Re: [PATCH] x86/mm/pat: take cpa_lock around large-page collapse

[Kiryl Shutsemau]

On Fri, Jun 26, 2026 at 06:32:11PM +0200, Denis V. Lunev wrote:
> 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 lock __change_page_attr() takes for the
> split path, so a concurrent walker can no longer hold a pointer into
> a table the collapse is about to free.
> 
> debug_pagealloc bypasses cpa_lock in __change_page_attr() (the direct
> map is 4K then, with no large pages to serialize), so the lock cannot
> order the two there. Skip the collapse in that config: it is only an
> optimization, and not freeing the tables leaves the unserialized walk
> nothing to race.
> 
> Fixes: 41d88484c71c ("x86/mm/pat: restore large ROX pages after fragmentation")
> Signed-off-by: Denis V. Lunev <den@xxxxxxxxxx>

Acked-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>

-- 
  Kiryl Shutsemau / Kirill A. Shutemov