[PATCH v2 0/4] s390/vfio-ap: Fix pre-existing bugs in vfio_ap device driver

[PATCH v2 0/4] s390/vfio-ap: Fix pre-existing bugs in vfio_ap device driver

[Anthony Krowiak]

The sashiko AI reported several pre-existing bugs in the vfio_ap device
driver code while reviewing unrelated patches. This series fixes four
such bugs.

This series is based on the 'next' branch in the 
https://git.kernel.org/pub/scm/linux/kernel/git/kvms390/linux.git
source tree.

Change log v1 => v2:
~~~~~~~~~~~~~~~~~~~
Patch 1/4: Fix leak of pinned NIB and registered NISC in 
           vfio_ap_irq_enable() 
* Removed call to pr_warn() in vfio_ap_irq_enable() for the default case
  in the switch statement to avoid flooding of the dmesg log. This was
  a pre-existing condition detected by sashiko and fixed here since it is
  in the code block changed by this patch. The VFIO_AP_DBF_WARN that occurs
  after the switch statement will display the error to the dbf log. 

Patch 3/4: Fix unbounded loop in apq_reset_check()
* Removed call to vfio_ap_free_aqic_resources() and provided a doc block
  explaining the rationale for allowing the leak of resources.

Patch 4/4: Use AP_DOMAINS for adm_add bitmap size in vfio_ap_mdev_cfg_add()
* Changed subject prefix to Use AP_DOMAINS for aqm/adm bitmap size
* Fixed two other locations where AP_DEVICES was used for the aqm/adm
  bitmap size.

Anthony Krowiak (4):
  s390/vfio-ap: Fix leak of pinned NIB and registered NISC in
    vfio_ap_irq_enable()
  s390/vfio-ap: Fix failure to release IRQ notification eventfd contexts
  s390/vfio-ap: Fix unbounded loop in apq_reset_check()
  s390/vfio-ap: Use AP_DOMAINS for adm_add bitmap size in
    vfio_ap_mdev_cfg_add()

 drivers/s390/crypto/vfio_ap_ops.c | 55 +++++++++++++++++++++++++++----
 1 file changed, 48 insertions(+), 7 deletions(-)

-- 
2.53.0

[PATCH v2 1/4] s390/vfio-ap: Fix leak of pinned NIB and registered NISC in vfio_ap_irq_enable()

[Anthony Krowiak]

The vfio_ap_irq_enable() function executes the PQAP(AQIC) instruction to
enable interrupts for an AP queue. A switch statement is used to examine
the status response code returned from the instruction to determine
whether it succeeded or failed and react accordingly. For the default case,
the vfio_ap_irq_disable function is invoked to disable interrupts for the
queue and clean up the AQIC resources (i.e., unpin the NIB and unregister
the NISC). There are a number of problems with this:

1. Neither the q->saved_iova nor q->saved_isc has been set, so the
   AQIC resources - assuming those values have been previously set - will
   be the NIB and NISC resources from a prior call; the NIB and NISC from
   the current call are therefore leaked.

2. Interrupts may never have been enabled. Sending a disable instruction to
   a queue that the hardware just told you is in a bad state (CHECKSTOPPED,
   DECONFIGURED, Q_NOT_AVAIL) is at best wasted work and at worst generates
   a further WARN_ONCE from inside vfio_ap_irq_disable's own default.

3. The hardware just rejected the new ap_aqic() enable attempt with an
