mm/page_table_check: fix crash on ZONE_DEVICE

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: mm/page_table_check: fix crash on ZONE_DEVICE Not all pages may apply to pgtable check. One example is ZONE_DEVICE pages: they map PFNs directly, and they don't allocate page_ext at all even if there's struct page around. One may reference devm_memremap_pages(). When both ZONE_DEVICE and page-table-check enabled, then try to map some dax memories, one can trigger kernel bug constantly now when the kernel was trying to inject some pfn maps on the dax device: kernel BUG at mm/page_table_check.c:55! While it's pretty legal to use set_pxx_at() for ZONE_DEVICE pages for page fault resolutions, skip all the checks if page_ext doesn't even exist in pgtable checker, which applies to ZONE_DEVICE but maybe more.

References

• https://git.kernel.org/stable/c/51897f99351fff7b57f4f141940fa93b4e90fd2b
• https://git.kernel.org/stable/c/84d3549d54f5ff9fa3281257be3019386f51d1a0
• https://git.kernel.org/stable/c/dec2382247860d2134c8d41e103e26460c099629
• https://git.kernel.org/stable/c/8bb592c2eca8fd2bc06db7d80b38da18da4a2f43