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Date: Fri, 14 Apr 2017 16:00:32 -0600
From: Alex Williamson <alex.williamson@redhat.com>
To: Kirti Wankhede <kwankhede@nvidia.com>
Cc: <kvm@vger.kernel.org>, <eric.auger@redhat.com>, <peterx@redhat.com>,
        <linux-kernel@vger.kernel.org>, <slp@redhat.com>
Subject: Re: [PATCH v3] vfio/type1: Remove locked page accounting workqueue
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On Fri, 14 Apr 2017 14:58:14 -0600
Alex Williamson <alex.williamson@redhat.com> wrote:

> On Sat, 15 Apr 2017 00:51:28 +0530
> Kirti Wankhede <kwankhede@nvidia.com> wrote:
> 
> > On 4/12/2017 12:58 AM, Alex Williamson wrote:  
> > > If the mmap_sem is contented then the vfio type1 IOMMU backend will
> > > defer locked page accounting updates to a workqueue task.  This has a
> > > few problems and depending on which side the user tries to play, they
> > > might be over-penalized for unmaps that haven't yet been accounted or
> > > race the workqueue to enter more mappings than they're allowed.  The
> > > original intent of this workqueue mechanism seems to be focused on
> > > reducing latency through the ioctl, but we cannot do so at the cost
> > > of correctness.  Remove this workqueue mechanism and update the
> > > callers to allow for failure.  We can also now recheck the limit under
> > > write lock to make sure we don't exceed it.
> > > 
> > > vfio_pin_pages_remote() also now necessarily includes an unwind path
> > > which we can jump to directly if the consecutive page pinning finds
> > > that we're exceeding the user's memory limits.  This avoids the
> > > current lazy approach which does accounting and mapping up to the
> > > fault, only to return an error on the next iteration to unwind the
> > > entire vfio_dma.
> > > 
> > > Cc: stable@vger.kernel.org
> > > Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> > > ---
> > > 
> > > v3: Update for comments from Peter
> > >     - Use task_rlimit() exclusively
> > >     - Discuss vfio_pin_pages_remote() exit branch in commitlog
> > > 
> > >  drivers/vfio/vfio_iommu_type1.c |   99 +++++++++++++++++----------------------
> > >  1 file changed, 44 insertions(+), 55 deletions(-)
> > > 
> > > diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> > > index 32d2633092a3..176ebcc0ffa2 100644
> > > --- a/drivers/vfio/vfio_iommu_type1.c
> > > +++ b/drivers/vfio/vfio_iommu_type1.c
> > > @@ -246,69 +246,43 @@ static int vfio_iova_put_vfio_pfn(struct vfio_dma *dma, struct vfio_pfn *vpfn)
> > >  	return ret;
> > >  }
> > >  
> > > -struct vwork {
> > > -	struct mm_struct	*mm;
> > > -	long			npage;
> > > -	struct work_struct	work;
> > > -};
> > > -
> > > -/* delayed decrement/increment for locked_vm */
> > > -static void vfio_lock_acct_bg(struct work_struct *work)
> > > -{
> > > -	struct vwork *vwork = container_of(work, struct vwork, work);
> > > -	struct mm_struct *mm;
> > > -
> > > -	mm = vwork->mm;
> > > -	down_write(&mm->mmap_sem);
> > > -	mm->locked_vm += vwork->npage;
> > > -	up_write(&mm->mmap_sem);
> > > -	mmput(mm);
> > > -	kfree(vwork);
> > > -}
> > > -
> > > -static void vfio_lock_acct(struct task_struct *task, long npage)
> > > +static int vfio_lock_acct(struct task_struct *task, long npage)
> > >  {
> > > -	struct vwork *vwork;
> > >  	struct mm_struct *mm;
> > >  	bool is_current;
> > > +	int ret;
> > >  
> > >  	if (!npage)
> > > -		return;
> > > +		return 0;
> > >  
> > >  	is_current = (task->mm == current->mm);
> > >  
> > >  	mm = is_current ? task->mm : get_task_mm(task);
> > >  	if (!mm)
> > > -		return; /* process exited */
> > > +		return -ESRCH; /* process exited */
> > >  
> > > -	if (down_write_trylock(&mm->mmap_sem)) {
> > > -		mm->locked_vm += npage;
> > > -		up_write(&mm->mmap_sem);
> > > -		if (!is_current)
> > > -			mmput(mm);
> > > -		return;
> > > -	}
> > > +	ret = down_write_killable(&mm->mmap_sem);
> > > +	if (!ret) {
> > > +		if (npage < 0) {
> > > +			mm->locked_vm += npage;
> > > +		} else {
> > > +			unsigned long limit;
> > >  
> > > -	if (is_current) {
> > > -		mm = get_task_mm(task);
> > > -		if (!mm)
> > > -			return;
> > > +			limit = task_rlimit(task, RLIMIT_MEMLOCK) >> PAGE_SHIFT;
> > > +
> > > +			if (mm->locked_vm + npage <= limit)
> > > +				mm->locked_vm += npage;
> > > +			else
> > > +				ret = -ENOMEM;
> > > +		}
> > > +    
> > 
> > Sorry if I'm late here on my review.
> > 
> > There are rlimit checks before calling vfio_lock_acct() while pinning
> > pages. I agree this is checked holding locks, so this check is more
> > robust, but still it feels redundant. I think you can remove checks from
> > vfio_pin_page_external() and vfio_pin_pages_remote().  
> 
> If we removed those pre-checks then a user/mdev vendor driver would be
> able to pin massive amounts of memory, potentially causing a DoS on the
> host (ex. trigger OOM), before we bother to test whether they really
> have permission to do so.  I think redundancy is better.

