To read physical RAM without relying on virtual page table CR3 contexts:

NTSTATUS ReadPhysicalAddress(PHYSICAL_ADDRESS physAddr, PVOID buffer, SIZE_T size) {
    PVOID pMapped = MmMapIoSpace(physAddr, size, MmNonCached);
    if (!pMapped) return STATUS_INSUFFICIENT_RESOURCES;
    RtlCopyMemory(buffer, pMapped, size);
    MmUnmapIoSpace(pMapped, size);
    return STATUS_SUCCESS;
}

Physical addresses are immutable across thread context switches, making physical memory mapping 100% immune to virtual CR3 directory base swapping.

This is identical to how PCIe DMA hardware reads memory. Physical RAM mapping is the cleanest kernel read technique.

Make sure you don't map non-existent physical addresses to avoid BugCheck 0x3B.