mm/page_alloc: don't call pfn_to_page() on possibly non-existent PFN in split_large_buddy()

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: mm/page_alloc: don't call pfn_to_page() on possibly non-existent PFN in split_large_buddy() In split_large_buddy(), we might call pfn_to_page() on a PFN that might not exist. In corner cases, such as when freeing the highest pageblock in the last memory section, this could result with CONFIG_SPARSEMEM && !CONFIG_SPARSEMEM_EXTREME in __pfn_to_section() returning NULL and and __section_mem_map_addr() dereferencing that NULL pointer. Let's fix it, and avoid doing a pfn_to_page() call for the first iteration, where we already have the page. So far this was found by code inspection, but let's just CC stable as the fix is easy.

References

• https://git.kernel.org/stable/c/4234ca9884bcae9e48ed38652d91696ad5cd591d
• https://git.kernel.org/stable/c/faeec8e23c10bd30e8aa759a2eb3018dae00f924