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

[Ido Schimmel <idosch@nvidia.com>]

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: stable@vger.kernel.org
> Reported-by: Vega <vega@nebusec.ai>
> Signed-off-by: Zhiling Zou <zhilinz@nebusec.ai>
> ---
> 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/

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.