Date:   Mon, 03 Apr 2017 14:02:29 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     alex.williamson@...hat.com, kvm@...r.kernel.org
Cc:     eric.auger@...hat.com, kwankhede@...dia.com,
        linux-kernel@...r.kernel.org, slp@...hat.com
Subject: [PATCH] vfio/type1: Remove locked page accounting workqueue

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 able to race the workqueue to enter more mappings
than they're allowed.  It's not entirely clear what motivated this
workqueue mechanism in the original vfio design, but it seems to
introduce more problems than it solves, so remove it 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.

Cc: stable@...r.kernel.org
Signed-off-by: Alex Williamson <alex.williamson@...hat.com>
---

Sergio had proposed a QEMU workaround for this:
https://lists.nongnu.org/archive/html/qemu-devel/2017-04/msg00244.html
Clearly the bug is in the kernel and I'm more inclined to fix it via
stable releases.  I also considered adding a flag in the type1 info
structure to indicate synchronous lock accounting, but then second
guessed using that to advertise the defect, especially if the workaround
is only to pause and try again.  Comments/suggestions?  Thanks,

Alex

 drivers/vfio/vfio_iommu_type1.c |   99 ++++++++++++++++++---------------------
 1 file changed, 45 insertions(+), 54 deletions(-)

diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
index 32d2633092a3..d93a88748d14 100644
--- a/drivers/vfio/vfio_iommu_type1.c
+++ b/drivers/vfio/vfio_iommu_type1.c
@@ -246,69 +246,45 @@ 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)
+static int vfio_lock_acct(struct task_struct *task, long npage)
 {
-	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)
-{
-	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)) {
+	ret = down_write_killable(&mm->mmap_sem);
+	if (ret)
+		return ret;
+
+	if (npage < 0) {
 		mm->locked_vm += npage;
-		up_write(&mm->mmap_sem);
-		if (!is_current)
-			mmput(mm);
-		return;
-	}
+	} else {
+		unsigned long limit;
+
+		limit = is_current ? rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT :
+				task_rlimit(task, RLIMIT_MEMLOCK) >> PAGE_SHIFT;
 
-	if (is_current) {
-		mm = get_task_mm(task);
-		if (!mm)
-			return;
+		if (mm->locked_vm + npage <= limit)
+			mm->locked_vm += npage;
+		else
+			ret = -ENOMEM;
 	}
 
-	/*
-	 * 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)) {
+	up_write(&mm->mmap_sem);
+
+	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 +381,7 @@ 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 +418,6 @@ 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 +434,25 @@ 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 +507,14 @@ 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:

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