vfio: Remove unnecessary uses of vfio_container.group_lock

The original intent of vfio_container.group_lock is to protect
vfio_container.group_list, however over time it's become a crutch to
prevent changes in container composition any time we call into the
iommu driver backend.  This introduces problems when we start to have
more complex interactions, for example when a user's DMA unmap request
triggers a notification to an mdev vendor driver, who responds by
attempting to unpin mappings within that request, re-entering the
iommu backend.  We incorrectly assume that the use of read-locks here
allow for this nested locking behavior, but a poorly timed write-lock
could in fact trigger a deadlock.

The current use of group_lock seems to fall into the trap of locking
code, not data.  Correct that by removing uses of group_lock that are
not directly related to group_list.  Note that the vfio type1 iommu
backend has its own mutex, vfio_iommu.lock, which it uses to protect
itself for each of these interfaces anyway.  The group_lock appears to
be a redundancy for these interfaces and type1 even goes so far as to
release its mutex to allow for exactly the re-entrant code path above.

Reported-by: Chuanxiao Dong <chuanxiao.dong@intel.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Acked-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Cc: stable@vger.kernel.org # v4.10+

1 files changed, 0 insertions, 38 deletions

diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
index 7597a377eb4e..330d50582f40 100644
--- a/drivers/vfio/vfio.c
+++ b/drivers/vfio/vfio.c
@@ -1175,15 +1175,11 @@ static long vfio_fops_unl_ioctl(struct file *filep,
                 ret = vfio_ioctl_set_iommu(container, arg);
                 break;
         default:
-                down_read(&container->group_lock);
-
                 driver = container->iommu_driver;
                 data = container->iommu_data;
 
                 if (driver) /* passthrough all unrecognized ioctls */
                         ret = driver->ops->ioctl(data, cmd, arg);
-
-                up_read(&container->group_lock);
         }
 
         return ret;
@@ -1237,15 +1233,11 @@ static ssize_t vfio_fops_read(struct file *filep, char __user *buf,
         struct vfio_iommu_driver *driver;
         ssize_t ret = -EINVAL;
 
-        down_read(&container->group_lock);
-
         driver = container->iommu_driver;
         if (likely(driver && driver->ops->read))
                 ret = driver->ops->read(container->iommu_data,
                                         buf, count, ppos);
 
-        up_read(&container->group_lock);
-
         return ret;
 }
 
@@ -1256,15 +1248,11 @@ static ssize_t vfio_fops_write(struct file *filep, const char __user *buf,
         struct vfio_iommu_driver *driver;
         ssize_t ret = -EINVAL;
 
-        down_read(&container->group_lock);
-
         driver = container->iommu_driver;
         if (likely(driver && driver->ops->write))
                 ret = driver->ops->write(container->iommu_data,
                                          buf, count, ppos);
 
-        up_read(&container->group_lock);
-
         return ret;
 }
 
@@ -1274,14 +1262,10 @@ static int vfio_fops_mmap(struct file *filep, struct vm_area_struct *vma)
         struct vfio_iommu_driver *driver;
         int ret = -EINVAL;
 
-        down_read(&container->group_lock);
-
         driver = container->iommu_driver;
         if (likely(driver && driver->ops->mmap))
                 ret = driver->ops->mmap(container->iommu_data, vma);
 
-        up_read(&container->group_lock);
-
         return ret;
 }
 
@@ -1993,8 +1977,6 @@ int vfio_pin_pages(struct device *dev, unsigned long *user_pfn, int npage,
                 goto err_pin_pages;
 
         container = group->container;
-        down_read(&container->group_lock);
-
         driver = container->iommu_driver;
         if (likely(driver && driver->ops->pin_pages))
                 ret = driver->ops->pin_pages(container->iommu_data, user_pfn,
@@ -2002,7 +1984,6 @@ int vfio_pin_pages(struct device *dev, unsigned long *user_pfn, int npage,
         else
                 ret = -ENOTTY;
 
-        up_read(&container->group_lock);
         vfio_group_try_dissolve_container(group);
 
 err_pin_pages:
@@ -2042,8 +2023,6 @@ int vfio_unpin_pages(struct device *dev, unsigned long *user_pfn, int npage)
                 goto err_unpin_pages;
 
         container = group->container;
-        down_read(&container->group_lock);
-
         driver = container->iommu_driver;
         if (likely(driver && driver->ops->unpin_pages))
                 ret = driver->ops->unpin_pages(container->iommu_data, user_pfn,
@@ -2051,7 +2030,6 @@ int vfio_unpin_pages(struct device *dev, unsigned long *user_pfn, int npage)
         else
                 ret = -ENOTTY;
 
-        up_read(&container->group_lock);
         vfio_group_try_dissolve_container(group);
 
 err_unpin_pages:
@@ -2073,8 +2051,6 @@ static int vfio_register_iommu_notifier(struct vfio_group *group,
                 return -EINVAL;
 
         container = group->container;
-        down_read(&container->group_lock);
-
         driver = container->iommu_driver;
         if (likely(driver && driver->ops->register_notifier))
                 ret = driver->ops->register_notifier(container->iommu_data,
@@ -2082,7 +2058,6 @@ static int vfio_register_iommu_notifier(struct vfio_group *group,
         else
                 ret = -ENOTTY;
 
-        up_read(&container->group_lock);
         vfio_group_try_dissolve_container(group);
 
         return ret;
@@ -2100,8 +2075,6 @@ static int vfio_unregister_iommu_notifier(struct vfio_group *group,
                 return -EINVAL;
 
         container = group->container;
-        down_read(&container->group_lock);
-
         driver = container->iommu_driver;
         if (likely(driver && driver->ops->unregister_notifier))
                 ret = driver->ops->unregister_notifier(container->iommu_data,
@@ -2109,7 +2082,6 @@ static int vfio_unregister_iommu_notifier(struct vfio_group *group,
         else
                 ret = -ENOTTY;
 
-        up_read(&container->group_lock);
         vfio_group_try_dissolve_container(group);
 
         return ret;
@@ -2127,7 +2099,6 @@ static int vfio_register_group_notifier(struct vfio_group *group,
                                         unsigned long *events,
                                         struct notifier_block *nb)
 {
-        struct vfio_container *container;
         int ret;
         bool set_kvm = false;
 
@@ -2145,9 +2116,6 @@ static int vfio_register_group_notifier(struct vfio_group *group,
         if (ret)
                 return -EINVAL;
 
-        container = group->container;
-        down_read(&container->group_lock);
-
         ret = blocking_notifier_chain_register(&group->notifier, nb);
 
         /*
@@ -2158,7 +2126,6 @@ static int vfio_register_group_notifier(struct vfio_group *group,
                 blocking_notifier_call_chain(&group->notifier,
                                         VFIO_GROUP_NOTIFY_SET_KVM, group->kvm);
 
-        up_read(&container->group_lock);
         vfio_group_try_dissolve_container(group);
 
         return ret;
@@ -2167,19 +2134,14 @@ static int vfio_register_group_notifier(struct vfio_group *group,
 static int vfio_unregister_group_notifier(struct vfio_group *group,
                                          struct notifier_block *nb)
 {
-        struct vfio_container *container;
         int ret;
 
         ret = vfio_group_add_container_user(group);
         if (ret)
                 return -EINVAL;
 
-        container = group->container;
-        down_read(&container->group_lock);
-
         ret = blocking_notifier_chain_unregister(&group->notifier, nb);
 
-        up_read(&container->group_lock);
         vfio_group_try_dissolve_container(group);
 
         return ret;