Page allocations don't have redzones. So when we are checking shadow for OOB accesses (in particular, memset/cpy) we can miss OOB if the next page happens to be allocated (for kernel/user/pagecache).

As the result such OOBs can lead to silent memory corruptions, which are very expensive to debug.

I think it's an overkill to add additional checks for normal memory accesses, but for memory_is_poisoned_n (memset/cpy) we could well check that all accessed pages belong to the same allocation:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/mm/kasan/generic.c?id=04aa64375f48a5d430b5550d9271f8428883e550#n123

Simiarly to how copy_to/from_user checks that the access does not cross object boundaries.