CVE-2021-47623: 
Linux Kernel vulnerability analysis and mitigation

In the Linux kernel, a vulnerability was discovered in the powerpc/fixmap component. The issue occurs when unmapping a fixmap entry by calling __set_fixmap() with FIXMAP_PAGE_CLEAR as flags. The powerpc __set_fixmap() implementation calls map_kernel_page(), which triggers a warning when called a second time for the same page (Kernel Git). This vulnerability was assigned CVE-2021-47623 and was fixed in 2021.

The vulnerability manifests when map_kernel_page() is called multiple times on the same page during fixmap unmapping operations. This triggers a VM debug warning at arch/powerpc/mm/pgtable.c:194 in the set_pte_at function. The issue was resolved by implementing a new unmap_kernel_page() function which properly clears an existing page table entry (PTE) and flushes the TLB range (Kernel Git).

When triggered, this vulnerability results in VM debug warnings and potential system instability on PowerPC systems. The issue affects the kernel's memory management subsystem, specifically the fixmap unmapping operations.

The vulnerability is triggered during normal system operations when unmapping fixmap entries. It primarily affects PowerPC systems and requires no special privileges to trigger, as it can occur during regular kernel operations like early_iounmap() calls (Kernel Git).

The issue was fixed by implementing a new unmap_kernel_page() function that properly handles the unmapping operation. The fix was included in the Linux kernel through a patch that modifies the fixmap handling code to use the new function when FIXMAP_PAGE_CLEAR is specified (Kernel Git).