   unexpected status. Disabling a previously-working IRQ config - assuming
   that is even possible - as a reaction to a failed enable attempt does
   not make sense; it is actively destructive, tearing down something that
   was working for no valid reason.

The fix is to unregister the NISC and an unpin the NIB in the default case
of the switch statement.

Fixes: ec89b55e3bce7 ("s390: ap: implement PAPQ AQIC interception in kernel")
Cc: stable@vger.kernel.org
Signed-off-by: Anthony Krowiak <akrowiak@linux.ibm.com>
---
 drivers/s390/crypto/vfio_ap_ops.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/s390/crypto/vfio_ap_ops.c b/drivers/s390/crypto/vfio_ap_ops.c
index 940c0ff668be..22a6ceaa7b56 100644
--- a/drivers/s390/crypto/vfio_ap_ops.c
+++ b/drivers/s390/crypto/vfio_ap_ops.c
@@ -503,9 +503,12 @@ static struct ap_queue_status vfio_ap_irq_enable(struct vfio_ap_queue *q,
 		vfio_unpin_pages(&q->matrix_mdev->vdev, nib, 1);
 		break;
 	default:
-		pr_warn("%s: apqn %04x: response: %02x\n", __func__, q->apqn,
-			status.response_code);
-		vfio_ap_irq_disable(q);
+		/* We could not modify IRQ settings: clear new configuration */
+		ret = kvm_s390_gisc_unregister(kvm, isc);
+		if (ret)
+			VFIO_AP_DBF_WARN("%s: kvm_s390_gisc_unregister: rc=%d isc=%d, apqn=%#04x\n",
+					 __func__, ret, isc, q->apqn);
+		vfio_unpin_pages(&q->matrix_mdev->vdev, nib, 1);
 		break;
 	}
 
-- 
2.53.0

[PATCH v2 2/4] s390/vfio-ap: Fix failure to release IRQ notification eventfd contexts

[Anthony Krowiak]

When userspace registers IRQ notification eventfds via the
VFIO_DEVICE_SET_IRQS ioctl, vfio_ap_set_request_irq() and
vfio_ap_set_cfg_change_irq() each call eventfd_ctx_fdget(), which
takes a reference on the eventfd_ctx and stores it in
matrix_mdev->req_trigger and matrix_mdev->cfg_chg_trigger
respectively.

These references are dropped only when userspace explicitly replaces
or clears them via a subsequent SET_IRQS call.  If the device is
closed without that explicit teardown - because the guest exits,
the VM process crashes, or the device file is simply closed -
neither vfio_ap_mdev_close_device() nor the remove path releases
these references.  The eventfd_ctx backing objects and their
associated file references therefore leak for the lifetime of the
kernel.

Fix this by introducing vfio_ap_mdev_release_eventfds() and calling
it from vfio_ap_mdev_close_device() after vfio_ap_mdev_unset_kvm().
The VFIO core guarantees that close_device is called before
vfio_unregister_group_dev() returns in the remove path, so fixing
close_device is sufficient to cover both teardown paths.

Note:
~~~~
The matrix_dev->mdevs lock must be held during the call to
vfio_ap_mdev_release_eventfds(). There is a small window between the calls
to vfio_ap_mdev_unset_kvm() which gets and releases the update locks
and the acquisition of the matrix_dev->mdevs_lock mutex during which
it is possible - although highly unlikely during normal operation - whereby
a concurrent SET_IRQS call can get in.

