NuytsTech Security

CVE-2025-39899

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

mm/userfaultfd: fix kmap_local LIFO ordering for CONFIG_HIGHPTE

With CONFIG_HIGHPTE on 32-bit ARM, move_pages_pte() maps PTE pages using
kmap_local_page(), which requires unmapping in Last-In-First-Out order.

The current code maps dst_pte first, then src_pte, but unmaps them in the
same order (dst_pte, src_pte), violating the LIFO requirement. This
causes the warning in kunmap_local_indexed():

WARNING: CPU: 0 PID: 604 at mm/highmem.c:622 kunmap_local_indexed+0x178/0x17c
addr != __fix_to_virt(FIX_KMAP_BEGIN + idx)

Fix this by reversing the unmap order to respect LIFO ordering.

This issue follows the same pattern as similar fixes:
– commit eca6828403b8 (“crypto: skcipher – fix mismatch between mapping and unmapping order”)
– commit 8cf57c6df818 (“nilfs2: eliminate staggered calls to kunmap in nilfs_rename”)

Both of which addressed the same fundamental requirement that kmap_local
operations must follow LIFO ordering.

More information : https://git.kernel.org/stable/c/9614d8bee66387501f48718fa306e17f2aa3f2f3

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