Air carrier crew reported a loss of control during a higher altitude cruise climb in an area of thunderstorms and severe turbulence. The crew regained aircraft control of speed and altitude then continued to destination.

Narrative

In preparation for this flight; flight deck crew discussed the WSI RPM turbulence that showed a bumpy ride for the most of the first few hours of the flight specifically in the Japanese airspace. The WSI RPM showed mostly yellow with very little orange areas scattered over the first part of the route and concentrated over Japan. After pilots briefing; dispatch briefing was conducted on the phone and talked about the possibility of a bumpy ride. No PIREPs or forecast for severe turbulence was noted by pilots; Dispatch; nor WSI. Once arrived at the airport Captain briefed the group of flight attendants in the gate area while waiting for the cleaners. Captain instructed the lead flight attendant at the gate to have the flight attendants and passengers remain seated until a call for release was made by the pilots.After departure and while passing through approximately 18;000 feet; the pilot monitoring inquired ATC about any turbulence reports ahead. As the ATC responded that there are no turbulence report ahead and with smooth rides in VMC; Captain requested the two relief pilots to call the flight attendants to temporarily get up to let the relief pilots out so they can begin their rest. This was done during smooth air and flight attendants were returned to their jump seats to comply with Captain's initial request to be seated for the climb. After about 15 minutes and approaching waypoint VAPGU a line of clouds were visually noticed up ahead with some possible gaps to navigate through. Weather radar was on with intensity adjusted to high. Returns from the weather radar were very minimal showing isolated greens with amber that could easily be avoided with small deviations. Pilots made multiple attempts to request a weather deviation but non-stop ATC conversations in Chinese was making it difficult. After a few calls a left deviation of heading 100 was approved by Dalian Control on VHF. As the flight progressed through the clouds; light to moderate turbulence began. After a few minutes; while in IMC; weather radar showed red returns up ahead and to the right. Considering wind direction and radar returns; further left deviations were requested from the same controller with furthest being heading 060. Approximately at XA:27 near Dalian VOR (DBL) and at 29;100 feet (8;900 m) severe turbulence began and continued on for at least 10 to 15 minutes. ATC transmissions were difficult to hear due to strong static caused by the nearby storm. ATC issued a climb instruction to climb and maintain 31;100 feet (9;500m). Pilots began to climb and after a few hundred feet a decision was made to request to descend back to 29;100 feet to have enough gap between the lowest and highest speed limits. Pilot monitoring cleared extra data from the EFB to identify current position and distance from Pyongyang FIR. Approximately 111nm was noted using the measuring tool on the EFB. Multiple ATC transmissions called the flight's call sign with unreadable instructions or request. Every ATC transmission was responded by pilot monitoring with call sign and 'say again' or call sign 'Unable to read we are in severe turbulence; standby'. Meanwhile; pilot flying was responding to aircraft speed oscillation and severe turbulence. On at least three occasions; while in clean configuration; aircraft speed dropped to amber band with stick shaker being activated and accelerated immediately to 10-20 knots passed into the VMO red band with overspeed warning message and aural warning activated. Speed bug was set to the middle of the both speed limits at approximately 290/300 KIAS to be able to stabilize the speed in a safe margin from stalling and over speeding. Combination of some thrust adjustments and full speed breaks were used to avoid overspeed but based on the severity of the situation; no other control input was available to avoid overspeed. Approximately 20-30 nm north of SANKO waypoint a different controller on the same frequency instructed to 'turn right immediately'. Pilot monitoring responded with call sign and 'turn right immediately' and a southerly heading was picked. Exactly at this time some gaps and a part of the sky was visible and VMC condition returned after 2-3 minutes. ATC instructed to fly direct to AGAVO waypoint and later asked to maintain 5 nm right of track.After conditions returned to normal; Captain checked with the lead flight attendant and an assessment was made that there were no injuries or damages. This was reported to the ATC. A PIREP was sent over the ACARS to report the severe turbulence over DBL FL290 with national weather service. Dispatch was contacted via ACARS to report the severe turbulence and passenger/crew condition. The pilot monitoring looked up the fault code in the manual. This was entered into maintenance logbook by Captain with a short description of the severe turbulence and overspeed and hit 'send'. There was possibly no confirmation message for this maintenance write up. FOM adverse weather section was reviewed together and eventually the flight attendants were released. The seat belt sign was on from the gate until 20-30 minutes after returning to the original route after deviations.Unrelated to the turbulence event; a sick passenger was reported to the Captain by flight attendants 20-40 minutes after releasing flight attendants. The condition of the passenger was unrelated to the severe turbulence and according to the passenger; they had a cardiac arrest days ago and was released from hospital after a 10 day recovery period. Flight attendants being unable to use the video contact function with medlink; Dispatch was contacted on SAT comm by Captain to talk with medlink doctor and due to passenger feeling normal a decision was made to continue to ZZZ. Captain and Dispatch discussed the turbulence event on SAT comm after the medlink call was over.After arriving at ZZZ; chief pilot was contacted by Captain and a second verbal request had to be made with maintenance to enter the maintenance write up regarding the severe turbulence and overspeed.