[PATCH net v2] net/iucv: take a private, writable frame before rewriting it in place

[Bryam Vargas via B4 Relay]

From: Bryam Vargas <hexlabsecurity@xxxxxxxxx>

afiucv_hs_rcv() rewrites the frame in place -- EBCASC() on four name
fields, then afiucv_swap_src_dest() swaps them and pushes an Ethernet
header -- and can hand the same skb to dev_queue_xmit(), without making
it private first, so a packet socket that also gets the frame reads the
rewritten names. An ETH_P_ALL tap runs first and leaves af_iucv a
cloned skb; a socket opened for ETH_P_AF_IUCV sits on a ptype_specific
list, walked after ptype_base[], so deliver_skb() takes a reference and
af_iucv runs on an skb that is shared but not cloned.

Unshare, then cow the head, in that order: skb_cow_head() can reach
pskb_expand_head(), which has BUG_ON(skb_shared()). Asking for ETH_HLEN
also covers the unchecked push in afiucv_swap_src_dest(); on the ordinary
path eth_type_trans() has already pulled that much, so it only compares.

Fixes: 3881ac441f64 ("af_iucv: add HiperSockets transport")
Closes: https://sashiko.dev/#/patchset/20260813-b4-disp-60433a46-v1-1-509e1200533e@xxxxxxxxx?part=1
Signed-off-by: Bryam Vargas <hexlabsecurity@xxxxxxxxx>
Reviewed-by: Alexandra Winter <wintera@xxxxxxxxxxxxx>
---
Sorry for the long silence since August: a major earthquake hit where I
live, and I was finishing a postgrad program. Back now.

v2: the old 2/2 alone. 1/2 is dropped; Alexandra's 80230a18c164
    ("net/iucv: filter frames in afiucv_hs_rcv() by ingress device")
    replaced it and is in mainline. Rebased on net, no code change,
    carries Alexandra's R-b. Kept on net; Alexandra agreed on v1 to
    treat it as a fix. The commit message now covers both cases: v1
    described the ETH_P_ALL tap, where af_iucv gets a cloned skb, and
    the v1 thread added the ETH_P_AF_IUCV socket, where it gets a shared
    one. My reply there said v1 had the delivery order backwards. It
    didn't; both cases are real, and the A/B below shows each call is
    needed.

v1: https://lore.kernel.org/all/20260815-b4-disp-dc82fde4-v1-0-e83b10b22ce9@xxxxxxxxx/

Tested on s390x under QEMU TCG, KASAN and lockdep on. No z/VM there, so
af_iucv gets its frames from lo (80230a18c164 filters after the
EBCASC(), so lo frames still hit the rewrite), and a packet socket
reads each one back. Unpatched, the tap and the ETH_P_AF_IUCV socket
both get the 48 name bytes already in ASCII, and a SYN with no listener
never reaches that socket as a 0xfbfb frame. Patched, every frame
arrives as sent. Drop skb_share_check() and the ETH_P_AF_IUCV socket is
broken again; drop skb_cow_head() and both are. FWIW no splat in any
boot, patched or not -- on lo the damage is in what the other receivers
read. Builds clean for s390 with W=1. Repro and logs on request.
---
 net/iucv/af_iucv.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/net/iucv/af_iucv.c b/net/iucv/af_iucv.c
index db261ecd19af..493dc156e9f5 100644
--- a/net/iucv/af_iucv.c
+++ b/net/iucv/af_iucv.c
@@ -2072,11 +2072,20 @@ static int afiucv_hs_rcv(struct sk_buff *skb, struct net_device *dev,
 	int err = NET_RX_SUCCESS;
 	char nullstring[8];
 
+	skb = skb_share_check(skb, GFP_ATOMIC);
+	if (!skb)
+		return NET_RX_SUCCESS;
+
 	if (!pskb_may_pull(skb, sizeof(*trans_hdr))) {
 		kfree_skb(skb);
 		return NET_RX_SUCCESS;
 	}
 
+	if (skb_cow_head(skb, ETH_HLEN)) {
+		kfree_skb(skb);
+		return NET_RX_SUCCESS;
+	}
+
 	trans_hdr = iucv_trans_hdr(skb);
 	EBCASC(trans_hdr->destAppName, sizeof(trans_hdr->destAppName));
 	EBCASC(trans_hdr->destUserID, sizeof(trans_hdr->destUserID));

---
base-commit: 6d25ffca055a77787c21a36b66c253f76239411b
change-id: 20261008-b4-disp-bf46f1ba-0f5bc3fc02c4

Best regards,
--  
Bryam Vargas <hexlabsecurity@xxxxxxxxx>