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The ISS has a mandatory 45 day minimum supply of food and other essentials needed by the crew. In practice it's more like 6 months. Cargo supplies are launched using several independently-developed cargo ships: Cygnus, Progress, and Cargo Dragon. They're all in regular rotation to top up supplies. The Starliner astronauts could stay up as long as is needed if a Crew Dragon needs to be launched to take them home.

There is a policy of requiring a "lifeboat" seat for every single person on board the ISS. During the Shuttle era where shuttle missions swapped out crew, this role was filled by an extra Soyuz that docked uncrewed. Nowadays it's filled by the original vehicle that brought up that specific crew member. If Starliner can't be used to return, that rule will have to be waived temporarily. If that happens then a Crew Dragon is the only vehicle likely to be used to bring them home. They're proven reliable vehicles that can do automated docking. In theory a Soyuz can dock uncrewed as well and is also an incredibly safe way to return to Earth (though it is a bit of a roller coaster ride). In practice, given the geopolitical climate, I think it would be... unlikely. By default it's Crew Dragon.

There's also the issue of spacesuit compatibility. During re-entry, the spacesuits themselves are connected to the vehicle's backup life support systems. If the vehicle pressurization fails or if there's other life support problems that don't compromise the rest of the vehicle, they'll still be able to safely land. Starliner and Crew Dragon suits don't talk to each other. Some sort of adaptor hardware would have to be built.

Which brings up the matter of docking port schedules. It's going to be really tricky to finagle schedules and vehicle availability. Docking schedules are typically planned years prior.

So in a nutshell they're completely safe so long as there's no event that requires evacuation of the ISS. And even then they'd probably pile in to the nearest Crew Dragon, try to line the cargo floor with something thick and soft and light, and basically bareback it down to Earth. But because there's a lifeboat rule they're going to want them home quick.