Commit 83d116c5 authored by Jia He's avatar Jia He Committed by Catalin Marinas
Browse files

mm: fix double page fault on arm64 if PTE_AF is cleared 

When we tested pmdk unit test [1] vmmalloc_fork TEST3 on arm64 guest, there
will be a double page fault in __copy_from_user_inatomic of cow_user_page.

To reproduce the bug, the cmd is as follows after you deployed everything:
make -C src/test/vmmalloc_fork/ TEST_TIME=60m check

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 fixes 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: default avatarJia He <justin.he@arm.com>
Reported-by: default avatarYibo Cai <Yibo.Cai@arm.com>
Reviewed-by: default avatarCatalin Marinas <catalin.marinas@arm.com>
Acked-by: default avatarKirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: default avatarCatalin Marinas <catalin.marinas@arm.com>
parent f2c4e597

mm/memory.c

+89 −15
Original line number Diff line number Diff line
@@ -118,6 +118,18 @@ int randomize_va_space __read_mostly = 
					2;

#endif



#ifndef arch_faults_on_old_pte

static inline bool arch_faults_on_old_pte(void)

{

	/*

	 * Those arches which don't have hw access flag feature need to

	 * implement their own helper. By default, "true" means pagefault

	 * will be hit on old pte.

	 */

	return true;

}

#endif



static int __init disable_randmaps(char *s)

{

	randomize_va_space = 0;
@@ -2145,19 +2157,57 @@ 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
@@ -2165,12 +2215,24 @@ static inline void cow_user_page(struct page *dst, struct page *src, unsigned lo 
	 * in which case we just give up and fill the result with

	 * zeroes.

	 */

		if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE))

	if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE)) {

		/*

		 * Give a warn in case there can be some obscure

		 * use-case

		 */

		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);

	} else

		copy_user_highpage(dst, src, va, vma);



	return ret;

}



static gfp_t __get_fault_gfp_mask(struct vm_area_struct *vma)
@@ -2327,7 +2389,19 @@ 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))

CRA Git | Maintained and supported by SUSTech CRA and CCSE