CVE-2026-64563: rhashtable: clear stale iter->p on table restart

From: Greg Kroah-Hartman <[email protected]>

Description
===========

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

rhashtable: clear stale iter->p on table restart

rhashtable_walk_start_check() has two restart paths when resuming a walk.
When iter->walker.tbl is valid, it re-validates iter->p against the table
and sets iter->p = NULL if the object is gone.  When iter->walker.tbl is
NULL (table was freed during resize), it resets slot and skip but forgets
to clear iter->p.

rhashtable_walk_next() then dereferences the stale iter->p, reading
freed memory.  This is a use-after-free.

Any caller that does multi-fragment rhashtable walks across
walk_stop/walk_start boundaries is affected.  Concrete cases include
netlink_diag (__netlink_diag_dump in net/netlink/diag.c) and TIPC
(tipc_nl_sk_walk in net/tipc/socket.c).

Crash stack (netlink_diag):
  BUG: KASAN: slab-use-after-free in rhashtable_walk_next+0x365/0x3c0
  Read of size 8 at addr ffff88801a9d2438 (freed kmalloc-2k, offset 1080)
  Call Trace:
   rhashtable_walk_next+0x365/0x3c0 (lib/rhashtable.c:1016)
   __netlink_diag_dump+0x160/0x760 (net/netlink/diag.c:122)
   netlink_diag_dump+0xc2/0x240
   netlink_dump+0x5bc/0x1270
   netlink_recvmsg+0x7a3/0x980
   sock_recvmsg+0x1bc/0x200
   __sys_recvfrom+0x1d4/0x2c0

The Linux kernel CVE team has assigned CVE-2026-64563 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 4.18 with commit 5d240a8936f6a1d3ece06701e8c4d830a2eca8a8 and fixed in 6.18.42 with commit 3ff7c1dbf722cf3fa538672452ba182318e0fcc3
	Issue introduced in 4.18 with commit 5d240a8936f6a1d3ece06701e8c4d830a2eca8a8 and fixed in 7.1.6 with commit 4169d9fb92f313ff8e7e83d733c1ecdcc93eebd3
	Issue introduced in 4.18 with commit 5d240a8936f6a1d3ece06701e8c4d830a2eca8a8 and fixed in 7.2-rc5 with commit 8173f7e2ce67e6ca1d4763f3da14e5b01ce77456

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-64563
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	lib/rhashtable.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/3ff7c1dbf722cf3fa538672452ba182318e0fcc3
	https://git.kernel.org/stable/c/4169d9fb92f313ff8e7e83d733c1ecdcc93eebd3
	https://git.kernel.org/stable/c/8173f7e2ce67e6ca1d4763f3da14e5b01ce77456