CVE-2026-93066

Summary

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

x86/mm/pat: Take cpa_lock around large-page collapse

Loading and unloading modules concurrently on several CPUs on a KASAN build, with a short delay injected at the CPA page-table lookup to widen the window, faults within minutes:

BUG: KASAN: use-after-free in __change_page_attr+0x7cc/0x7e0 Write of size 8 at addr ffff888181139718 by task modprobe … The buggy address belongs to the physical page: pfn:0x181139 … page_type: f2(table)

cpa_collapse_large_pages() rebuilds a leaf PMD from its 4K PTEs and frees the old PTE-table pages, while __change_page_attr() fetches a PTE pointer from a lockless lookup_address_in_pgd_attr() and writes it with set_pte_atomic() only later. When module text is served from a shared large ROX mapping the two run on the same PMD:

CPU A (module load) CPU B (module finalize)

execmem_make_temp_rw set_memory_nx __change_page_attr split 2M -> 4K table P kpte = &P[i] (lockless) execmem_restore_rox set_memory_rox (CPA_COLLAPSE) cpa_collapse_large_pages rebuild leaf PMD flush_tlb_all pagetable_free(P) set_pte_atomic(kpte, …) -> writes into freed P

P is a page-table page (page_type: table), reused at once, so the write corrupts whatever got the page next: a bad-pte or bad-page splat, or a fatal fault once P has been turned into read-only text.

The flush_tlb_all() before the free does not close this: its IPI only serializes against page-table walkers that run with interrupts off (e.g. GUP-fast); the walk in __change_page_attr() runs with interrupts on, so nothing stops it from holding a stale pointer into P.

Serialize the collapse - the PMD rebuild, TLB flush and PTE-table free - under cpa_lock, the same lock __change_page_attr() now takes unconditionally since commit ("x86/mm/pat: stop gating cpa_lock on debug_pagealloc_enabled()"), so a concurrent walker can no longer hold a pointer into a table the collapse is about to free.

Affected Software

Weaknesses

References

• https://git.kernel.org/stable/c/591b6fac9df3f43522e8ff6eed8662f8b6f1a124
• https://git.kernel.org/stable/c/fb97d1d6c20692e15247fa5c8b894b2325e38aee
• https://git.kernel.org/stable/c/1aac65f3e651334259ecb2a5f5ddb81c01f02599

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