Re: [PATCH] mm: Recheck page table entry with page table lock held

[Kirill A. Shutemov]

On Thu, Sep 20, 2018 at 02:54:08PM +0530, Aneesh Kumar K.V wrote:
> We clear the pte temporarily during read/modify/write update of the pte. If we
> take a page fault while the pte is cleared, the application can get SIGBUS. One
> such case is with remap_pfn_range without a backing vm_ops->fault callback.
> do_fault will return SIGBUS in that case.

It would be nice to show the path that clears pte temporarily.

> Fix this by taking page table lock and rechecking for pte_none.
> 
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx>
> ---
>  mm/memory.c | 31 +++++++++++++++++++++++++++----
>  1 file changed, 27 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index c467102a5cbc..c2f933184303 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3745,10 +3745,33 @@ static vm_fault_t do_fault(struct vm_fault *vmf)
>  	struct vm_area_struct *vma = vmf->vma;
>  	vm_fault_t ret;
>  
> -	/* The VMA was not fully populated on mmap() or missing VM_DONTEXPAND */
> -	if (!vma->vm_ops->fault)
> -		ret = VM_FAULT_SIGBUS;
> -	else if (!(vmf->flags & FAULT_FLAG_WRITE))
> +	/*
> +	 * The VMA was not fully populated on mmap() or missing VM_DONTEXPAND
> +	 */
> +	if (!vma->vm_ops->fault) {
> +
> +		/*
> +		 * pmd entries won't be marked none during a R/M/W cycle.
> +		 */
> +		if (unlikely(pmd_none(*vmf->pmd)))
> +			ret = VM_FAULT_SIGBUS;
> +		else {
> +			vmf->ptl = pte_lockptr(vmf->vma->vm_mm, vmf->pmd);
> +			/*
> +			 * Make sure this is not a temporary clearing of pte
> +			 * by holding ptl and checking again. A R/M/W update
> +			 * of pte involves: take ptl, clearing the pte so that
> +			 * we don't have concurrent modification by hardware
> +			 * followed by an update.
> +			 */
> +			spin_lock(vmf->ptl);
> +			if (unlikely(pte_none(*vmf->pte)))
> +				ret = VM_FAULT_SIGBUS;
> +			else
> +				ret = VM_FAULT_NOPAGE;

We return 0 if we did nothing in fault path.

-- 
 Kirill A. Shutemov