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

Desmond Cheong Zhi Xi Wed, 30 Jun 2021 05:29:08 -0700

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.vet...@ffwll.ch>
Signed-off-by: Desmond Cheong Zhi Xi <desmondcheon...@gmail.com>


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 isinverting 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, thisshould 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.cb/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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Re: [Intel-gfx] [PATCH v5 3/3] drm: protect drm_master pointers in drm_lease.c

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