Re: [PATCH v4 3/3] mm: fix double page fault on arm64 if PTE_AF is cleared

[Jia He]

Hi Kirill

[On behalf of justin.he@xxxxxxx because some mails are filted...]

On 2019/9/18 22:00, Kirill A. Shutemov wrote:
> On Wed, Sep 18, 2019 at 09:19:14PM +0800, Jia He wrote:
> > When we tested pmdk unit test [1] vmmalloc_fork TEST1 in arm64 guest, there
> > will be a double page fault in __copy_from_user_inatomic of cow_user_page.
> >
> > Below call trace is from arm64 do_page_fault for debugging purpose
> > [  110.016195] Call trace:
> > [  110.016826]  do_page_fault+0x5a4/0x690
> > [  110.017812]  do_mem_abort+0x50/0xb0
> > [  110.018726]  el1_da+0x20/0xc4
> > [  110.019492]  __arch_copy_from_user+0x180/0x280
> > [  110.020646]  do_wp_page+0xb0/0x860
> > [  110.021517]  __handle_mm_fault+0x994/0x1338
> > [  110.022606]  handle_mm_fault+0xe8/0x180
> > [  110.023584]  do_page_fault+0x240/0x690
> > [  110.024535]  do_mem_abort+0x50/0xb0
> > [  110.025423]  el0_da+0x20/0x24
> >
> > The pte info before __copy_from_user_inatomic is (PTE_AF is cleared):
> > [ffff9b007000] pgd=000000023d4f8003, pud=000000023da9b003, pmd=000000023d4b3003, pte=360000298607bd3
> >
> > As told by Catalin: "On arm64 without hardware Access Flag, copying from
> > user will fail because the pte is old and cannot be marked young. So we
> > always end up with zeroed page after fork() + CoW for pfn mappings. we
> > don't always have a hardware-managed access flag on arm64."
> >
> > This patch fix it by calling pte_mkyoung. Also, the parameter is
> > changed because vmf should be passed to cow_user_page()
> >
> > [1] https://github.com/pmem/pmdk/tree/master/src/test/vmmalloc_fork
> >
> > Reported-by: Yibo Cai <Yibo.Cai@xxxxxxx>
> > Signed-off-by: Jia He <justin.he@xxxxxxx>
> > ---
> >   mm/memory.c | 35 ++++++++++++++++++++++++++++++-----
> >   1 file changed, 30 insertions(+), 5 deletions(-)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index e2bb51b6242e..d2c130a5883b 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -118,6 +118,13 @@ int randomize_va_space __read_mostly =
> >   					2;
> >   #endif
> >   
> > +#ifndef arch_faults_on_old_pte
> > +static inline bool arch_faults_on_old_pte(void)
> > +{
> > +	return false;
> > +}
> > +#endif
> > +
> >   static int __init disable_randmaps(char *s)
> >   {
> >   	randomize_va_space = 0;
> > @@ -2140,8 +2147,12 @@ static inline int pte_unmap_same(struct mm_struct *mm, pmd_t *pmd,
> >   	return same;
> >   }
> >   
> > -static inline void cow_user_page(struct page *dst, struct page *src, unsigned long va, struct vm_area_struct *vma)
> > +static inline void cow_user_page(struct page *dst, struct page *src,
> > +				 struct vm_fault *vmf)
> >   {
> > +	struct vm_area_struct *vma = vmf->vma;
> > +	unsigned long addr = vmf->address;
> > +
> >   	debug_dma_assert_idle(src);
> >   
> >   	/*
> > @@ -2152,20 +2163,34 @@ static inline void cow_user_page(struct page *dst, struct page *src, unsigned lo
> >   	 */
> >   	if (unlikely(!src)) {
> >   		void *kaddr = kmap_atomic(dst);
> > -		void __user *uaddr = (void __user *)(va & PAGE_MASK);
> > +		void __user *uaddr = (void __user *)(addr & PAGE_MASK);
> > +		pte_t entry;
> >   
> >   		/*
> >   		 * This really shouldn't fail, because the page is there
> >   		 * in the page tables. But it might just be unreadable,
> >   		 * in which case we just give up and fill the result with
> > -		 * zeroes.
> > +		 * zeroes. On architectures with software "accessed" bits,
> > +		 * we would take a double page fault here, so mark it
> > +		 * accessed here.
> >   		 */
> > +		if (arch_faults_on_old_pte() && !pte_young(vmf->orig_pte)) {
> > +			spin_lock(vmf->ptl);
> > +			if (likely(pte_same(*vmf->pte, vmf->orig_pte))) {
> > +				entry = pte_mkyoung(vmf->orig_pte);
> > +				if (ptep_set_access_flags(vma, addr,
> > +							  vmf->pte, entry, 0))
> > +					update_mmu_cache(vma, addr, vmf->pte);
> > +			}
> I don't follow.
>
> So if pte has changed under you, you don't set the accessed bit, but never
> the less copy from the user.
>
> What makes you think it will not trigger the same problem?
>
> I think we need to make cow_user_page() fail in this case and caller --
> wp_page_copy() -- return zero. If the fault was solved by other thread, we
> are fine. If not userspace would re-fault on the same address and we will
> handle the fault from the second attempt.

Thanks for the pointing. How about make cow_user_page() be returned

VM_FAULT_RETRY? Then in do_page_fault(), it can retry the page fault?

---
Cheers,
Justin (Jia He)

> > +			spin_unlock(vmf->ptl);
> > +		}
> > +
> >   		if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE))
> >   			clear_page(kaddr);
> >   		kunmap_atomic(kaddr);
> >   		flush_dcache_page(dst);
> >   	} else
> > -		copy_user_highpage(dst, src, va, vma);
> > +		copy_user_highpage(dst, src, addr, vma);
> >   }
> >   
> >   static gfp_t __get_fault_gfp_mask(struct vm_area_struct *vma)
> > @@ -2318,7 +2343,7 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf)
> >   				vmf->address);
> >   		if (!new_page)
> >   			goto oom;
> > -		cow_user_page(new_page, old_page, vmf->address, vma);
> > +		cow_user_page(new_page, old_page, vmf);
> >   	}
> >   
> >   	if (mem_cgroup_try_charge_delay(new_page, mm, GFP_KERNEL, &memcg, false))
> > --
> > 2.17.1
--