mm/mseal: update VMA end correctly on merge
In the Linux kernel, the following vulnerability has been resolved:
mm/mseal: update VMA end correctly on merge
Previously we stored the end of the current VMA in curr_end, and then upon
iterating to the next VMA updated curr_start to curr_end to advance to the
next VMA.
However, this doesn't take into account the fact that a VMA might be
updated due to a merge by vma_modify_flags(), which can result in curr_end
being stale and thus, upon setting curr_start to curr_end, ending up with
an incorrect curr_start on the next iteration.
Resolve the issue by setting curr_end to vma->vm_end unconditionally to
ensure this value remains updated should this occur.
While we're here, eliminate this entire class of bug by simply setting
const curr_[start/end] to be clamped to the input range and VMAs, which
also happens to simplify the logic.