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

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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: 41d88484c7 ("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
This commit is contained in:
Denis V. Lunev 2026-07-15 20:34:52 +02:00 committed by Dave Hansen
parent 5fce67641a
commit 1aac65f3e6
1 changed files with 7 additions and 1 deletions
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8
arch/x86/mm/pat/set_memory.c
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@ -417,6 +417,8 @@ static void cpa_collapse_large_pages(struct cpa_data *cpa)
int collapsed = 0;
int i;
spin_lock(&cpa_lock);
if (cpa->flags & (CPA_PAGES_ARRAY | CPA_ARRAY)) {
for (i = 0; i < cpa->numpages; i++)
collapsed += collapse_large_pages(__cpa_addr(cpa, i),
@ -430,8 +432,10 @@ static void cpa_collapse_large_pages(struct cpa_data *cpa)
collapsed += collapse_large_pages(addr, &pgtables);
}
if (!collapsed)
if (!collapsed) {
spin_unlock(&cpa_lock);
return;
}
flush_tlb_all();
@ -439,6 +443,8 @@ static void cpa_collapse_large_pages(struct cpa_data *cpa)
list_del(&ptdesc->pt_list);
pagetable_free(ptdesc);
}
spin_unlock(&cpa_lock);
}
static void cpa_flush(struct cpa_data *cpa, int cache)