[PATCH net v2] net: dsa: microchip: linearize skb for tail-tagging switches

The pointer arithmentic for accessing the tail tag only works
for linear skbs.

For nonlinear skbs, it reads uninitialized memory inside the
skb headroom, essentially randomizing the tag. I have observed
it gets set to 6 most of the time.

Example where ksz9477_rcv thinks that the packet from port 1 comes from port 6
(which does not exist for the ksz9896 that's in use), dropping the packet.
Debug prints added by me (not included in this patch):

	[  256.645337] ksz9477_rcv:323 tag0=6
	[  256.645349] skb len=47 headroom=78 headlen=0 tailroom=0
	               mac=(64,14) mac_len=14 net=(78,0) trans=78
	               shinfo(txflags=0 nr_frags=1 gso(size=0 type=0 segs=0))
	               csum(0x0 start=0 offset=0 ip_summed=0 complete_sw=0 valid=0 level=0)
	               hash(0x0 sw=0 l4=0) proto=0x00f8 pkttype=1 iif=3
	               priority=0x0 mark=0x0 alloc_cpu=0 vlan_all=0x0
	               encapsulation=0 inner(proto=0x0000, mac=0, net=0, trans=0)
	[  256.645377] dev name=end1 feat=0x0002e10200114bb3
	[  256.645386] skb headroom: 00000000: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
	[  256.645395] skb headroom: 00000010: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
	[  256.645403] skb headroom: 00000020: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
	[  256.645411] skb headroom: 00000030: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
	[  256.645420] skb headroom: 00000040: ff ff ff ff ff ff 00 1c 19 f2 e2 db 08 06
	[  256.645428] skb frag:     00000000: 00 01 08 00 06 04 00 01 00 1c 19 f2 e2 db 0a 02
	[  256.645436] skb frag:     00000010: 00 83 00 00 00 00 00 00 0a 02 a0 2f 00 00 00 00
	[  256.645444] skb frag:     00000020: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 01
	[  256.645452] ksz_common_rcv:92 dsa_conduit_find_user returned NULL

Call skb_linearize before trying to access the tag.

This patch fixes ksz9477_rcv which is used by the ksz9896 I have at
hand, and also applies the same fix to ksz8795_rcv which seems to have
the same problem.

Signed-off-by: Jakob Unterwurzacher <redacted>
Cc: stable@vger.kernel.org
Fixes: 016e43a26bab ("net: dsa: ksz: Add KSZ8795 tag code")
Fixes: 8b8010fb7876 ("dsa: add support for Microchip KSZ tail tagging)
---
v1: https://lore.kernel.org/netdev/20250509071820.4100022-1-jakob.unterwurzacher@cherry.de/ (local)
v2: add Fixes tags, Cc stable, "[PATCH net]" prefix

 net/dsa/tag_ksz.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/net/dsa/tag_ksz.c b/net/dsa/tag_ksz.c
index 281bbac5539d..55ef093fe66b 100644
--- a/net/dsa/tag_ksz.c
+++ b/net/dsa/tag_ksz.c

@@ -140,7 +140,12 @@ static struct sk_buff *ksz8795_xmit(struct sk_buff *skb, struct net_device *dev)
 
 static struct sk_buff *ksz8795_rcv(struct sk_buff *skb, struct net_device *dev)
 {
-	u8 *tag = skb_tail_pointer(skb) - KSZ_EGRESS_TAG_LEN;
+	u8 *tag;
+
+	if (skb_linearize(skb))
+		return NULL;
+
+	tag = skb_tail_pointer(skb) - KSZ_EGRESS_TAG_LEN;
 
 	return ksz_common_rcv(skb, dev, tag[0] & KSZ8795_TAIL_TAG_EG_PORT_M,
 			      KSZ_EGRESS_TAG_LEN);

@@ -311,8 +316,13 @@ static struct sk_buff *ksz9477_xmit(struct sk_buff *skb,
 
 static struct sk_buff *ksz9477_rcv(struct sk_buff *skb, struct net_device *dev)
 {
+	u8 *tag;
+
+	if (skb_linearize(skb))
+		return NULL;
+
 	/* Tag decoding */
-	u8 *tag = skb_tail_pointer(skb) - KSZ_EGRESS_TAG_LEN;
+	tag = skb_tail_pointer(skb) - KSZ_EGRESS_TAG_LEN;
 	unsigned int port = tag[0] & KSZ9477_TAIL_TAG_EG_PORT_M;
 	unsigned int len = KSZ_EGRESS_TAG_LEN;
 

-- 
2.39.5