LoongArch: Return NULL from huge_pte_offset() for invalid PMD

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: LoongArch: Return NULL from huge_pte_offset() for invalid PMD LoongArch's huge_pte_offset() currently returns a pointer to a PMD slot even if the underlying entry points to invalid_pte_table (indicating no mapping). Callers like smaps_hugetlb_range() fetch this invalid entry value (the address of invalid_pte_table) via this pointer. The generic is_swap_pte() check then incorrectly identifies this address as a swap entry on LoongArch, because it satisfies the "!pte_present() && !pte_none()" conditions. This misinterpretation, combined with a coincidental match by is_migration_entry() on the address bits, leads to kernel crashes in pfn_swap_entry_to_page(). Fix this at the architecture level by modifying huge_pte_offset() to check the PMD entry's content using pmd_none() before returning. If the entry is invalid (i.e., it points to invalid_pte_table), return NULL instead of the pointer to the slot.

References

• https://git.kernel.org/stable/c/34256805720993e37adf6127371a1265aea8376a
• https://git.kernel.org/stable/c/2ca9380b12711afe95b3589bd82b59623b3c96b3
• https://git.kernel.org/stable/c/51424fd171cee6a33f01f7c66b8eb23ac42289d4
• https://git.kernel.org/stable/c/b49f085cd671addbda4802d6b9382513f7dd0f30
• https://git.kernel.org/stable/c/bd51834d1cf65a2c801295d230c220aeebf87a73