CVE-2025-40245: Undefined Security Weakness

In the Linux kernel, the following vulnerability has been resolved:

nios2: ensure that memblock.current_limit is set when setting pfn limits

On nios2, with CONFIG_FLATMEM set, the kernel relies on memblock_get_current_limit() to determine the limits of mem_map, in particular for max_low_pfn. Unfortunately, memblock.current_limit is only default initialized to MEMBLOCK_ALLOC_ANYWHERE at this point of the bootup, potentially leading to situations where max_low_pfn can erroneously exceed the value of max_pfn and, thus, the valid range of available DRAM.

This can in turn cause kernel-level paging failures, e.g.:

[ 76.900000] Unable to handle kernel paging request at virtual address 20303000 [ 76.900000] ea = c0080890, ra = c000462c, cause = 14 [ 76.900000] Kernel panic - not syncing: Oops [ 76.900000] ---[ end Kernel panic - not syncing: Oops ]---

This patch fixes this by pre-calculating memblock.current_limit based on the upper limits of the available memory ranges via adjust_lowmem_bounds, a simplified version of the equivalent implementation within the arm architecture.