Re: [Intel-gfx] [PATCH v2] vfio: fix deadlock between group lock and kvm lock

Alex Williamson
2023-02-01T23:27:42Z

On Wed, 1 Feb 2023 14:20:10 -0500
Matthew Rosato <mjrosato@linux.ibm.com> wrote:

> After 51cdc8bc120e, we have another deadlock scenario between the
> kvm->lock and the vfio group_lock with two different codepaths acquiring
> the locks in different order. Specifically in vfio_open_device, vfio
> holds the vfio group_lock when issuing device->ops->open_device but some
> drivers (like vfio-ap) need to acquire kvm->lock during their open_device
> routine; Meanwhile, kvm_vfio_release will acquire the kvm->lock first
> before calling vfio_file_set_kvm which will acquire the vfio group_lock.
>
> To resolve this, let's remove the need for the vfio group_lock from the
> kvm_vfio_release codepath. This is done by introducing a new spinlock to
> protect modifications to the vfio group kvm pointer, and acquiring a kvm
> ref from within vfio while holding this spinlock, with the reference held
> until the last close for the device in question.
>
> Fixes: 51cdc8bc120e ("kvm/vfio: Fix potential deadlock on vfio group_lock")
> Reported-by: Anthony Krowiak <akrowiak@linux.ibm.com>
> Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
> Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
> ---
> Changes from v1:
> * use spin_lock instead of spin_lock_irqsave (Jason)
> * clear device->kvm_put as part of vfio_kvm_put_kvm (Yi)
> * Re-arrange code to avoid referencing the group contents from within
> vfio_main (Kevin) which meant moving most of the code in this patch
> to group.c along with getting/dropping of the dev_set lock
> ---
> drivers/vfio/group.c | 90 +++++++++++++++++++++++++++++++++++++---
> drivers/vfio/vfio.h | 1 +
> drivers/vfio/vfio_main.c | 11 ++---
> include/linux/vfio.h | 2 +-
> 4 files changed, 91 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/vfio/group.c b/drivers/vfio/group.c
> index bb24b2f0271e..52f434861294 100644
> --- a/drivers/vfio/group.c
> +++ b/drivers/vfio/group.c
> @@ -13,6 +13,9 @@
> #include <linux/vfio.h>
> #include <linux/iommufd.h>
> #include <linux/anon_inodes.h>
> +#ifdef CONFIG_HAVE_KVM
> +#include <linux/kvm_host.h>
> +#endif
> #include "vfio.h"
>
> static struct vfio {
> @@ -154,6 +157,55 @@ static int vfio_group_ioctl_set_container(struct vfio_group *group,
> return ret;
> }
>
> +#ifdef CONFIG_HAVE_KVM
> +static bool vfio_kvm_get_kvm_safe(struct vfio_device *device, struct kvm *kvm)

I'm tempted to name these vfio_device_get_kvm_safe() and only pass the
vfio_device, where of course we can get the kvm pointer from the group
internally.

> +{
> + void (*pfn)(struct kvm *kvm);
> + bool (*fn)(struct kvm *kvm);
> + bool ret;
> +

We should assert_lockdep_held(&device->dev_set->lock) in both of these
since that seems to be what's protecting device->kvm and
device->put_kvm.

If we change as above to get the kvm pointer from the group within this
function, we can also move the kvm_ref_lock here, which seems to
simplify the caller quite a bit.

> + pfn = symbol_get(kvm_put_kvm);
> + if (WARN_ON(!pfn))
> + return false;
> +
> + fn = symbol_get(kvm_get_kvm_safe);
> + if (WARN_ON(!fn)) {
> + symbol_put(kvm_put_kvm);
> + return false;
> + }
> +
> + ret = fn(kvm);
> + if (ret)
> + device->put_kvm = pfn;
> + else
> + symbol_put(kvm_put_kvm);
> +
> + symbol_put(kvm_get_kvm_safe);
> +
> + return ret;
> +}
> +
> +static void vfio_kvm_put_kvm(struct vfio_device *device)
> +{
> + if (WARN_ON(!device->kvm || !device->put_kvm))
> + return;

It simplifies the caller if we can use this even in the !device->kvm
case.

