[PATCH net v6 1/1] ipmr: only copy pktinfo to cache reports

ipmr_cache_report() prepares IP_PKTINFO on the original skb. The
ipv4_pktinfo_prepare() helper updates skb->cb and clears the pktinfo fields
when packet info is disabled. Since the skb can continue through the data
path, preserve and restore its original control block around the helper.

The original implementation copies the entire control block to the report
skb. For synthetic RTM_GETROUTE packets, NETLINK_CB data is interpreted as
IPCB by the raw IPv4 receive path. With IP_RECVOPTS or IP_RETOPTS enabled,
corrupted option offsets cause out-of-bounds reads or a stack buffer
overflow. WHOLEPKT and WRVIFWHOLE reports also inherit stale IP option
offsets when their header is prepended.

Clear the report skb control block. Only prepare and copy pktinfo when the
input skb has an IPv4 header. Synthetic route-query skbs use NETLINK_CB,
not IPCB, so leave their report control block zeroed. Restore pkt->cb after
preparing pktinfo for regular IPv4 packets.

Fixes: bb7403655b3c ("ipmr: support IP_PKTINFO on cache report IGMP msg")
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Reported-by: Vega <redacted>
Link: https://lore.kernel.org/netdev/20260903113055.GA90787@shredder/ (local)
Suggested-by: Ido Schimmel <idosch@nvidia.com>
Suggested-by: Eric Dumazet <redacted>
Assisted-by: LLM
Signed-off-by: Zhiling Zou <redacted>
---
changes in v6:
- Skip ipv4_pktinfo_prepare() for synthetic route-query skbs, whose cb
  contains NETLINK_CB rather than IPCB, and leave report skb->cb zeroed.
- v5 Link: https://lore.kernel.org/netdev/192b5c65be8b370c1e6e4f6a31edf03d2606f427.1792215075.git.zhilinz@nebusec.ai/ (local)

changes in v5:
- Save and restore pkt->cb around ipv4_pktinfo_prepare().
- Prepare pktinfo after both report branches, clear skb->cb, and copy only
  the pktinfo fields; this also clears inherited IP options.
- Add the Fixes tag for the older WHOLEPKT/WRVIFWHOLE issue.
- v4 Link: https://lore.kernel.org/netdev/d8a36c317f6216d20f9cfd7e49f1888f62c35f7c.1788417128.git.zhilinz@nebusec.ai/ (local)

changes in v4:
- Clear IPCB(skb)->opt when prepending the 20-byte WHOLEPKT/WRVIFWHOLE
  header, so stale option offsets are not inherited from the original packet.
- v3 Link: https://lore.kernel.org/all/e93d0d2fa5725ddd06b20e3e4223ab68b93b48d1.1785719031.git.zhilinz@nebusec.ai/ (local)

changes in v3:
- Go back to the v1 scope and stop changing the common path.
- Remove memset(), since alloc_skb() already clears the control block.
- v2 Link: https://lore.kernel.org/all/1b809975e5bd9c0a1dd6fdd1db534e701fe5a4b6.1785379072.git.zhilinz@nebusec.ai/ (local)

changes in v2:
- Reword the commit message to say the entire 48-byte skb control block
  is copied.
- Mention that IP_RECVOPTS/IP_RETOPTS on the mrouted socket was unintended.
- Move the pktinfo copy to the common path to cover both report branches.
- v1 Link: https://lore.kernel.org/all/5bc7cd71c2d671b305f25497d88ae8a0aa663c08.1784894076.git.zhilinz@nebusec.ai/ (local)

 net/ipv4/ipmr.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/ipmr.c b/net/ipv4/ipmr.c
index b9c544d48c452..6635ea6b9209a 100644
--- a/net/ipv4/ipmr.c
+++ b/net/ipv4/ipmr.c

@@ -1057,6 +1057,7 @@ static void ipmr_cache_resolve(struct net *net, struct mr_table *mrt,
 static int ipmr_cache_report(const struct mr_table *mrt,
 			     struct sk_buff *pkt, vifi_t vifi, int assert)
 {
+	unsigned char pkt_cb[sizeof(pkt->cb)];
 	const int ihl = ip_hdrlen(pkt);
 	struct sock *mroute_sk;
 	struct igmphdr *igmp;

@@ -1112,8 +1113,6 @@ static int ipmr_cache_report(const struct mr_table *mrt,
 		msg = (struct igmpmsg *)skb_network_header(skb);
 		msg->im_vif = vifi;
 		msg->im_vif_hi = vifi >> 8;
-		ipv4_pktinfo_prepare(mroute_sk, pkt, false);
-		memcpy(skb->cb, pkt->cb, sizeof(skb->cb));
 		/* Add our header.
 		 * Note that code, csum and group fields are cleared.
 		 */

@@ -1124,6 +1123,15 @@ static int ipmr_cache_report(const struct mr_table *mrt,
 		skb->transport_header = skb->network_header;
 	}
 
+	memset(skb->cb, 0, sizeof(skb->cb));
+	/* Route queries from ipmr_get_route() carry NETLINK_CB, not IPCB. */
+	if (ip_hdr(pkt)->version) {
+		memcpy(pkt_cb, pkt->cb, sizeof(pkt_cb));
+		ipv4_pktinfo_prepare(mroute_sk, pkt, false);
+		*PKTINFO_SKB_CB(skb) = *PKTINFO_SKB_CB(pkt);
+		memcpy(pkt->cb, pkt_cb, sizeof(pkt_cb));
+	}
+
 	igmpmsg_netlink_event(mrt, skb);
 
 	/* Deliver to mrouted */

-- 
2.55.0