tag_ksz.c
+++ b/net/dsa/tag_ksz.c
@@ -22,7 +22,7 @@ static struct sk_buff *ksz_common_xmit(struct sk_buff *skb,
 	padlen = (skb->len >= ETH_ZLEN) ? 0 : ETH_ZLEN - skb->len;
-	if (skb_tailroom(skb) >= padlen + len) {
+	if (skb_tailroom(skb) >= padlen + len && !skb_cloned(skb)) {
 		/* Let dsa_slave_xmit() free skb */
 		if (__skb_put_padto(skb, skb->len + padlen, false))
 			return NULL;
@@ -45,7 +45,7 @@ static struct sk_buff *ksz_common_xmit(struct sk_buff *skb,
 		/* Let skb_put_padto() free nskb, and let dsa_slave_xmit() free
 		 * skb
-		if (skb_put_padto(nskb, nskb->len + padlen))
+		if (skb_put_padto(nskb, ETH_ZLEN + len))
 			return NULL;
Christian Eggers
Embedded software developer

Arnold & Richter Cine Technik GmbH & Co. Betriebs KG
Sitz: Muenchen - Registergericht: Amtsgericht Muenchen - Handelsregisternummer: HRA 57918
Persoenlich haftender Gesellschafter: Arnold & Richter Cine Technik GmbH
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