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 <[email protected]>
Suggested-by: Jason Gunthorpe <[email protected]>
Signed-off-by: Matthew Rosato <[email protected]>
---
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)
+{
+ void (*pfn)(struct kvm *kvm);
+ bool (*fn)(struct kvm *kvm);
+ bool ret;
+
+ 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;
+
+ 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);
+ if (ret && device->kvm && device->open_count == 0) {
+ vfio_kvm_put_kvm(device);
+ device->kvm = NULL;
+ }
+
+ mutex_unlock(&device->dev_set->lock);
+
out_unlock:
mutex_unlock(&device->group->group_lock);
return ret;
@@ -180,7 +250,16 @@ 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->kvm && device->open_count == 0) {
+ vfio_kvm_put_kvm(device);
+ device->kvm = NULL;
+ }
+
+ mutex_unlock(&device->dev_set->lock);
mutex_unlock(&device->group->group_lock);
}
@@ -450,6 +529,7 @@ 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 +883,9 @@ 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;
--
2.39.1
On Wed, 1 Feb 2023 14:20:10 -0500
Matthew Rosato <[email protected]> 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 <[email protected]>
> Suggested-by: Jason Gunthorpe <[email protected]>
> Signed-off-by: Matthew Rosato <[email protected]>
> ---
> 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;
> From: Alex Williamson <[email protected]>
> Sent: Thursday, February 2, 2023 7:28 AM
>
> On Wed, 1 Feb 2023 14:20:10 -0500
> Matthew Rosato <[email protected]> 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 <[email protected]>
> > Suggested-by: Jason Gunthorpe <[email protected]>
> > Signed-off-by: Matthew Rosato <[email protected]>
> > ---
> > 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.
Existing code set device->kvm in the vfio_device_first_open() which is
called by vfio_device_open(). After above change, seems not necessary
to pass kvm pointer into the call chain. Isn't it?
Regards,
Yi Liu
> From: Liu, Yi L <[email protected]>
> Sent: Thursday, February 2, 2023 11:47 AM
> > > 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.
>
> Existing code set device->kvm in the vfio_device_first_open() which is
> called by vfio_device_open(). After above change, seems not necessary
> to pass kvm pointer into the call chain. Isn't it?
>
Looks so.
> From: Alex Williamson <[email protected]>
> Sent: Thursday, February 2, 2023 7:28 AM
> >
> > +#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.
>
I have a different thought. In the end the cdev series also need the similar
safe get/put logic then it's better to keep it in vfio_main.c called by
the group/cdev path individually.
On Thu, 2 Feb 2023 03:46:59 +0000
"Liu, Yi L" <[email protected]> wrote:
> > From: Alex Williamson <[email protected]>
> > Sent: Thursday, February 2, 2023 7:28 AM
> >
> > On Wed, 1 Feb 2023 14:20:10 -0500
> > Matthew Rosato <[email protected]> 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 <[email protected]>
> > > Suggested-by: Jason Gunthorpe <[email protected]>
> > > Signed-off-by: Matthew Rosato <[email protected]>
> > > ---
> > > 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.
>
> Existing code set device->kvm in the vfio_device_first_open() which is
> called by vfio_device_open(). After above change, seems not necessary
> to pass kvm pointer into the call chain. Isn't it?
Yes, we can get it from the device. I didn't check how much this
bloats the patch though. As a fix, it might make sense to save that
refactoring for a follow-on patch. Thanks,
Alex
> -----Original Message-----
> From: Alex Williamson <[email protected]>
> Sent: Thursday, February 2, 2023 12:15 PM
> To: Liu, Yi L <[email protected]>
> Cc: Matthew Rosato <[email protected]>; [email protected];
> [email protected]; [email protected]; [email protected];
> [email protected]; [email protected];
> [email protected]; [email protected]; [email protected];
> [email protected]; [email protected]; [email protected];
> [email protected]; Wang, Zhi A <[email protected]>;
> Christopherson,, Sean <[email protected]>; Tian, Kevin
> <[email protected]>; [email protected]; [email protected];
> [email protected]; [email protected]; linux-
> [email protected]
> Subject: Re: [PATCH v2] vfio: fix deadlock between group lock and kvm lock
>
> On Thu, 2 Feb 2023 03:46:59 +0000
> "Liu, Yi L" <[email protected]> wrote:
>
> > > From: Alex Williamson <[email protected]>
> > > Sent: Thursday, February 2, 2023 7:28 AM
> > >
> > > On Wed, 1 Feb 2023 14:20:10 -0500
> > > Matthew Rosato <[email protected]> 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 <[email protected]>
> > > > Suggested-by: Jason Gunthorpe <[email protected]>
> > > > Signed-off-by: Matthew Rosato <[email protected]>
> > > > ---
> > > > 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.
