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Subject[PATCH v9 3/3] mm: fix double page fault on arm64 if PTE_AF is cleared
Date
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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