RE: [PATCH v3 2/2] mm: fix double page fault on arm64 if PTE_AF is cleared

From: Justin He (Arm Technology China)
Date: Mon Sep 16 2019 - 05:35:44 EST


Hi Kirill
> -----Original Message-----
> From: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>
> Sent: 2019å9æ16æ 17:16
> To: Justin He (Arm Technology China) <Justin.He@xxxxxxx>
> Cc: Catalin Marinas <Catalin.Marinas@xxxxxxx>; Will Deacon
> <will@xxxxxxxxxx>; Mark Rutland <Mark.Rutland@xxxxxxx>; James Morse
> <James.Morse@xxxxxxx>; Marc Zyngier <maz@xxxxxxxxxx>; Matthew
> Wilcox <willy@xxxxxxxxxxxxx>; Kirill A. Shutemov
> <kirill.shutemov@xxxxxxxxxxxxxxx>; linux-arm-kernel@xxxxxxxxxxxxxxxxxxx;
> linux-kernel@xxxxxxxxxxxxxxx; linux-mm@xxxxxxxxx; Punit Agrawal
> <punitagrawal@xxxxxxxxx>; Anshuman Khandual
> <Anshuman.Khandual@xxxxxxx>; Jun Yao <yaojun8558363@xxxxxxxxx>;
> Alex Van Brunt <avanbrunt@xxxxxxxxxx>; Robin Murphy
> <Robin.Murphy@xxxxxxx>; Thomas Gleixner <tglx@xxxxxxxxxxxxx>;
> Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>; JÃrÃme Glisse
> <jglisse@xxxxxxxxxx>; Ralph Campbell <rcampbell@xxxxxxxxxx>;
> hejianet@xxxxxxxxx
> Subject: Re: [PATCH v3 2/2] mm: fix double page fault on arm64 if PTE_AF
> is cleared
>
> On Sat, Sep 14, 2019 at 12:32:39AM +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 | 30 +++++++++++++++++++++++++-----
> > 1 file changed, 25 insertions(+), 5 deletions(-)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index e2bb51b6242e..a64af6495f71 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;
> > @@ -2140,7 +2147,8 @@ 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)
> > {
> > debug_dma_assert_idle(src);
> >
> > @@ -2152,20 +2160,32 @@ 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 *)(vmf->address &
> 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. If PTE_AF is cleared on arm64, it might
> > + * cause double page fault. So makes pte young here
> > */
> > + if (arch_faults_on_old_pte() && !pte_young(vmf->orig_pte))
> {
> > + spin_lock(vmf->ptl);
> > + entry = pte_mkyoung(vmf->orig_pte);
>
> Should't you re-validate that orig_pte after re-taking ptl? It can be
> stale by now.
Thanks, do you mean flush_cache_page(vma, vmf->address, pte_pfn(vmf->orig_pte))
before pte_mkyoung?

--
Cheers,
Justin (Jia He)


>
> > + if (ptep_set_access_flags(vmf->vma, vmf->address,
> > + vmf->pte, entry, 0))
> > + update_mmu_cache(vmf->vma, vmf-
> >address,
> > + vmf->pte);
> > + 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, vmf->address, vmf->vma);
> > }
> >
> > static gfp_t __get_fault_gfp_mask(struct vm_area_struct *vma)
> > @@ -2318,7 +2338,7 @@ 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
> >
> >
>
> --
> Kirill A. Shutemov
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