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

12 May 2025

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 jakob.unterwurzacher@cherry.de
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...
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



    