[PATCH v4 0/2] drm: address potential UAF bugs with drm_master ptrs

[PATCH v4 0/2] drm: address potential UAF bugs with drm_master ptrs

[Desmond Cheong Zhi Xi]

This patch series addresses potential use-after-free errors when dereferencing pointers to struct drm_master. These were identified after one such bug was caught by Syzbot in drm_getunique():
https://syzkaller.appspot.com/bug?id=148d2f1dfac64af52ffd27b661981a540724f803

The series is broken up into two patches:

1. Implement a locked version of drm_is_current_master() function that's used within drm_auth.c.

2. Identify areas in drm_lease.c where pointers to struct drm_master are dereferenced, and ensure that the master pointers are not freed during use.

Changes in v3 -> v4:
- Patch 1:
Move the call to drm_is_current_master in drm_mode_getconnector out from the section locked by &dev->mode_config.mutex. As suggested by Daniel Vetter. This avoids a circular lock lock dependency as reported here https://patchwork.freedesktop.org/patch/440406/

Additionally, inside drm_is_current_master, instead of grabbing &fpriv->master->dev->master_mutex, we grab &fpriv->minor->dev->master_mutex to avoid dereferencing a null ptr if fpriv->master is not set.

- Patch 2:
Modify kerneldoc formatting.

Additionally, add a file_priv->master NULL check inside drm_file_get_master, and handle the NULL result accordingly in drm_lease.c. As suggested by Daniel Vetter.

Changes in v2 -> v3:
- Patch 1:
Move the definition of drm_is_current_master and the _locked version higher up in drm_auth.c to avoid needing a forward declaration of drm_is_current_master_locked. As suggested by Daniel Vetter.

- Patch 2:
Instead of leaking drm_device.master_mutex into drm_lease.c to protect drm_master pointers, add a new drm_file_get_master() function that returns drm_file->master while increasing its reference count, to prevent drm_file->master from being freed. As suggested by Daniel Vetter.

Changes in v1 -> v2:
- Patch 2:
Move the lock and assignment before the DRM_DEBUG_LEASE in drm_mode_get_lease_ioctl, as suggested by Emil Velikov.

Desmond Cheong Zhi Xi (2):
  drm: add a locked version of drm_is_current_master
  drm: protect drm_master pointers in drm_lease.c

 drivers/gpu/drm/drm_auth.c      | 76 ++++++++++++++++++++++++--------
 drivers/gpu/drm/drm_connector.c |  5 ++-
 drivers/gpu/drm/drm_lease.c     | 77 ++++++++++++++++++++++++---------
 include/drm/drm_auth.h          |  1 +
 include/drm/drm_file.h          | 15 +++++--
 5 files changed, 131 insertions(+), 43 deletions(-)

-- 
2.25.1

[PATCH v4 1/2] drm: add a locked version of drm_is_current_master

[Desmond Cheong Zhi Xi]

While checking the master status of the DRM file in
drm_is_current_master(), the device's master mutex should be
held. Without the mutex, the pointer fpriv->master may be freed
concurrently by another process calling drm_setmaster_ioctl(). This
could lead to use-after-free errors when the pointer is subsequently
dereferenced in drm_lease_owner().

The callers of drm_is_current_master() from drm_auth.c hold the
device's master mutex, but external callers do not. Hence, we implement
drm_is_current_master_locked() to be used within drm_auth.c, and
modify drm_is_current_master() to grab the device's master mutex
before checking the master status.

Additionally, to avoid a circular lock dependency by introducing
drm_device.master_mutex into drm_is_current_master(), we move the call
to drm_is_current_master() in drm_mode_getconnector out from the
section locked by &dev->mode_config.mutex.

