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CVE-2025-21851 is a Linux kernel eBPF arena-map flaw affecting AArch64 kernels configured with 64 KB pages. During arena-map teardown, arena_map_free() passes the kernel virtual-memory start address returned by bpf_arena_get_kern_vm_start() to apply_to_existing_page_range(). The address can be non-page-aligned because of the GUARD_SZ calculation. This can cause apply_to_existing_page_range() to reach apply_to_pte_range() with an unaligned address, resulting in a soft lockup and, in observed testing, a segmentation fault. The correction rounds GUARD_SZ up to twice the page size, ensuring that the subsequent division in bpf_arena_get_kern_vm_start() produces a page-aligned address.
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Linux kernel BPF arena free soft-lockup flaw.
A Linux kernel BPF arena-map flaw on AArch64 systems using 64 KB pages. An unaligned address passed by arena_map_free() to apply_to_existing_page_range() can reach apply_to_pte_range() and cause a segmentation fault and soft lockup. The fix rounds GUARD_SZ to twice PAGE_SIZE so bpf_arena_get_kern_vm_start() returns a page-aligned address.
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