Perhaps _external is overly redundant since it's only doing one page
increments, _remote could go well into the above scenario w/o the
redundancy.  Thanks,

Alex

> > Also while checking the limit, !lock_cap checks is not considered here.
> > That would mean that there code would impose limit check even without
> > lock capability?  
> 
> That's a bug!  Thanks,
> 
> Alex
> 
> > > +		up_write(&mm->mmap_sem);
> > >  	}
> > >  
> > > -	/*
> > > -	 * Couldn't get mmap_sem lock, so must setup to update
> > > -	 * mm->locked_vm later. If locked_vm were atomic, we
> > > -	 * wouldn't need this silliness
> > > -	 */
> > > -	vwork = kmalloc(sizeof(struct vwork), GFP_KERNEL);
> > > -	if (WARN_ON(!vwork)) {
> > > +	if (!is_current)
> > >  		mmput(mm);
> > > -		return;
> > > -	}
> > > -	INIT_WORK(&vwork->work, vfio_lock_acct_bg);
> > > -	vwork->mm = mm;
> > > -	vwork->npage = npage;
> > > -	schedule_work(&vwork->work);
> > > +
> > > +	return ret;
> > >  }
> > >  
> > >  /*
> > > @@ -405,7 +379,7 @@ static int vaddr_get_pfn(struct mm_struct *mm, unsigned long vaddr,
> > >  static long vfio_pin_pages_remote(struct vfio_dma *dma, unsigned long vaddr,
> > >  				  long npage, unsigned long *pfn_base)
> > >  {
> > > -	unsigned long limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
> > > +	unsigned long pfn = 0, limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
> > >  	bool lock_cap = capable(CAP_IPC_LOCK);
> > >  	long ret, pinned = 0, lock_acct = 0;
> > >  	bool rsvd;
> > > @@ -442,8 +416,6 @@ static long vfio_pin_pages_remote(struct vfio_dma *dma, unsigned long vaddr,
> > >  	/* Lock all the consecutive pages from pfn_base */
> > >  	for (vaddr += PAGE_SIZE, iova += PAGE_SIZE; pinned < npage;
> > >  	     pinned++, vaddr += PAGE_SIZE, iova += PAGE_SIZE) {
> > > -		unsigned long pfn = 0;
> > > -
> > >  		ret = vaddr_get_pfn(current->mm, vaddr, dma->prot, &pfn);
> > >  		if (ret)
> > >  			break;
> > > @@ -460,14 +432,25 @@ static long vfio_pin_pages_remote(struct vfio_dma *dma, unsigned long vaddr,
> > >  				put_pfn(pfn, dma->prot);
> > >  				pr_warn("%s: RLIMIT_MEMLOCK (%ld) exceeded\n",
> > >  					__func__, limit << PAGE_SHIFT);
> > > -				break;
> > > +				ret = -ENOMEM;
> > > +				goto unpin_out;
> > >  			}
> > >  			lock_acct++;
> > >  		}
> > >  	}
> > >  
> > >  out:
> > > -	vfio_lock_acct(current, lock_acct);
> > > +	ret = vfio_lock_acct(current, lock_acct);
> > > +
> > > +unpin_out:
> > > +	if (ret) {
> > > +		if (!rsvd) {
> > > +			for (pfn = *pfn_base ; pinned ; pfn++, pinned--)
> > > +				put_pfn(pfn, dma->prot);
> > > +		}
> > > +
> > > +		return ret;
> > > +	}
> > >  
> > >  	return pinned;
> > >  }
> > > @@ -522,8 +505,14 @@ static int vfio_pin_page_external(struct vfio_dma *dma, unsigned long vaddr,
> > >  		goto pin_page_exit;
> > >  	}
> > >  
> > > -	if (!rsvd && do_accounting)
> > > -		vfio_lock_acct(dma->task, 1);
> > > +	if (!rsvd && do_accounting) {
> > > +		ret = vfio_lock_acct(dma->task, 1);
> > > +		if (ret) {
> > > +			put_pfn(*pfn_base, dma->prot);
> > > +			goto pin_page_exit;
> > > +		}
> > > +	}
> > > +
> > >  	ret = 1;
> > >  
> > >  pin_page_exit:
> > >     
>