Re: [Question] race during kasan_populate_vmalloc_pte

[mawupeng]

Hi maintainers,

kingly ping.

On 2024/6/18 14:40, Wupeng Ma wrote:
> Hi maintainers,
> 
> During our testing, we discovered that kasan vmalloc may trigger a false
> vmalloc-out-of-bounds warning due to a race between kasan_populate_vmalloc_pte
> and kasan_depopulate_vmalloc_pte.
> 
> cpu0				cpu1				cpu2
>   kasan_populate_vmalloc_pte	kasan_populate_vmalloc_pte	kasan_depopulate_vmalloc_pte
> 								spin_unlock(&init_mm.page_table_lock);
>   pte_none(ptep_get(ptep))
>   // pte is valid here, return here
> 								pte_clear(&init_mm, addr, ptep);
> 				pte_none(ptep_get(ptep))
> 				// pte is none here try alloc new pages
> 								spin_lock(&init_mm.page_table_lock);
> kasan_poison
> // memset kasan shadow region to 0
> 				page = __get_free_page(GFP_KERNEL);
> 				__memset((void *)page, KASAN_VMALLOC_INVALID, PAGE_SIZE);
> 				pte = pfn_pte(PFN_DOWN(__pa(page)), PAGE_KERNEL);
> 				spin_lock(&init_mm.page_table_lock);
> 				set_pte_at(&init_mm, addr, ptep, pte);
> 				spin_unlock(&init_mm.page_table_lock);
> 
> 
> Since kasan shadow memory in cpu0 is set to 0xf0 which means it is not
> initialized after the race in cpu1. Consequently, a false vmalloc-out-of-bounds
> warning is triggered when a user attempts to access this memory region.
> 
> The root cause of this problem is the pte valid check at the start of
> kasan_populate_vmalloc_pte should be removed since it is not protected by
> page_table_lock. However, this may result in severe performance degradation
> since pages will be frequently allocated and freed.
> 
> Is there have any thoughts on how to solve this issue?
> 
> Thank you.