Re: [PATCH v5 3/3] drm: protect drm_master pointers in drm_lease.c

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On 30/6/21 12:07 am, Daniel Vetter wrote:
On Tue, Jun 29, 2021 at 11:37:06AM +0800, Desmond Cheong Zhi Xi wrote:
Currently, direct copies of drm_file->master pointers should be
protected by drm_device.master_mutex when being dereferenced. This is
because drm_file->master is not invariant for the lifetime of
drm_file. If drm_file is not the creator of master, then
drm_file->is_master is false, and a call to drm_setmaster_ioctl will
invoke drm_new_set_master, which then allocates a new master for
drm_file and puts the old master.

Thus, without holding drm_device.master_mutex, the old value of
drm_file->master could be freed while it is being used by another
concurrent process.

In drm_lease.c, there are multiple instances where drm_file->master is
accessed and dereferenced while drm_device.master_mutex is not
held. This makes drm_lease.c vulnerable to use-after-free bugs.

We address this issue in 3 ways:

1. Clarify in the kerneldoc that drm_file->master is protected by
drm_device.master_mutex.

2. Add a new drm_file_get_master() function that calls drm_master_get
on drm_file->master while holding on to drm_device.master_mutex. Since
drm_master_get increments the reference count of master, this
prevents master from being freed until we unreference it with
drm_master_put.

3. In each case where drm_file->master is directly accessed and
eventually dereferenced in drm_lease.c, we wrap the access in a call
to the new drm_file_get_master function, then unreference the master
pointer once we are done using it.

Reported-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>

Series looks very nice, let's see what intel-gfx-ci says. You should get a
mail, but results are also here:

https://patchwork.freedesktop.org/series/91969/#rev2

One tiny comment below.

---
  drivers/gpu/drm/drm_auth.c  | 25 ++++++++++++
  drivers/gpu/drm/drm_lease.c | 77 +++++++++++++++++++++++++++----------
  include/drm/drm_auth.h      |  1 +
  include/drm/drm_file.h      | 15 ++++++--
  4 files changed, 95 insertions(+), 23 deletions(-)

diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
index ab1863c5a5a0..c36a0b72be26 100644
--- a/drivers/gpu/drm/drm_auth.c
+++ b/drivers/gpu/drm/drm_auth.c
@@ -384,6 +384,31 @@ struct drm_master *drm_master_get(struct drm_master *master)
  }
  EXPORT_SYMBOL(drm_master_get);
+/**
+ * drm_file_get_master - reference &drm_file.master of @file_priv
+ * @file_priv: DRM file private
+ *
+ * Increments the reference count of @file_priv's &drm_file.master and returns
+ * the &drm_file.master. If @file_priv has no &drm_file.master, returns NULL.
+ *
+ * Master pointers returned from this function should be unreferenced using
+ * drm_master_put().
+ */
+struct drm_master *drm_file_get_master(struct drm_file *file_priv)
+{
+	struct drm_master *master = NULL;
+
+	mutex_lock(&file_priv->minor->dev->master_mutex);
+	if (!file_priv->master)
+		goto unlock;
+	master = drm_master_get(file_priv->master);
+
+unlock:
+	mutex_unlock(&file_priv->minor->dev->master_mutex);
+	return master;
+}
+EXPORT_SYMBOL(drm_file_get_master);
+
  static void drm_master_destroy(struct kref *kref)
  {
  	struct drm_master *master = container_of(kref, struct drm_master, refcount);
diff --git a/drivers/gpu/drm/drm_lease.c b/drivers/gpu/drm/drm_lease.c
index 00fb433bcef1..cdcc87fa9685 100644
--- a/drivers/gpu/drm/drm_lease.c
+++ b/drivers/gpu/drm/drm_lease.c
@@ -106,10 +106,19 @@ static bool _drm_has_leased(struct drm_master *master, int id)
   */
  bool _drm_lease_held(struct drm_file *file_priv, int id)
  {
-	if (!file_priv || !file_priv->master)
+	bool ret;
+	struct drm_master *master;
+
+	if (!file_priv)
  		return true;
- return _drm_lease_held_master(file_priv->master, id);
+	master = drm_file_get_master(file_priv);
+	if (master == NULL)
+		return true;
+	ret = _drm_lease_held_master(master, id);
+	drm_master_put(&master);
+
+	return ret;
  }
/**
@@ -128,13 +137,20 @@ bool drm_lease_held(struct drm_file *file_priv, int id)
  	struct drm_master *master;
  	bool ret;
- if (!file_priv || !file_priv->master || !file_priv->master->lessor)
+	if (!file_priv)
  		return true;
- master = file_priv->master;
+	master = drm_file_get_master(file_priv);
+	if (master == NULL)
+		return true;
+	if (!master->lessor) {
+		drm_master_put(&master);
+		return true;
+	}
  	mutex_lock(&master->dev->mode_config.idr_mutex);
  	ret = _drm_lease_held_master(master, id);
  	mutex_unlock(&master->dev->mode_config.idr_mutex);
+	drm_master_put(&master);
  	return ret;
  }
@@ -154,10 +170,16 @@ uint32_t drm_lease_filter_crtcs(struct drm_file *file_priv, uint32_t crtcs_in)
  	int count_in, count_out;
  	uint32_t crtcs_out = 0;
- if (!file_priv || !file_priv->master || !file_priv->master->lessor)
+	if (!file_priv)
  		return crtcs_in;
- master = file_priv->master;
+	master = drm_file_get_master(file_priv);
+	if (master == NULL)
+		return crtcs_in;
+	if (!master->lessor) {
+		drm_master_put(&master);
+		return crtcs_in;
+	}
  	dev = master->dev;
count_in = count_out = 0;
@@ -176,6 +198,7 @@ uint32_t drm_lease_filter_crtcs(struct drm_file *file_priv, uint32_t crtcs_in)
  		count_in++;
  	}
  	mutex_unlock(&master->dev->mode_config.idr_mutex);
+	drm_master_put(&master);
  	return crtcs_out;
  }
@@ -489,7 +512,7 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
  	size_t object_count;
  	int ret = 0;
  	struct idr leases;
-	struct drm_master *lessor = lessor_priv->master;
+	struct drm_master *lessor;
  	struct drm_master *lessee = NULL;
  	struct file *lessee_file = NULL;
  	struct file *lessor_file = lessor_priv->filp;
@@ -501,12 +524,6 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
  	if (!drm_core_check_feature(dev, DRIVER_MODESET))
  		return -EOPNOTSUPP;
- /* Do not allow sub-leases */
-	if (lessor->lessor) {
-		DRM_DEBUG_LEASE("recursive leasing not allowed\n");
-		return -EINVAL;
-	}
-
  	/* need some objects */
  	if (cl->object_count == 0) {
  		DRM_DEBUG_LEASE("no objects in lease\n");
@@ -518,12 +535,22 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
  		return -EINVAL;
  	}
+ lessor = drm_file_get_master(lessor_priv);
+	/* Do not allow sub-leases */
+	if (lessor->lessor) {
+		DRM_DEBUG_LEASE("recursive leasing not allowed\n");
+		ret = -EINVAL;
+		goto out_lessor;
+	}
+
  	object_count = cl->object_count;
object_ids = memdup_user(u64_to_user_ptr(cl->object_ids),
  			array_size(object_count, sizeof(__u32)));
-	if (IS_ERR(object_ids))
-		return PTR_ERR(object_ids);
+	if (IS_ERR(object_ids)) {
+		ret = PTR_ERR(object_ids);
+		goto out_lessor;
+	}
idr_init(&leases); @@ -534,14 +561,15 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
