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.

Fixes: 41d88484c71c ("x86/mm/pat: restore large ROX pages after fragmentation")
Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Link: https://patch.msgid.link/20260715183453.2381141-1-den@openvz.org

0 files changed, 0 insertions, 0 deletions