Re: [Question] race during kasan_populate_vmalloc_pte

[Alexander Potapenko <glider@google.com>]

On Fri, Jul 12, 2024 at 4:08 AM mawupeng <mawupeng1@huawei.com> wrote:
>
> 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.

I am going to take a closer look at this issue. Any chance you have a
reproducer for it?