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Before entering the guest, we check whether our VMID is still part of the current generation. In order to avoid taking a lock, we start with checking that the generation is still current, and only if not current do we take the lock, recheck, and update the generation and VMID. This leaves open a small race: A vcpu can bump up the global generation number as well as the VM's, but has not updated the VMID itself yet. At that point another vcpu from the same VM comes in, checks the generation (and finds it not needing anything), and jumps into the guest. At this point, we end-up with two vcpus belonging to the same VM running with two different VMIDs. Eventually, the VMID used by the second vcpu will get reassigned, and things will really go wrong... A simple solution would be to drop this initial check, and always take the lock. This is likely to cause performance issues. A middle ground is to convert the spinlock to a rwlock, and only take the read lock on the fast path. If the check fails at that point, drop it and acquire the write lock, rechecking the condition. This ensures that the above scenario doesn't occur. Cc: stable@vger.kernel.org Reported-by: Mark Rutland <mark.rutland@arm.com> Tested-by: Shannon Zhao <zhaoshenglong@huawei.com> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com> |
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