> +
> + device->put_kvm(device->kvm);
> + device->put_kvm = NULL;
> + symbol_put(kvm_put_kvm);
> +}
> +
> +#else
> +static bool vfio_kvm_get_kvm_safe(struct vfio_device *device, struct kvm *kvm)
> +{
> + return false;
> +}
> +
> +static void vfio_kvm_put_kvm(struct vfio_device *device)
> +{
> +}
> +#endif
> +
> static int vfio_device_group_open(struct vfio_device *device)
> {
> int ret;
> @@ -164,14 +216,32 @@ static int vfio_device_group_open(struct vfio_device *device)
> goto out_unlock;
> }
>
> + mutex_lock(&device->dev_set->lock);
> +
> /*
> - * Here we pass the KVM pointer with the group under the lock. If the
> - * device driver will use it, it must obtain a reference and release it
> - * during close_device.
> + * Before the first device open, get the KVM pointer currently
> + * associated with the group (if there is one) and obtain a reference
> + * now that will be held until the open_count reaches 0 again. Save
> + * the pointer in the device for use by drivers.
> */
> + if (device->open_count == 0) {
> + spin_lock(&device->group->kvm_ref_lock);
> + if (device->group->kvm &&
> + vfio_kvm_get_kvm_safe(device, device->group->kvm))
> + device->kvm = device->group->kvm;
> + spin_unlock(&device->group->kvm_ref_lock);
> + }
> +
> ret = vfio_device_open(device, device->group->iommufd,
> device->group->kvm);

We're using device->group->kvm outside of kvm_ref_lock here, it should
be using device->kvm.

>
> + if (ret && device->kvm && device->open_count == 0) {

Slightly redundant, if device->open_count == 0 here, we can infer ret
is non-zero.

I fiddled with it a little further, see if you like anything from the
version below and incorporate what you do. Thanks,

Alex

diff --git a/drivers/vfio/group.c b/drivers/vfio/group.c
index bb24b2f0271e..5121a34e1489 100644
--- a/drivers/vfio/group.c
+++ b/drivers/vfio/group.c
@@ -13,6 +13,9 @@
#include <linux/vfio.h>
#include <linux/iommufd.h>
#include <linux/anon_inodes.h>
+#ifdef CONFIG_HAVE_KVM
+#include <linux/kvm_host.h>
+#endif
#include "vfio.h"

static struct vfio {
@@ -154,6 +157,64 @@ static int vfio_group_ioctl_set_container(struct vfio_group *group,
return ret;
}

+#ifdef CONFIG_HAVE_KVM
+static void vfio_device_get_kvm_safe(struct vfio_device *device)
+{
+ void (*pfn)(struct kvm *kvm);
+ bool (*fn)(struct kvm *kvm);
+ bool ret;
+
+ lockdep_assert_held(&device->dev_set->lock);
+
+ spin_lock(&device->group->kvm_ref_lock);
+ if (!device->group->kvm)
+ goto unlock;
+
+ pfn = symbol_get(kvm_put_kvm);
+ if (WARN_ON(!pfn))
+ goto unlock;
+
+ fn = symbol_get(kvm_get_kvm_safe);
+ if (WARN_ON(!fn)) {
+ symbol_put(kvm_put_kvm);
+ goto unlock;
+ }
+
+ ret = fn(device->group->kvm);
+ symbol_put(kvm_get_kvm_safe);
+ if (!ret) {
+ symbol_put(kvm_put_kvm);
+ goto unlock;
+ }
+
+ device->put_kvm = pfn;
+ device->kvm = device->group->kvm;
+unlock:
+ spin_unlock(&device->group->kvm_ref_lock);
+}
+
+static void vfio_device_put_kvm(struct vfio_device *device)
+{
+ lockdep_assert_held(&device->dev_set->lock);
+
+ if (!device->kvm)
+ return;
+
+ if (WARN_ON(!device->put_kvm))
+ goto clear;
+
+ device->put_kvm(device->kvm);
+ device->put_kvm = NULL;
+ symbol_put(kvm_put_kvm);
+
+clear:
+ device->kvm = NULL;
+}
+#else
+static void vfio_device_get_kvm_safe(struct vfio_device *device) {}
+static void vfio_device_put_kvm(struct vfio_device *device) {}
+#endif
+
static int vfio_device_group_open(struct vfio_device *device)
{
int ret;
@@ -164,13 +225,23 @@ static int vfio_device_group_open(struct vfio_device *device)
goto out_unlock;
}

+ mutex_lock(&device->dev_set->lock);
+
/*
- * Here we pass the KVM pointer with the group under the lock. If the
- * device driver will use it, it must obtain a reference and release it
- * during close_device.
+ * Before the first device open, get the KVM pointer currently
+ * associated with the group (if there is one) and obtain a reference
+ * now that will be held until the open_count reaches 0 again. Save
+ * the pointer in the device for use by drivers.
*/
- ret = vfio_device_open(device, device->group->iommufd,
- device->group->kvm);
+ if (device->open_count == 0)
+ vfio_device_get_kvm_safe(device);
+
+ ret = vfio_device_open(device, device->group->iommufd, device->kvm);
+
+ if (device->open_count == 0)
+ vfio_device_put_kvm(device);
+
+ mutex_unlock(&device->dev_set->lock);

out_unlock:
mutex_unlock(&device->group->group_lock);
@@ -180,7 +251,14 @@ static int vfio_device_group_open(struct vfio_device *device)
void vfio_device_group_close(struct vfio_device *device)
{
mutex_lock(&device->group->group_lock);
+ mutex_lock(&device->dev_set->lock);
+
vfio_device_close(device, device->group->iommufd);
+
+ if (device->open_count == 0)
+ vfio_device_put_kvm(device);
+
+ mutex_unlock(&device->dev_set->lock);
mutex_unlock(&device->group->group_lock);
}