Failing to avoid this lock dependency produces the following lockdep
splat:

======================================================
WARNING: possible circular locking dependency detected
5.13.0-rc7-CI-CI_DRM_10254+ #1 Not tainted
------------------------------------------------------
kms_frontbuffer/1087 is trying to acquire lock:
ffff88810dcd01a8 (&dev->master_mutex){+.+.}-{3:3}, at: drm_is_current_master+0x1b/0x40
but task is already holding lock:
ffff88810dcd0488 (&dev->mode_config.mutex){+.+.}-{3:3}, at: drm_mode_getconnector+0x1c6/0x4a0
which lock already depends on the new lock.
the existing dependency chain (in reverse order) is:
-> #2 (&dev->mode_config.mutex){+.+.}-{3:3}:
       __mutex_lock+0xab/0x970
       drm_client_modeset_probe+0x22e/0xca0
       __drm_fb_helper_initial_config_and_unlock+0x42/0x540
       intel_fbdev_initial_config+0xf/0x20 [i915]
       async_run_entry_fn+0x28/0x130
       process_one_work+0x26d/0x5c0
       worker_thread+0x37/0x380
       kthread+0x144/0x170
       ret_from_fork+0x1f/0x30
-> #1 (&client->modeset_mutex){+.+.}-{3:3}:
       __mutex_lock+0xab/0x970
       drm_client_modeset_commit_locked+0x1c/0x180
       drm_client_modeset_commit+0x1c/0x40
       __drm_fb_helper_restore_fbdev_mode_unlocked+0x88/0xb0
       drm_fb_helper_set_par+0x34/0x40
       intel_fbdev_set_par+0x11/0x40 [i915]
       fbcon_init+0x270/0x4f0
       visual_init+0xc6/0x130
       do_bind_con_driver+0x1e5/0x2d0
       do_take_over_console+0x10e/0x180
       do_fbcon_takeover+0x53/0xb0
       register_framebuffer+0x22d/0x310
       __drm_fb_helper_initial_config_and_unlock+0x36c/0x540
       intel_fbdev_initial_config+0xf/0x20 [i915]
       async_run_entry_fn+0x28/0x130
       process_one_work+0x26d/0x5c0
       worker_thread+0x37/0x380
       kthread+0x144/0x170
       ret_from_fork+0x1f/0x30
-> #0 (&dev->master_mutex){+.+.}-{3:3}:
       __lock_acquire+0x151e/0x2590
       lock_acquire+0xd1/0x3d0
       __mutex_lock+0xab/0x970
       drm_is_current_master+0x1b/0x40
       drm_mode_getconnector+0x37e/0x4a0
       drm_ioctl_kernel+0xa8/0xf0
       drm_ioctl+0x1e8/0x390
       __x64_sys_ioctl+0x6a/0xa0
       do_syscall_64+0x39/0xb0
       entry_SYSCALL_64_after_hwframe+0x44/0xae
other info that might help us debug this:
Chain exists of: &dev->master_mutex --> &client->modeset_mutex --> &dev->mode_config.mutex
 Possible unsafe locking scenario:
       CPU0                    CPU1
       ----                    ----
  lock(&dev->mode_config.mutex);
                               lock(&client->modeset_mutex);
                               lock(&dev->mode_config.mutex);
  lock(&dev->master_mutex);
*** DEADLOCK ***
1 lock held by kms_frontbuffer/1087:
 #0: ffff88810dcd0488 (&dev->mode_config.mutex){+.+.}-{3:3}, at: drm_mode_getconnector+0x1c6/0x4a0
stack backtrace:
CPU: 7 PID: 1087 Comm: kms_frontbuffer Not tainted 5.13.0-rc7-CI-CI_DRM_10254+ #1
Hardware name: Intel Corporation Ice Lake Client Platform/IceLake U DDR4 SODIMM PD RVP TLC, BIOS ICLSFWR1.R00.3234.A01.1906141750 06/14/2019
Call Trace:
 dump_stack+0x7f/0xad
 check_noncircular+0x12e/0x150
 __lock_acquire+0x151e/0x2590
 lock_acquire+0xd1/0x3d0
 __mutex_lock+0xab/0x970
 drm_is_current_master+0x1b/0x40
 drm_mode_getconnector+0x37e/0x4a0
 drm_ioctl_kernel+0xa8/0xf0
 drm_ioctl+0x1e8/0x390
 __x64_sys_ioctl+0x6a/0xa0
 do_syscall_64+0x39/0xb0
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
---
 drivers/gpu/drm/drm_auth.c      | 51 +++++++++++++++++++++------------
 drivers/gpu/drm/drm_connector.c |  5 +++-
 2 files changed, 36 insertions(+), 20 deletions(-)

diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
index f00e5abdbbf4..ab1863c5a5a0 100644
--- a/drivers/gpu/drm/drm_auth.c
+++ b/drivers/gpu/drm/drm_auth.c
@@ -61,6 +61,35 @@
  * trusted clients.
  */
 