  	if (ret) {
  		DRM_DEBUG_LEASE("lease object lookup failed: %i\n", ret);
  		idr_destroy(&leases);
-		return ret;
+		goto out_lessor;
  	}
/* Allocate a file descriptor for the lease */
  	fd = get_unused_fd_flags(cl->flags & (O_CLOEXEC | O_NONBLOCK));
  	if (fd < 0) {
  		idr_destroy(&leases);
-		return fd;
+		ret = fd;
+		goto out_lessor;
  	}
DRM_DEBUG_LEASE("Creating lease\n");
@@ -577,6 +605,7 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
  	/* Hook up the fd */
  	fd_install(fd, lessee_file);
+ drm_master_put(&lessor);
  	DRM_DEBUG_LEASE("drm_mode_create_lease_ioctl succeeded\n");
  	return 0;
@@ -586,6 +615,8 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
  out_leases:
  	put_unused_fd(fd);
+out_lessor:
+	drm_master_put(&lessor);
  	DRM_DEBUG_LEASE("drm_mode_create_lease_ioctl failed: %d\n", ret);
  	return ret;
  }
@@ -608,7 +639,7 @@ int drm_mode_list_lessees_ioctl(struct drm_device *dev,
  	struct drm_mode_list_lessees *arg = data;
  	__u32 __user *lessee_ids = (__u32 __user *) (uintptr_t) (arg->lessees_ptr);
  	__u32 count_lessees = arg->count_lessees;
-	struct drm_master *lessor = lessor_priv->master, *lessee;
+	struct drm_master *lessor, *lessee;
  	int count;
  	int ret = 0;
@@ -619,6 +650,7 @@ int drm_mode_list_lessees_ioctl(struct drm_device *dev,
  	if (!drm_core_check_feature(dev, DRIVER_MODESET))
  		return -EOPNOTSUPP;
+ lessor = drm_file_get_master(lessor_priv);
  	DRM_DEBUG_LEASE("List lessees for %d\n", lessor->lessee_id);
mutex_lock(&dev->mode_config.idr_mutex);
@@ -642,6 +674,7 @@ int drm_mode_list_lessees_ioctl(struct drm_device *dev,
  		arg->count_lessees = count;
mutex_unlock(&dev->mode_config.idr_mutex);
+	drm_master_put(&lessor);
return ret;
  }
@@ -661,7 +694,7 @@ int drm_mode_get_lease_ioctl(struct drm_device *dev,
  	struct drm_mode_get_lease *arg = data;
  	__u32 __user *object_ids = (__u32 __user *) (uintptr_t) (arg->objects_ptr);
  	__u32 count_objects = arg->count_objects;
-	struct drm_master *lessee = lessee_priv->master;
+	struct drm_master *lessee;
  	struct idr *object_idr;
  	int count;
  	void *entry;
@@ -675,6 +708,7 @@ int drm_mode_get_lease_ioctl(struct drm_device *dev,
  	if (!drm_core_check_feature(dev, DRIVER_MODESET))
  		return -EOPNOTSUPP;
+ lessee = drm_file_get_master(lessee_priv);
  	DRM_DEBUG_LEASE("get lease for %d\n", lessee->lessee_id);
mutex_lock(&dev->mode_config.idr_mutex);
@@ -702,6 +736,7 @@ int drm_mode_get_lease_ioctl(struct drm_device *dev,
  		arg->count_objects = count;
mutex_unlock(&dev->mode_config.idr_mutex);
+	drm_master_put(&lessee);
return ret;
  }
@@ -720,7 +755,7 @@ int drm_mode_revoke_lease_ioctl(struct drm_device *dev,
  				void *data, struct drm_file *lessor_priv)
  {
  	struct drm_mode_revoke_lease *arg = data;
-	struct drm_master *lessor = lessor_priv->master;
+	struct drm_master *lessor;
  	struct drm_master *lessee;
  	int ret = 0;
@@ -730,6 +765,7 @@ int drm_mode_revoke_lease_ioctl(struct drm_device *dev,
  	if (!drm_core_check_feature(dev, DRIVER_MODESET))
  		return -EOPNOTSUPP;
+ lessor = drm_file_get_master(lessor_priv);
  	mutex_lock(&dev->mode_config.idr_mutex);
lessee = _drm_find_lessee(lessor, arg->lessee_id);
@@ -750,6 +786,7 @@ int drm_mode_revoke_lease_ioctl(struct drm_device *dev,
fail:
  	mutex_unlock(&dev->mode_config.idr_mutex);
+	drm_master_put(&lessor);
return ret;
  }
diff --git a/include/drm/drm_auth.h b/include/drm/drm_auth.h
index 6bf8b2b78991..f99d3417f304 100644
--- a/include/drm/drm_auth.h
+++ b/include/drm/drm_auth.h
@@ -107,6 +107,7 @@ struct drm_master {
  };
struct drm_master *drm_master_get(struct drm_master *master);
+struct drm_master *drm_file_get_master(struct drm_file *file_priv);
  void drm_master_put(struct drm_master **master);
  bool drm_is_current_master(struct drm_file *fpriv);
diff --git a/include/drm/drm_file.h b/include/drm/drm_file.h
index b81b3bfb08c8..e9931fca4ab7 100644
--- a/include/drm/drm_file.h
+++ b/include/drm/drm_file.h
@@ -226,9 +226,18 @@ struct drm_file {
  	/**
  	 * @master:
  	 *
-	 * Master this node is currently associated with. Only relevant if
-	 * drm_is_primary_client() returns true. Note that this only
-	 * matches &drm_device.master if the master is the currently active one.
+	 * Master this node is currently associated with. Protected by struct
+	 * &drm_device.master_mutex.
+	 *
+	 * Only relevant if drm_is_primary_client() returns true. Note that
+	 * this only matches &drm_device.master if the master is the currently
+	 * active one.
+	 *
+	 * When obtaining a copy of this pointer, it is recommended to either

