Le'ts Andrea and Mike
On Fri 19-03-21 22:24:28, Bui Quang Minh wrote:
> userfaultfd_writeprotect() use change_protection() to clear write bit in
> page table entries (pte/pmd). So, later write to this virtual address
> range causes a page fault, which is then handled by userspace program.
> However, change_protection() has no effect when there is no page table
> entries associated with that virtual memory range (a newly mapped memory
> range). As a result, later access to that memory range causes allocating a
> page table entry with write bit still set (due to VM_WRITE flag in
> vma->vm_flags).
>
> Add checks for VM_UFFD_WP in vma->vm_flags when allocating new page table
> entry in missing page table entry page fault path.
>From the above it is not really clear whether this is a usability
problem or a bug of the interface.
> Signed-off-by: Bui Quang Minh <minhquangbui99@xxxxxxxxx>
> ---
> mm/huge_memory.c | 12 ++++++++++++
> mm/memory.c | 10 ++++++++++
> 2 files changed, 22 insertions(+)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index ae907a9c2050..9bb16a55a48c 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -636,6 +636,11 @@ static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf,
>
> entry = mk_huge_pmd(page, vma->vm_page_prot);
> entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
> + if (userfaultfd_wp(vma)) {
> + entry = pmd_wrprotect(entry);
> + entry = pmd_mkuffd_wp(entry);
> + }
> +
> page_add_new_anon_rmap(page, vma, haddr, true);
> lru_cache_add_inactive_or_unevictable(page, vma);
> pgtable_trans_huge_deposit(vma->vm_mm, vmf->pmd, pgtable);
> @@ -643,6 +648,13 @@ static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf,
> update_mmu_cache_pmd(vma, vmf->address, vmf->pmd);
> add_mm_counter(vma->vm_mm, MM_ANONPAGES, HPAGE_PMD_NR);
> mm_inc_nr_ptes(vma->vm_mm);
> +
> + if (userfaultfd_huge_pmd_wp(vma, *vmf->pmd)) {
> + spin_unlock(vmf->ptl);
> + count_vm_event(THP_FAULT_ALLOC);
> + count_memcg_event_mm(vma->vm_mm, THP_FAULT_ALLOC);
> + return handle_userfault(vmf, VM_UFFD_WP);
> + }
> spin_unlock(vmf->ptl);
> count_vm_event(THP_FAULT_ALLOC);
> count_memcg_event_mm(vma->vm_mm, THP_FAULT_ALLOC);
> diff --git a/mm/memory.c b/mm/memory.c
> index 5efa07fb6cdc..b835746545bf 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3564,6 +3564,11 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
> if (vma->vm_flags & VM_WRITE)
> entry = pte_mkwrite(pte_mkdirty(entry));
>
> + if (userfaultfd_wp(vma)) {
> + entry = pte_wrprotect(entry);
> + entry = pte_mkuffd_wp(entry);
> + }
> +
> vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, vmf->address,
> &vmf->ptl);
> if (!pte_none(*vmf->pte)) {
> @@ -3590,6 +3595,11 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
>
> /* No need to invalidate - it was non-present before */
> update_mmu_cache(vma, vmf->address, vmf->pte);
> +
> + if (userfaultfd_pte_wp(vma, *vmf->pte)) {
> + pte_unmap_unlock(vmf->pte, vmf->ptl);
> + return handle_userfault(vmf, VM_UFFD_WP);
> + }
> unlock:
> pte_unmap_unlock(vmf->pte, vmf->ptl);
> return ret;
> --
> 2.25.1
--
Michal Hocko
SUSE Labs
