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SubjectRE: [PATCH v9 3/3] mm: fix double page fault on arm64 if PTE_AF is cleared
Date
Hi Matthew and Kirill
I didn't add your previous r-b and a-b tag since I refactored the cow_user_page
and changed the ptl range in v9. Please have a review, thanks


--
Cheers,
Justin (Jia He)



> -----Original Message-----
> From: Jia He <justin.he@arm.com>
> Sent: 2019年9月25日 10:59
> To: Catalin Marinas <Catalin.Marinas@arm.com>; Will Deacon
> <will@kernel.org>; Mark Rutland <Mark.Rutland@arm.com>; James Morse
> <James.Morse@arm.com>; Marc Zyngier <maz@kernel.org>; Matthew
> Wilcox <willy@infradead.org>; Kirill A. Shutemov
> <kirill.shutemov@linux.intel.com>; linux-arm-kernel@lists.infradead.org;
> linux-kernel@vger.kernel.org; linux-mm@kvack.org; Suzuki Poulose
> <Suzuki.Poulose@arm.com>
> Cc: Punit Agrawal <punitagrawal@gmail.com>; Anshuman Khandual
> <Anshuman.Khandual@arm.com>; Alex Van Brunt
> <avanbrunt@nvidia.com>; Robin Murphy <Robin.Murphy@arm.com>;
> Thomas Gleixner <tglx@linutronix.de>; Andrew Morton <akpm@linux-
> foundation.org>; Jérôme Glisse <jglisse@redhat.com>; Ralph Campbell
> <rcampbell@nvidia.com>; hejianet@gmail.com; Kaly Xin (Arm Technology
> China) <Kaly.Xin@arm.com>; nd <nd@arm.com>; Justin He (Arm
> Technology China) <Justin.He@arm.com>
> Subject: [PATCH v9 3/3] mm: fix double page fault on arm64 if PTE_AF is
> cleared
>
> 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()
>
> Add a WARN_ON_ONCE when __copy_from_user_inatomic() returns error
> in case there can be some obscure use-case.(by Kirill)
>
> [1] https://github.com/pmem/pmdk/tree/master/src/test/vmmalloc_fork
>
> Signed-off-by: Jia He <justin.he@arm.com>
> Reported-by: Yibo Cai <Yibo.Cai@arm.com>
> ---
> mm/memory.c | 99
> +++++++++++++++++++++++++++++++++++++++++++++--------
> 1 file changed, 84 insertions(+), 15 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index e2bb51b6242e..a0a381b36ff2 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;
> @@ -2140,32 +2147,82 @@ 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 bool cow_user_page(struct page *dst, struct page *src,
> + struct vm_fault *vmf)
> {
> + bool ret;
> + void *kaddr;
> + void __user *uaddr;
> + bool force_mkyoung;
> + struct vm_area_struct *vma = vmf->vma;
> + struct mm_struct *mm = vma->vm_mm;
> + unsigned long addr = vmf->address;
> +
> debug_dma_assert_idle(src);
>
> + if (likely(src)) {
> + copy_user_highpage(dst, src, addr, vma);
> + return true;
> + }
> +
> /*
> * If the source page was a PFN mapping, we don't have
> * a "struct page" for it. We do a best-effort copy by
> * just copying from the original user address. If that
> * fails, we just zero-fill it. Live with it.
> */
> - if (unlikely(!src)) {
> - void *kaddr = kmap_atomic(dst);
> - void __user *uaddr = (void __user *)(va & PAGE_MASK);
> + kaddr = kmap_atomic(dst);
> + uaddr = (void __user *)(addr & PAGE_MASK);
> +
> + /*
> + * On architectures with software "accessed" bits, we would
> + * take a double page fault, so mark it accessed here.
> + */
> + force_mkyoung = arch_faults_on_old_pte() && !pte_young(vmf-
> >orig_pte);
> + if (force_mkyoung) {
> + pte_t entry;
> +
> + vmf->pte = pte_offset_map_lock(mm, vmf->pmd, addr,
> &vmf->ptl);
> + if (!likely(pte_same(*vmf->pte, vmf->orig_pte))) {
> + /*
> + * Other thread has already handled the fault
> + * and we don't need to do anything. If it's
> + * not the case, the fault will be triggered
> + * again on the same address.
> + */
> + ret = false;
> + goto pte_unlock;
> + }
> +
> + entry = pte_mkyoung(vmf->orig_pte);
> + if (ptep_set_access_flags(vma, addr, vmf->pte, entry, 0))
> + update_mmu_cache(vma, addr, vmf->pte);
> + }
>
> + /*
> + * 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.
> + */
> + if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE)) {
> /*
> - * 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.
> + * Give a warn in case there can be some obscure
> + * use-case
> */
> - 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);
> + WARN_ON_ONCE(1);
> + clear_page(kaddr);
> + }
> +
> + ret = true;
> +
> +pte_unlock:
> + if (force_mkyoung)
> + pte_unmap_unlock(vmf->pte, vmf->ptl);
> + kunmap_atomic(kaddr);
> + flush_dcache_page(dst);
> +
> + return ret;
> }
>
> static gfp_t __get_fault_gfp_mask(struct vm_area_struct *vma)
> @@ -2318,7 +2375,19 @@ 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);
> +
> + if (!cow_user_page(new_page, old_page, vmf)) {
> + /*
> + * COW failed, if the fault was solved by other,
> + * it's fine. If not, userspace would re-fault on
> + * the same address and we will handle the fault
> + * from the second attempt.
> + */
> + put_page(new_page);
> + if (old_page)
> + put_page(old_page);
> + return 0;
> + }
> }
>
> if (mem_cgroup_try_charge_delay(new_page, mm, GFP_KERNEL,
> &memcg, false))
> --
> 2.17.1

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