I found this a bit confusing, since I generally don't think of
dereferencing the pointer as "taking a copy". That's more for the entire
datastructure when you have a memcpy() call, or kmemdup() or something
like that. Also "it is recommended" is a bit weak if you get a
use-after-free if you dont :-)

So instead "When dererencing this pointer either hold ... or use
drm_file_get_master() ...."

Cheers, Daniel

+	 * hold struct &drm_device.master_mutex for the duration of the
+	 * pointer's use, or to use drm_file_get_master() if struct
+	 * &drm_device.master_mutex is not currently held and there is no other
+	 * need to hold it. This prevents @master from being freed during use.
  	 *
  	 * See also @authentication and @is_master and the :ref:`section on
  	 * primary nodes and authentication <drm_primary_node>`.
--
2.25.1



Hi Daniel,

Thanks for the suggestion, I'll clarify the kerneldoc accordingly.

Regarding the results from intel-gfx-ci, it seems that the patch is inverting the lock hierarchy for
&dev->master_mutex --> &dev->mode_config.idr_mutex

Currently the dmesg warnings share a common call trace:
drm_file_get_master+0x1b/0x70
_drm_lease_held+0x21/0x70
__drm_mode_object_find+0xd1/0xe0

Looking at the functions that lock &dev->mode_config.idr_mutex, this should be the only instance of this lock order inversion.

I'm thinking the call to _drm_lease_held can be moved outside of the &dev->mode_config.idr_mutex lock in __drm_mode_object. Any thoughts?

diff --git a/drivers/gpu/drm/drm_mode_object.c b/drivers/gpu/drm/drm_mode_object.c
index b26588b52795..63d35f1f98dd 100644
--- a/drivers/gpu/drm/drm_mode_object.c
+++ b/drivers/gpu/drm/drm_mode_object.c
@@ -146,16 +146,18 @@ struct drm_mode_object *__drm_mode_object_find(struct drm_device *dev,
        if (obj && obj->id != id)
                obj = NULL;

-       if (obj && drm_mode_object_lease_required(obj->type) &&
-           !_drm_lease_held(file_priv, obj->id))
-               obj = NULL;
-
        if (obj && obj->free_cb) {
                if (!kref_get_unless_zero(&obj->refcount))
                        obj = NULL;
        }
        mutex_unlock(&dev->mode_config.idr_mutex);

+       if (obj && drm_mode_object_lease_required(obj->type) &&
+               !_drm_lease_held(file_priv, obj->id)) {
+               drm_mode_object_put(obj);
+               obj = NULL;
+       }
+
        return obj;
 }




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