It is possible to provide insane values for size in contigmalloc(9) request, which usually not reaches the phys allocator due to failing KVA allocation. But with the 4/4 i386, where 32bit architecture has almost 4G KVA, contigmalloc(1G) is not unreasonable outright and KVA might be available sometimes.
Then, the calculation of pa_end could wrap around, depending on the physical address, and the checks in vm_phys_alloc_seg_contig() would pass while the iteration in the loop after the 'done' label goes out of the vm_page_array bounds.
Fix it by detecting the wrap.