Re: [PATCH net v2 1/1] ipmr: only copy pktinfo to cache reports

On Thu, Jul 30, 2026 at 11:17:07AM +0800, Zhiling Zou wrote:

ipmr_cache_report() builds reports for mrouted from a packet that may be a
synthetic RTM_GETROUTE query. That query skb stores the netlink requester
portid in NETLINK_CB(), but the report is delivered to a raw IPv4 socket,
whose receive path interprets skb->cb as IPCB().

Commit bb7403655b3c ("ipmr: support IP_PKTINFO on cache report IGMP
msg") added IP_PKTINFO support by calling ipv4_pktinfo_prepare() on the
original packet and then copying the entire 48-byte skb control block to
the report skb. For synthetic route-query packets, this copies
NETLINK_CB() bytes into IPCB() and lets a controlled portid corrupt
IPCB(skb)->opt. With IP_RECVOPTS or IP_RETOPTS enabled, the raw socket
receive path can then copy past the short report packet or overflow the
stack option buffer.

Keep the IP_PKTINFO support, but initialize the report skb control buffer
and copy only the pktinfo fields prepared by ipv4_pktinfo_prepare(). Do
this in the common path so both short reports and whole-packet reports are
covered.

This removes the accidental IP_RECVOPTS/IP_RETOPTS behavior on the
mrouted socket, but that behavior was only a side effect of copying the
entire control block and was never intended to work for these reports.

Fixes: bb7403655b3c ("ipmr: support IP_PKTINFO on cache report IGMP msg")
Cc: <redacted>
Reported-by: Vega <redacted>
Signed-off-by: Zhiling Zou <redacted>
---
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 never intended
- move the pktinfo copy to the common path so both report branches are covered
- v1 Link: https://lore.kernel.org/all/5bc7cd71c2d671b305f25497d88ae8a0aa663c08.1784894076.git.zhilinz@nebusec.ai/ (local)

Sashiko keeps complaining about more bugs with this code. I think it's
better to fix one bug at a time (with different Fixes tags). Let's go
back to v1, but please remove the memset() given alloc_skb() takes care
of clearing the control block. I will fix the remaining problems after
your patch is accepted.