From 1aac65f3e651334259ecb2a5f5ddb81c01f02599 Mon Sep 17 00:00:00 2001 From: "Denis V. Lunev" Date: Wed, 15 Jul 2026 20:34:52 +0200 Subject: [PATCH] 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 Signed-off-by: Dave Hansen Acked-by: Kiryl Shutsemau (Meta) Link: https://patch.msgid.link/20260715183453.2381141-1-den@openvz.org --- arch/x86/mm/pat/set_memory.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c index e9b408343c5d..b1e780a465b5 100644 --- a/arch/x86/mm/pat/set_memory.c +++ b/arch/x86/mm/pat/set_memory.c @@ -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)