> >
> > Existing code set device->kvm in the vfio_device_first_open() which is
> > called by vfio_device_open(). After above change, seems not necessary
> > to pass kvm pointer into the call chain. Isn't it?
>
> Yes, we can get it from the device. I didn't check how much this
> bloats the patch though. As a fix, it might make sense to save that
> refactoring for a follow-on patch. Thanks,
65 lines diff file. ???? follow-on path works well.
diff --git a/drivers/vfio/group.c b/drivers/vfio/group.c
index bb24b2f0271e..9e04e55c838f 100644
--- a/drivers/vfio/group.c
+++ b/drivers/vfio/group.c
@@ -169,8 +169,7 @@ static int vfio_device_group_open(struct vfio_device *device)
* device driver will use it, it must obtain a reference and release it
* during close_device.
*/
- ret = vfio_device_open(device, device->group->iommufd,
- device->group->kvm);
+ ret = vfio_device_open(device, device->group->iommufd);
out_unlock:
mutex_unlock(&device->group->group_lock);
diff --git a/drivers/vfio/vfio.h b/drivers/vfio/vfio.h
index f8219a438bfb..4ece6cb4cf2e 100644
--- a/drivers/vfio/vfio.h
+++ b/drivers/vfio/vfio.h
@@ -19,7 +19,7 @@ struct vfio_container;
void vfio_device_put_registration(struct vfio_device *device);
bool vfio_device_try_get_registration(struct vfio_device *device);
int vfio_device_open(struct vfio_device *device,
- struct iommufd_ctx *iommufd, struct kvm *kvm);
+ struct iommufd_ctx *iommufd);
void vfio_device_close(struct vfio_device *device,
struct iommufd_ctx *iommufd);
diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
index 5177bb061b17..45a7d6d38e2e 100644
--- a/drivers/vfio/vfio_main.c
+++ b/drivers/vfio/vfio_main.c
@@ -345,7 +345,7 @@ static bool vfio_assert_device_open(struct vfio_device *device)
}
static int vfio_device_first_open(struct vfio_device *device,
- struct iommufd_ctx *iommufd, struct kvm *kvm)
+ struct iommufd_ctx *iommufd)
{
int ret;
@@ -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)
@@ -396,14 +395,14 @@ static void vfio_device_last_close(struct vfio_device *device,
}
int vfio_device_open(struct vfio_device *device,
- struct iommufd_ctx *iommufd, struct kvm *kvm)
+ struct iommufd_ctx *iommufd)
{
int ret = 0;
mutex_lock(&device->dev_set->lock);
device->open_count++;
if (device->open_count == 1) {
- ret = vfio_device_first_open(device, iommufd, kvm);
+ ret = vfio_device_first_open(device, iommufd);
if (ret)
device->open_count--;
}
On 2/1/23 11:10 PM, Tian, Kevin wrote:
>> From: Alex Williamson <[email protected]>
>> Sent: Thursday, February 2, 2023 7:28 AM
>>>
>>> +#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.
>>
>
> I have a different thought. In the end the cdev series also need the similar
> safe get/put logic then it's better to keep it in vfio_main.c called by
> the group/cdev path individually.
Ah, I hadn't considered the cdev series - OK, I can move the functions back into vfio_main and externalize both via drivers/vfio/vfio.h so they can be called from group.c for this fix and then available to vfio_main.c already for cdev.