UBUNTU-CVE-2026-90193: Medium severity ubuntu/linux-hwe-edge vulnerability

In the Linux kernel, the following vulnerability has been resolved: mailbox: qcom-cpucp: fix PREEMPTRT self-deadlock in IRQ handler qcomcpucpmboxirqfn() calls mboxchanreceiveddata() while holding chan->lock. Under PREEMPTRT, spinlockirqsave() is converted to an rtspinlock (rtmutex-based), which tracks ownership and can sleep. The callback chain triggered by mboxchanreceiveddata() eventually reaches mailboxclearchannel() -> mboxsendmessage() -> addtorbuf(), which attempts to re-acquire the same chan->lock. Since rtmutex detects the re-entrant lock attempt by the same owner, the thread blocks waiting for a lock it already holds, causing a permanent deadlock. This deadlock manifests as 'irq/N-apsscpucpmbox' stuck in D state with the following call trace: rtspinlock -> mboxsendmessage -> mailboxclearchannel -> scmirxcallback -> mboxchanreceiveddata [<- held chan->lock here] Fix by saving chan->cl locally and clearing the HW interrupt register inside the lock, then invoking mboxchanreceiveddata() after releasing the lock. This preserves the mutual exclusion for chan->cl access while avoiding the lock re-entrancy that causes the PREEMPTRT deadlock.