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+

diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
index 7597a377eb4e459cee02d74e09a320b0f55b0526..330d50582f400e7cd93d3242d91270e0d1fdbf66 100644 (file)
--- 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;