New user-space memory is allocated either from the already allocated process memory, or using the mmap system call
Note that memory allocated may not be physically allocated:
Kernel uses demand fault paging to allocate the physical page (the physical page is allocated when access to the virtual address generates a page fault)
... or may have been swapped out, which also induces a page fault
User space memory allocation is allowed to over-commit memory (more than available physical memory) => can lead to out of memory
OOM killer enters in action and selects a process to kill to retrieve some memory