Re: [PATCH v6 1/1] kasan: Avoid sleepable page allocation from atomic context

[Harry Yoo]

On Thu, May 08, 2025 at 04:15:46PM +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.
> 
> On s390 if make arch_enter_lazy_mmu_mode() -> preempt_enable()
> and arch_leave_lazy_mmu_mode() -> preempt_disable(), such crash
> occurs:
> 
> [    0.663336] BUG: sleeping function called from invalid context at ./include/linux/sched/mm.h:321
> [    0.663348] in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 2, name: kthreadd
> [    0.663358] preempt_count: 1, expected: 0
> [    0.663366] RCU nest depth: 0, expected: 0
> [    0.663375] no locks held by kthreadd/2.
> [    0.663383] Preemption disabled at:
> [    0.663386] [<0002f3284cbb4eda>] apply_to_pte_range+0xfa/0x4a0
> [    0.663405] CPU: 0 UID: 0 PID: 2 Comm: kthreadd Not tainted 6.15.0-rc5-gcc-kasan-00043-gd76bb1ebb558-dirty #162 PREEMPT
> [    0.663408] Hardware name: IBM 3931 A01 701 (KVM/Linux)
> [    0.663409] Call Trace:
> [    0.663410]  [<0002f3284c385f58>] dump_stack_lvl+0xe8/0x140
> [    0.663413]  [<0002f3284c507b9e>] __might_resched+0x66e/0x700
> [    0.663415]  [<0002f3284cc4f6c0>] __alloc_frozen_pages_noprof+0x370/0x4b0
> [    0.663419]  [<0002f3284ccc73c0>] alloc_pages_mpol+0x1a0/0x4a0
> [    0.663421]  [<0002f3284ccc8518>] alloc_frozen_pages_noprof+0x88/0xc0
> [    0.663424]  [<0002f3284ccc8572>] alloc_pages_noprof+0x22/0x120
> [    0.663427]  [<0002f3284cc341ac>] get_free_pages_noprof+0x2c/0xc0
> [    0.663429]  [<0002f3284cceba70>] kasan_populate_vmalloc_pte+0x50/0x120
> [    0.663433]  [<0002f3284cbb4ef8>] apply_to_pte_range+0x118/0x4a0
> [    0.663435]  [<0002f3284cbc7c14>] apply_to_pmd_range+0x194/0x3e0
> [    0.663437]  [<0002f3284cbc99be>] __apply_to_page_range+0x2fe/0x7a0
> [    0.663440]  [<0002f3284cbc9e88>] apply_to_page_range+0x28/0x40
> [    0.663442]  [<0002f3284ccebf12>] kasan_populate_vmalloc+0x82/0xa0
> [    0.663445]  [<0002f3284cc1578c>] alloc_vmap_area+0x34c/0xc10
> [    0.663448]  [<0002f3284cc1c2a6>] __get_vm_area_node+0x186/0x2a0
> [    0.663451]  [<0002f3284cc1e696>] __vmalloc_node_range_noprof+0x116/0x310
> [    0.663454]  [<0002f3284cc1d950>] __vmalloc_node_noprof+0xd0/0x110
> [    0.663457]  [<0002f3284c454b88>] alloc_thread_stack_node+0xf8/0x330
> [    0.663460]  [<0002f3284c458d56>] dup_task_struct+0x66/0x4d0
> [    0.663463]  [<0002f3284c45be90>] copy_process+0x280/0x4b90
> [    0.663465]  [<0002f3284c460940>] kernel_clone+0xd0/0x4b0
> [    0.663467]  [<0002f3284c46115e>] kernel_thread+0xbe/0xe0
> [    0.663469]  [<0002f3284c4e440e>] kthreadd+0x50e/0x7f0
> [    0.663472]  [<0002f3284c38c04a>] __ret_from_fork+0x8a/0xf0
> [    0.663475]  [<0002f3284ed57ff2>] ret_from_fork+0xa/0x38
> 
> 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@xxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: 3c5c3cfb9ef4 ("kasan: support backing vmalloc space with real shadow memory")
> 
> Signed-off-by: Alexander Gordeev <agordeev@xxxxxxxxxxxxx>
> ---

FWIW, this patch looks good to me.
Reviewed-by: Harry Yoo <harry.yoo@xxxxxxxxxx>

With a minor suggestion below.

>  mm/kasan/shadow.c | 77 ++++++++++++++++++++++++++++++++++++++---------
>  1 file changed, 63 insertions(+), 14 deletions(-)
> 
> diff --git a/mm/kasan/shadow.c b/mm/kasan/shadow.c
> index 88d1c9dcb507..660cc2148575 100644
> --- a/mm/kasan/shadow.c
> +++ b/mm/kasan/shadow.c
> @@ -292,30 +292,81 @@ void __init __weak kasan_populate_early_vm_area_shadow(void *start,

... snip ...

> +static int __kasan_populate_vmalloc(unsigned long start, unsigned long end)
> +{
> +	unsigned long nr_populated, nr_pages, nr_total = PFN_UP(end - start);
> +	struct vmalloc_populate_data data;
> +	int ret;
> +
> +	data.pages = (struct page **)__get_free_page(GFP_KERNEL | __GFP_ZERO);
> +	if (!data.pages)
> +		return -ENOMEM;
> +
> +	while (nr_total) {
> +		nr_pages = min(nr_total, PAGE_SIZE / sizeof(data.pages[0]));
> +		nr_populated = alloc_pages_bulk(GFP_KERNEL, nr_pages, data.pages);
> +		if (nr_populated != nr_pages) {
> +			free_pages_bulk(data.pages, nr_populated);
> +			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);
> +		free_pages_bulk(data.pages, nr_pages);

A minor suggestion:

I think this free_pages_bulk() can be moved outside the loop
(but with PAGE_SIZE / sizeof(data.pages[0]) instead of nr_pages),
because alloc_pages_bulk() simply skips allocating pages for any
non-NULL entries.

If some pages in the array were not used, it doesn't have to be freed;
on the next iteration of the loop alloc_pages_bulk() can skip
allocating pages for the non-NULL entries.

> +		if (ret)
> +			return ret;
> +
> +		start += nr_pages * PAGE_SIZE;
> +		nr_total -= nr_pages;
> +	}
> +
> +	free_page((unsigned long)data.pages);
> +
>  	return 0;
>  }

-- 
Cheers,
Harry / Hyeonggon