CVE-2026-23255: net: add proper RCU protection to /proc/net/ptype

In the Linux kernel, the following vulnerability has been resolved:

net: add proper RCU protection to /proc/net/ptype

Yin Fengwei reported an RCU stall in ptypeseqshow() and provided a patch.

Real issue is that ptypeseqnext() and ptypeseqshow() violate RCU rules.

ptypeseqshow() runs under rcureadlock(), and reads pt->dev to get device name without any barrier.

At the same time, concurrent writers can remove a packettype structure (which is correctly freed after an RCU grace period) and clear pt->dev without an RCU grace period.

Define ptypeiterstate to carry a dev pointer along seqnetprivate:

struct ptypeiterstate { struct seqnetprivate p; struct netdevice dev; // added in this patch };

We need to record the device pointer in ptypegetidx() and ptypeseqnext() so that ptypeseqshow() is safe against concurrent pt->dev changes.

We also need to add full RCU protection in ptypeseqnext(). (Missing READONCE() when reading list.next values)

Many thanks to Dong Chenchen for providing a repro.