riscv: mm: Fix the out of bound issue of vmemmap address

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: riscv: mm: Fix the out of bound issue of vmemmap address In sparse vmemmap model, the virtual address of vmemmap is calculated as: ((struct page *)VMEMMAP_START - (phys_ram_base >> PAGE_SHIFT)). And the struct page's va can be calculated with an offset: (vmemmap + (pfn)). However, when initializing struct pages, kernel actually starts from the first page from the same section that phys_ram_base belongs to. If the first page's physical address is not (phys_ram_base >> PAGE_SHIFT), then we get an va below VMEMMAP_START when calculating va for it's struct page. For example, if phys_ram_base starts from 0x82000000 with pfn 0x82000, the first page in the same section is actually pfn 0x80000. During init_unavailable_range(), we will initialize struct page for pfn 0x80000 with virtual address ((struct page *)VMEMMAP_START - 0x2000), which is below VMEMMAP_START as well as PCI_IO_END. This commit fixes this bug by introducing a new variable 'vmemmap_start_pfn' which is aligned with memory section size and using it to calculate vmemmap address instead of phys_ram_base.

References

• https://git.kernel.org/stable/c/04350304428063da6a55a8a4597d409dc69148b2
• https://git.kernel.org/stable/c/92f08673d3f1893191323572f60e3c62f2e57c2f
• https://git.kernel.org/stable/c/a4a7ac3d266008018f05fae53060fcb331151a14
• https://git.kernel.org/stable/c/d2bd51954ac8377c2f1eb1813e694788998add66
• https://git.kernel.org/stable/c/f754f27e98f88428aaf6be6e00f5cbce97f62d4b