[PATCH 2/2] scsi: target: tcmu: Fix memory leak caused by wrong uio usage

[Bodo Stroesser <bostroesser@gmail.com>]

When user deletes a tcmu device via configFS, tcmu calls
uio_unregister_device(). During that call uio resets its pointer
to struct uio_info provided by tcmu. That means, after
uio_unregister_device uio will no longer execute any of the
callbacks tcmu had set in uio_info.

Especially, if userspace daemon still holds the corresponding uio
device open or mmap'ed while tcmu calls uio_unregister_device(),
uio will not call tcmu_release() when userspace finally closes
and munmaps the uio device.

Since tcmu does refcounting for the tcmu device in tcmu_open()
and tcmu_release(), in the decribed case refcount does not drop
to 0 and tcmu does not free tcmu device's resources.
In extrem cases this can cause memory leaking of up to 1 GB for
a single tcmu device.

After uio_unregister_device(), uio will reject every open, read,
write, mmap from userspace with -EOI. But userspace daemon can
still access the mmap'ed command ring and data area. Therefore
tcmu should wait until userspace munmaps the uio device before
it frees the resources, as we don't want to cause SIGSEGV or
SIGBUS to user space.

That said, current refcounting during tcmu_open and tcmu_release
does not work correctly, and refcounting better should be done in
the open and close callouts of the vm_operations_struct, which
tcmu assigns to each mmap of the uio device (because it wants its
own page fault handler).

This patch fixes the memory leak by removing refcounting from
tcmu_open and tcmu_close, and instead adding new tcmu_vma_open()
and tcmu_vma_close() handlers that only do refcounting.

Signed-off-by: Bodo Stroesser <bostroesser@gmail.com>
---
 drivers/target/target_core_user.c | 29 ++++++++++++++++++++++++++---
 1 file changed, 26 insertions(+), 3 deletions(-)

diff --git a/drivers/target/target_core_user.c b/drivers/target/target_core_user.c
index 6d010cf22879..bf73cd5f4b04 100644
--- a/drivers/target/target_core_user.c
+++ b/drivers/target/target_core_user.c
@@ -1643,6 +1643,8 @@ static void tcmu_dev_kref_release(struct kref *kref)
 	bitmap_free(udev->data_bitmap);
 	mutex_unlock(&udev->cmdr_lock);
 
+	pr_debug("dev_kref_release\n");
+
 	call_rcu(&dev->rcu_head, tcmu_dev_call_rcu);
 }
 
@@ -1758,6 +1760,25 @@ static struct page *tcmu_try_get_block_page(struct tcmu_dev *udev, uint32_t dbi)
 	return page;
 }
 
+static void tcmu_vma_open(struct vm_area_struct *vma)
+{
+	struct tcmu_dev *udev = vma->vm_private_data;
+
+	pr_debug("vma_open\n");
+
+	kref_get(&udev->kref);
+}
+
+static void tcmu_vma_close(struct vm_area_struct *vma)
+{
+	struct tcmu_dev *udev = vma->vm_private_data;
+
+	pr_debug("vma_close\n");
+
+	/* release ref from tcmu_vma_open */
+	kref_put(&udev->kref, tcmu_dev_kref_release);
+}
+
 static vm_fault_t tcmu_vma_fault(struct vm_fault *vmf)
 {
 	struct tcmu_dev *udev = vmf->vma->vm_private_data;
@@ -1796,6 +1817,8 @@ static vm_fault_t tcmu_vma_fault(struct vm_fault *vmf)
 }
 
 static const struct vm_operations_struct tcmu_vm_ops = {
+	.open = tcmu_vma_open,
+	.close = tcmu_vma_close,
 	.fault = tcmu_vma_fault,
 };
 
@@ -1812,6 +1835,8 @@ static int tcmu_mmap(struct uio_info *info, struct vm_area_struct *vma)
 	if (vma_pages(vma) != (udev->ring_size >> PAGE_SHIFT))
 		return -EINVAL;
 
+	tcmu_vma_open(vma);
+
 	return 0;
 }
 
@@ -1824,7 +1849,6 @@ static int tcmu_open(struct uio_info *info, struct inode *inode)
 		return -EBUSY;
 
 	udev->inode = inode;
-	kref_get(&udev->kref);
 
 	pr_debug("open\n");
 
@@ -1838,8 +1862,7 @@ static int tcmu_release(struct uio_info *info, struct inode *inode)
 	clear_bit(TCMU_DEV_BIT_OPEN, &udev->flags);
 
 	pr_debug("close\n");
-	/* release ref from open */
-	kref_put(&udev->kref, tcmu_dev_kref_release);
+
 	return 0;
 }
 
-- 
2.12.3