Taking matrix_dev->mdevs_lock around vfio_ap_mdev_release_eventfds()
is sufficient to make this race-free. The SET_IRQS ioctl path writes
req_trigger and cfg_chg_trigger only from vfio_ap_mdev_ioctl(), which
holds mdevs_lock for its entire duration and always calls
eventfd_ctx_put() on the previous value before storing the new one.

Any number of concurrent SET_IRQS calls during the window between
vfio_ap_mdev_unset_kvm() and the acquisition of mdevs_lock are
therefore safe: each ioctl invocation puts the reference it found and
installs a new one, leaving exactly one live reference in the field
when it releases the lock. When release_eventfds subsequently acquires
mdevs_lock it finds that single surviving reference and puts it.
Conversely, a SET_IRQS call that loses the race and blocks on
mdevs_lock will find the field NULL after release_eventfds finishes,
take ownership of the reference it just created, and install it into a
field that will never be read again - a transient leak. To close that
final case, callers must ensure no new SET_IRQS ioctls can be issued
after close_device() is called, which the VFIO core guarantees by
releasing the device file before invoking close_device().

Fixes: bf48961f6f48e ("s390/vfio-ap: realize the VFIO_DEVICE_SET_IRQS ioctl")
Cc: stable@vger.kernel.org
Signed-off-by: Anthony Krowiak <akrowiak@linux.ibm.com>
Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
---
 drivers/s390/crypto/vfio_ap_ops.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/drivers/s390/crypto/vfio_ap_ops.c b/drivers/s390/crypto/vfio_ap_ops.c
index 22a6ceaa7b56..3f99b239fa95 100644
--- a/drivers/s390/crypto/vfio_ap_ops.c
+++ b/drivers/s390/crypto/vfio_ap_ops.c
@@ -2064,12 +2064,28 @@ static int vfio_ap_mdev_open_device(struct vfio_device *vdev)
 	return vfio_ap_mdev_set_kvm(matrix_mdev, vdev->kvm);
 }
 
+static void vfio_ap_mdev_release_eventfds(struct ap_matrix_mdev *matrix_mdev)
+{
+	if (matrix_mdev->req_trigger) {
+		eventfd_ctx_put(matrix_mdev->req_trigger);
+		matrix_mdev->req_trigger = NULL;
+	}
+	if (matrix_mdev->cfg_chg_trigger) {
+		eventfd_ctx_put(matrix_mdev->cfg_chg_trigger);
+		matrix_mdev->cfg_chg_trigger = NULL;
+	}
+}
+
 static void vfio_ap_mdev_close_device(struct vfio_device *vdev)
 {
 	struct ap_matrix_mdev *matrix_mdev =
 		container_of(vdev, struct ap_matrix_mdev, vdev);
 
 	vfio_ap_mdev_unset_kvm(matrix_mdev);
+
+	mutex_lock(&matrix_dev->mdevs_lock);
+	vfio_ap_mdev_release_eventfds(matrix_mdev);
+	mutex_unlock(&matrix_dev->mdevs_lock);
 }
 
 static void vfio_ap_mdev_request(struct vfio_device *vdev, unsigned int count)
-- 
2.53.0

[PATCH v2 3/4] s390/vfio-ap: Fix unbounded loop in apq_reset_check()

[Anthony Krowiak]

The apq_reset_check() worker polls ap_tapq() in a while(true) loop
waiting for a queue reset to complete. When ap_tapq() returns
AP_RESPONSE_BUSY or AP_RESPONSE_RESET_IN_PROGRESS,
apq_status_check() returns -EBUSY and the loop continues after
sleeping AP_RESET_MAX_WAIT (20ms). There is no upper bound on how
many times the loop iterates, so if the hardware continuously
returns a busy response the worker runs indefinitely.

This is particularly harmful because several callers of
vfio_ap_mdev_reset_queues() and vfio_ap_mdev_reset_qlist() call
flush_work() on each queue's reset_work while holding one or more
of the global matrix_dev locks (guests_lock, mdevs_lock) or the
KVM lock. An indefinitely spinning worker permanently blocks all
of those locks, hanging mdev removal, KVM guest teardown, and the
VFIO_DEVICE_RESET ioctl path.

Fix this by introducing AP_RESET_MAX_WAIT (2000ms) and breaking out
of the poll loop when elapsed time reaches that threshold. On
timeout the final busy status is written back to q->reset_status
so that callers inspecting reset_status.response_code after
flush_work() see a non-zero value and can return an appropriate
error. vfio_ap_free_aqic_resources() is called before returning
to release any KVM ISC registration and pinned NIB page,
consistent with all other early-exit paths in the function.

Fixes: dd174833e44e ("s390/vfio-ap: remove upper limit on wait for queue reset to complete")
Cc: stable@vger.kernel.org
Signed-off-by: Anthony Krowiak <akrowiak@linux.ibm.com>
---
 drivers/s390/crypto/vfio_ap_ops.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/drivers/s390/crypto/vfio_ap_ops.c b/drivers/s390/crypto/vfio_ap_ops.c
index 3f99b239fa95..7a9b5448e90a 100644
--- a/drivers/s390/crypto/vfio_ap_ops.c
+++ b/drivers/s390/crypto/vfio_ap_ops.c
@@ -31,6 +31,7 @@
 #define AP_QUEUE_IN_USE "in use"
 
