UBUNTU-CVE-2026-89486: Use After Free

In the Linux kernel, the following vulnerability has been resolved: ipmi: Fix use-after-free of cmdrcvr in ipmidestroyuser() Commit 9e91f8a6c868 ("ipmi:msghandler: Remove srcu for the ipmiinterfaces list") dropped the synchronizercu() between unlinking the command receivers from intf->cmdrcvrs and freeing them, updating only the comment that explains why the barrier is needed. The cmdrcvrs list is still traversed under plain RCU: findcmdrcvr() walks it inside rcureadlock(), and handleipmbgetmsgcmd() borrows rcvr->user from that lookup within the same read-side section. Without the grace period, ipmidestroyuser() can kfree() a cmdrcvr while a reader still holds a pointer to it, causing a use-after-free. The rework only made srcu unnecessary for the interfaces list; the cmdrcvrs list still relies on plain RCU. Restore the synchronizercu() before freeing the receivers.