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Date:   Mon, 22 Mar 2021 11:14:37 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Bui Quang Minh <minhquangbui99@gmail.com>
Cc:     akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Andrea Arcangeli <aarcange@redhat.com>,
        Mike Rapoport <rppt@kernel.org>
Subject: Re: [PATCH] userfaultfd: Write protect when virtual memory range has
 no page table entry
Message-ID: <YFhuDf6L7nkUoT7q@dhcp22.suse.cz>
References: <20210319152428.52683-1-minhquangbui99@gmail.com>
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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@gmail.com>
> ---
>  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