 #define AP_RESET_INTERVAL		20	/* Reset sleep interval (20ms)		*/
+#define AP_RESET_MAX_WAIT		2000	/* Maximum wait for reset (2000ms)	*/
 
 static int vfio_ap_mdev_reset_queues(struct ap_matrix_mdev *matrix_mdev);
 static int vfio_ap_mdev_reset_qlist(struct list_head *qlist);
@@ -1971,6 +1972,27 @@ static void apq_reset_check(struct work_struct *reset_work)
 					      status.response_code,
 					      status.queue_empty,
 					      status.irq_enabled);
+			if (elapsed >= AP_RESET_MAX_WAIT) {
+				/*
+				 * Timed out waiting for reset to complete.
+				 *
+				 * The AQIC resources associated with this queue - the pinned page
+				 * containing the NIB and the registered guest ISC - cannot be freed
+				 * here. The NIB is the active DMA target for AP interrupt delivery
+				 * until the reset completes; freeing the pinned page while the
+				 * hardware may still write to it would result in a use-after-free
+				 * kernel crash.
+				 *
+				 * If the reset eventually completes, interrupts will be terminated
+				 * and the pinned NIB page and ISC registration will be leaked. This
+				 * is preferable to either a use-after-free or waiting indefinitely:
+				 * apq_reset_check() holds the matrix_dev->mdevs_lock mutex, which
+				 * serializes access to all mdev objects system-wide, so blocking
+				 * here would stall all other guests using AP queues.
+				 */
+				memcpy(&q->reset_status, &status, sizeof(status));
+				return;
+			}
 		} else {
 			if (q->reset_status.response_code == AP_RESPONSE_RESET_IN_PROGRESS ||
 			    q->reset_status.response_code == AP_RESPONSE_BUSY ||
-- 
2.53.0

[PATCH v2 4/4] s390/vfio-ap: Use AP_DOMAINS for adm_add bitmap size in vfio_ap_mdev_cfg_add()

[Anthony Krowiak]

Domain and control domain bitmaps are sized by the AP_DOMAINS constant, not
AP_DEVICES. The two constants are both 256 today so there is no functional
impact, but using the wrong constant is inconsistent with every operation
on aqm/adm bitmaps.

Use AP_DOMAINS to keep the code consistent and correct in case
the two constants ever diverge.

Note:
This patch was submitted in response to a sashiko review comment pointing
out there are other functions besides vfio_ap_mdev_cfg_add(), so there are
fixes included here for those also. The subject line was kept the same
since this is in v2 of this patch.

Signed-off-by: Anthony Krowiak <akrowiak@linux.ibm.com>
---
 drivers/s390/crypto/vfio_ap_ops.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/s390/crypto/vfio_ap_ops.c b/drivers/s390/crypto/vfio_ap_ops.c
index 7a9b5448e90a..a41d55146db1 100644
--- a/drivers/s390/crypto/vfio_ap_ops.c
+++ b/drivers/s390/crypto/vfio_ap_ops.c
@@ -1414,7 +1414,7 @@ static void vfio_ap_mdev_hot_unplug_domain(struct ap_matrix_mdev *matrix_mdev,
 {
 	DECLARE_BITMAP(apqis, AP_DOMAINS);
 
-	bitmap_zero(apqis, AP_DEVICES);
+	bitmap_zero(apqis, AP_DOMAINS);
 	set_bit_inv(apqi, apqis);
 	vfio_ap_mdev_hot_unplug_domains(matrix_mdev, apqis);
 }
@@ -2738,11 +2738,11 @@ static void vfio_ap_mdev_on_cfg_remove(struct ap_config_info *cur_config_info,
 	do_remove |= bitmap_andnot(aqrem,
 				   (unsigned long *)prev_config_info->aqm,
 				   (unsigned long *)cur_config_info->aqm,
-				   AP_DEVICES);
+				   AP_DOMAINS);
 	do_remove |= bitmap_andnot(cdrem,
 				   (unsigned long *)prev_config_info->adm,
 				   (unsigned long *)cur_config_info->adm,
-				   AP_DEVICES);
+				   AP_DOMAINS);
 
 	if (do_remove)
 		vfio_ap_mdev_cfg_remove(aprem, aqrem, cdrem);
@@ -2853,7 +2853,7 @@ static void vfio_ap_mdev_cfg_add(unsigned long *apm_add, unsigned long *aqm_add,
 		bitmap_and(matrix_mdev->aqm_add,
 			   matrix_mdev->matrix.aqm, aqm_add, AP_DOMAINS);
 		bitmap_and(matrix_mdev->adm_add,
-			   matrix_mdev->matrix.adm, adm_add, AP_DEVICES);
+			   matrix_mdev->matrix.adm, adm_add, AP_DOMAINS);
 
 		mutex_unlock(&matrix_dev->mdevs_lock);
 	}
-- 
2.53.0