On Tue, Apr 29, 2025 at 06:08:41PM +0200, Alexander Gordeev wrote:

apply_to_pte_range() enters the lazy MMU mode and then invokes kasan_populate_vmalloc_pte() callback on each page table walk iteration. However, the callback can go into sleep when trying to allocate a single page, e.g. if an architecutre disables preemption on lazy MMU mode enter.

Should we add a comment that pte_fn_t must not sleep in apply_to_pte_range()?

On s390 if make arch_enter_lazy_mmu_mode() -> preempt_enable() and arch_leave_lazy_mmu_mode() -> preempt_disable(), such crash occurs:

[  553.332108] preempt_count: 1, expected: 0
[  553.332117] no locks held by multipathd/2116.
[  553.332128] CPU: 24 PID: 2116 Comm: multipathd Kdump: loaded Tainted:
[  553.332139] Hardware name: IBM 3931 A01 701 (LPAR)
[  553.332146] Call Trace:
[  553.332152]  [<00000000158de23a>] dump_stack_lvl+0xfa/0x150
[  553.332167]  [<0000000013e10d12>] __might_resched+0x57a/0x5e8
[  553.332178]  [<00000000144eb6c2>] __alloc_pages+0x2ba/0x7c0
[  553.332189]  [<00000000144d5cdc>] __get_free_pages+0x2c/0x88
[  553.332198]  [<00000000145663f6>] kasan_populate_vmalloc_pte+0x4e/0x110
[  553.332207]  [<000000001447625c>] apply_to_pte_range+0x164/0x3c8
[  553.332218]  [<000000001448125a>] apply_to_pmd_range+0xda/0x318
[  553.332226]  [<000000001448181c>] __apply_to_page_range+0x384/0x768
[  553.332233]  [<0000000014481c28>] apply_to_page_range+0x28/0x38
[  553.332241]  [<00000000145665da>] kasan_populate_vmalloc+0x82/0x98
[  553.332249]  [<00000000144c88d0>] alloc_vmap_area+0x590/0x1c90
[  553.332257]  [<00000000144ca108>] __get_vm_area_node.constprop.0+0x138/0x260
[  553.332265]  [<00000000144d17fc>] __vmalloc_node_range+0x134/0x360
[  553.332274]  [<0000000013d5dbf2>] alloc_thread_stack_node+0x112/0x378
[  553.332284]  [<0000000013d62726>] dup_task_struct+0x66/0x430
[  553.332293]  [<0000000013d63962>] copy_process+0x432/0x4b80
[  553.332302]  [<0000000013d68300>] kernel_clone+0xf0/0x7d0
[  553.332311]  [<0000000013d68bd6>] __do_sys_clone+0xae/0xc8
[  553.332400]  [<0000000013d68dee>] __s390x_sys_clone+0xd6/0x118
[  553.332410]  [<0000000013c9d34c>] do_syscall+0x22c/0x328
[  553.332419]  [<00000000158e7366>] __do_syscall+0xce/0xf0
[  553.332428]  [<0000000015913260>] system_call+0x70/0x98

Instead of allocating single pages per-PTE, bulk-allocate the shadow memory prior to applying kasan_populate_vmalloc_pte() callback on a page range.

Suggested-by: Andrey Ryabinin ryabinin.a.a@gmail.com Cc: stable@vger.kernel.org Fixes: 3c5c3cfb9ef4 ("kasan: support backing vmalloc space with real shadow memory")

Signed-off-by: Alexander Gordeev agordeev@linux.ibm.com

mm/kasan/shadow.c | 65 +++++++++++++++++++++++++++++++++++------------ 1 file changed, 49 insertions(+), 16 deletions(-)

diff --git a/mm/kasan/shadow.c b/mm/kasan/shadow.c index 88d1c9dcb507..ea9a06715a81 100644 --- a/mm/kasan/shadow.c +++ b/mm/kasan/shadow.c @@ -292,30 +292,65 @@ void __init __weak kasan_populate_early_vm_area_shadow(void *start, { } +struct vmalloc_populate_data {

• unsigned long start;
• struct page **pages;

+};

static int kasan_populate_vmalloc_pte(pte_t *ptep, unsigned long addr,

• 		      void *unused)
  

• 		      void *_data)
  

{

• unsigned long page;

• struct vmalloc_populate_data *data = _data;
• struct page *page;
• unsigned long pfn; pte_t pte;

if (likely(!pte_none(ptep_get(ptep)))) return 0;

• page = __get_free_page(GFP_KERNEL);
• if (!page)

• return -ENOMEM;
  

• __memset((void *)page, KASAN_VMALLOC_INVALID, PAGE_SIZE);
• pte = pfn_pte(PFN_DOWN(__pa(page)), PAGE_KERNEL);

• page = data->pages[PFN_DOWN(addr - data->start)];
• pfn = page_to_pfn(page);
• __memset(pfn_to_virt(pfn), KASAN_VMALLOC_INVALID, PAGE_SIZE);
• pte = pfn_pte(pfn, PAGE_KERNEL);

spin_lock(&init_mm.page_table_lock);

• if (likely(pte_none(ptep_get(ptep)))) {

• if (likely(pte_none(ptep_get(ptep)))) set_pte_at(&init_mm, addr, ptep, pte);

• page = 0;
  

With this patch, now if the pte is already set, the page is leaked?

Should we set data->pages[PFN_DOWN(addr - data->start)] = NULL and free non-null elements later in __kasan_populate_vmalloc()?

• } spin_unlock(&init_mm.page_table_lock);
• if (page)

• free_page(page);
  

• return 0;

+}

+static int __kasan_populate_vmalloc(unsigned long start, unsigned long end) +{

• unsigned long nr_pages, nr_total = PFN_UP(end - start);
• struct vmalloc_populate_data data;
• int ret;
• data.pages = (struct page **)__get_free_page(GFP_KERNEL);
• if (!data.pages)

• return -ENOMEM;
  

• while (nr_total) {

• nr_pages = min(nr_total, PAGE_SIZE / sizeof(data.pages[0]));
  

• __memset(data.pages, 0, nr_pages * sizeof(data.pages[0]));
  

• if (nr_pages != alloc_pages_bulk(GFP_KERNEL, nr_pages, data.pages)) {
  

When the return value of alloc_pages_bulk() is less than nr_pages, you still need to free pages in the array unless nr_pages is zero.

• 	free_page((unsigned long)data.pages);
  

• 	return -ENOMEM;
  

• }
  

• data.start = start;
  

• ret = apply_to_page_range(&init_mm, start, nr_pages * PAGE_SIZE,
  

• 			  kasan_populate_vmalloc_pte, &data);
  

• if (ret)
  

• 	return ret;
  

• start += nr_pages * PAGE_SIZE;
  

• nr_total -= nr_pages;
  

• }
• free_page((unsigned long)data.pages);
• return 0;

} @@ -348,9 +383,7 @@ int kasan_populate_vmalloc(unsigned long addr, unsigned long size) shadow_start = PAGE_ALIGN_DOWN(shadow_start); shadow_end = PAGE_ALIGN(shadow_end);

• ret = apply_to_page_range(&init_mm, shadow_start,

• 		  shadow_end - shadow_start,
  

• 		  kasan_populate_vmalloc_pte, NULL);
  

• ret = __kasan_populate_vmalloc(shadow_start, shadow_end); if (ret) return ret;

2.45.2