In the Linux kernel, the following vulnerability has been resolved: net: ipa: fix stalled modem TX queue after runtime resume ipa_start_xm

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

net: ipa: fix stalled modem TX queue after runtime resume

ipa_start_xmit() unconditionally stops the TX queue before calling
pm_runtime_get(), relying on the wake scheduled by runtime resume
(ipa_modem_wake_queue_work()) to restart it once power is ACTIVE.
But that work is queued from within the runtime resume callback,
before the device's power state reaches RPM_ACTIVE, so it can run
while the device is still RPM_RESUMING. The wake is then consumed
too early: the transmit it restarts stops the queue again,
pm_runtime_get() returns -EINPROGRESS without arranging any future
wake (deferred_resume exists only for RPM_SUSPENDING), and after the
resume completes nothing is left to wake the queue. Transmit stalls
permanently: packets pile up in the qdisc behind the stopped queue,
the device runtime-suspends, and since the netdev registers no
ndo_tx_timeout the watchdog never fires. Observed on SM7635
(Fairphone 6) as the cellular data path going permanently deaf
within hours, RX included, since nothing resumes the suspended
endpoints.

Close the window by making the wake work wait for the resume to
complete (pm_runtime_get_sync()) before waking the queue. Every
queue stop is then guaranteed a later wake that happens while power
is ACTIVE; a transmit racing a new suspend/resume cycle re-schedules
the work. If the device could not be resumed, wake the queue anyway
so pending packets are dropped by the transmit path rather than
stranded.

The STARTED power flag used to narrow this window: a wake running
before the transmit path's stop suppressed that stop, but only once,
as the flag was cleared by the first stop it absorbed. Removing the
flag made a single transmit during an in-flight resume sufficient to
strand the queue, which is the form observed.

With an accelerated reproducer (autosuspend delay shortened to 5 ms,
~20 packets/s of TX), an unpatched kernel stalled three times in
230 s / 4380 packets; with this patch the same test ran 3601 s /
70298 packets without a stall.