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

[Aneesh Kumar K.V]

On 9/20/18 4:35 PM, Kirill A. Shutemov wrote:
> 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.


we do that in the ptep_modify_prot_start/ptep_modify_prot_commit. Also 
in hugetlb_change_protection. The hugetlb case many not be relevant 
because that cannot be backed by a vma without vma->vm_ops.

What will hit this will be mprotect of a remap_pfn_range address?

> > 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.

I didn't get that. If we find the pte not none, we return so that we 
retry the access. Are you suggesting VM_FAULT_NOPAGE is not the right 
return for that?

-aneesh