@@ -450,6 +528,7 @@ static struct vfio_group *vfio_group_alloc(struct iommu_group *iommu_group,

refcount_set(&group->drivers, 1);
mutex_init(&group->group_lock);
+ spin_lock_init(&group->kvm_ref_lock);
INIT_LIST_HEAD(&group->device_list);
mutex_init(&group->device_lock);
group->iommu_group = iommu_group;
@@ -803,9 +882,9 @@ void vfio_file_set_kvm(struct file *file, struct kvm *kvm)
if (!vfio_file_is_group(file))
return;

- mutex_lock(&group->group_lock);
+ spin_lock(&group->kvm_ref_lock);
group->kvm = kvm;
- mutex_unlock(&group->group_lock);
+ spin_unlock(&group->kvm_ref_lock);
}
EXPORT_SYMBOL_GPL(vfio_file_set_kvm);

diff --git a/drivers/vfio/vfio.h b/drivers/vfio/vfio.h
index f8219a438bfb..20c6bc249cb8 100644
--- a/drivers/vfio/vfio.h
+++ b/drivers/vfio/vfio.h
@@ -74,6 +74,7 @@ struct vfio_group {
struct file *opened_file;
struct blocking_notifier_head notifier;
struct iommufd_ctx *iommufd;
+ spinlock_t kvm_ref_lock;
};

int vfio_device_set_group(struct vfio_device *device,
diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
index 5177bb061b17..14dbf781ea8c 100644
--- a/drivers/vfio/vfio_main.c
+++ b/drivers/vfio/vfio_main.c
@@ -361,7 +361,6 @@ static int vfio_device_first_open(struct vfio_device *device,
if (ret)
goto err_module_put;

- device->kvm = kvm;
if (device->ops->open_device) {
ret = device->ops->open_device(device);
if (ret)
@@ -370,7 +369,6 @@ static int vfio_device_first_open(struct vfio_device *device,
return 0;

err_unuse_iommu:
- device->kvm = NULL;
if (iommufd)
vfio_iommufd_unbind(device);
else
@@ -387,7 +385,6 @@ static void vfio_device_last_close(struct vfio_device *device,

if (device->ops->close_device)
device->ops->close_device(device);
- device->kvm = NULL;
if (iommufd)
vfio_iommufd_unbind(device);
else
@@ -400,14 +397,14 @@ int vfio_device_open(struct vfio_device *device,
{
int ret = 0;

- mutex_lock(&device->dev_set->lock);
+ lockdep_assert_held(&device->dev_set->lock);
+
device->open_count++;
if (device->open_count == 1) {
ret = vfio_device_first_open(device, iommufd, kvm);
if (ret)
device->open_count--;
}
- mutex_unlock(&device->dev_set->lock);

return ret;
}
@@ -415,12 +412,12 @@ int vfio_device_open(struct vfio_device *device,
void vfio_device_close(struct vfio_device *device,
struct iommufd_ctx *iommufd)
{
- mutex_lock(&device->dev_set->lock);
+ lockdep_assert_held(&device->dev_set->lock);
+
vfio_assert_device_open(device);
if (device->open_count == 1)
vfio_device_last_close(device, iommufd);
device->open_count--;
- mutex_unlock(&device->dev_set->lock);
}

/*
diff --git a/include/linux/vfio.h b/include/linux/vfio.h
index 35be78e9ae57..87ff862ff555 100644
--- a/include/linux/vfio.h
+++ b/include/linux/vfio.h
@@ -46,7 +46,6 @@ struct vfio_device {
struct vfio_device_set *dev_set;
struct list_head dev_set_list;
unsigned int migration_flags;
- /* Driver must reference the kvm during open_device or never touch it */
struct kvm *kvm;

/* Members below here are private, not for driver use */
@@ -58,6 +57,7 @@ struct vfio_device {
struct list_head group_next;
struct list_head iommu_entry;
struct iommufd_access *iommufd_access;
+ void (*put_kvm)(struct kvm *kvm);
#if IS_ENABLED(CONFIG_IOMMUFD)
struct iommufd_device *iommufd_device;
struct iommufd_ctx *iommufd_ictx;