+static bool drm_is_current_master_locked(struct drm_file *fpriv)
+{
+	lockdep_assert_held_once(&fpriv->minor->dev->master_mutex);
+
+	return fpriv->is_master && drm_lease_owner(fpriv->master) == fpriv->minor->dev->master;
+}
+
+/**
+ * drm_is_current_master - checks whether @priv is the current master
+ * @fpriv: DRM file private
+ *
+ * Checks whether @fpriv is current master on its device. This decides whether a
+ * client is allowed to run DRM_MASTER IOCTLs.
+ *
+ * Most of the modern IOCTL which require DRM_MASTER are for kernel modesetting
+ * - the current master is assumed to own the non-shareable display hardware.
+ */
+bool drm_is_current_master(struct drm_file *fpriv)
+{
+	bool ret;
+
+	mutex_lock(&fpriv->minor->dev->master_mutex);
+	ret = drm_is_current_master_locked(fpriv);
+	mutex_unlock(&fpriv->minor->dev->master_mutex);
+
+	return ret;
+}
+EXPORT_SYMBOL(drm_is_current_master);
+
 int drm_getmagic(struct drm_device *dev, void *data, struct drm_file *file_priv)
 {
 	struct drm_auth *auth = data;
@@ -223,7 +252,7 @@ int drm_setmaster_ioctl(struct drm_device *dev, void *data,
 	if (ret)
 		goto out_unlock;
 
-	if (drm_is_current_master(file_priv))
+	if (drm_is_current_master_locked(file_priv))
 		goto out_unlock;
 
 	if (dev->master) {
@@ -272,7 +301,7 @@ int drm_dropmaster_ioctl(struct drm_device *dev, void *data,
 	if (ret)
 		goto out_unlock;
 
-	if (!drm_is_current_master(file_priv)) {
+	if (!drm_is_current_master_locked(file_priv)) {
 		ret = -EINVAL;
 		goto out_unlock;
 	}
@@ -321,7 +350,7 @@ void drm_master_release(struct drm_file *file_priv)
 	if (file_priv->magic)
 		idr_remove(&file_priv->master->magic_map, file_priv->magic);
 
-	if (!drm_is_current_master(file_priv))
+	if (!drm_is_current_master_locked(file_priv))
 		goto out;
 
 	drm_legacy_lock_master_cleanup(dev, master);
@@ -342,22 +371,6 @@ void drm_master_release(struct drm_file *file_priv)
 	mutex_unlock(&dev->master_mutex);
 }
 
-/**
- * drm_is_current_master - checks whether @priv is the current master
- * @fpriv: DRM file private
- *
- * Checks whether @fpriv is current master on its device. This decides whether a
- * client is allowed to run DRM_MASTER IOCTLs.
- *
- * Most of the modern IOCTL which require DRM_MASTER are for kernel modesetting
- * - the current master is assumed to own the non-shareable display hardware.
- */
-bool drm_is_current_master(struct drm_file *fpriv)
-{
-	return fpriv->is_master && drm_lease_owner(fpriv->master) == fpriv->minor->dev->master;
-}
-EXPORT_SYMBOL(drm_is_current_master);
-
 /**
  * drm_master_get - reference a master pointer
  * @master: &struct drm_master
diff --git a/drivers/gpu/drm/drm_connector.c b/drivers/gpu/drm/drm_connector.c
index da39e7ff6965..a5f63bad3893 100644
--- a/drivers/gpu/drm/drm_connector.c
+++ b/drivers/gpu/drm/drm_connector.c
@@ -2414,6 +2414,7 @@ int drm_mode_getconnector(struct drm_device *dev, void *data,
 	struct drm_mode_modeinfo u_mode;
 	struct drm_mode_modeinfo __user *mode_ptr;
 	uint32_t __user *encoder_ptr;
+	bool is_current_master;
 
 	if (!drm_core_check_feature(dev, DRIVER_MODESET))
 		return -EOPNOTSUPP;
@@ -2444,9 +2445,11 @@ int drm_mode_getconnector(struct drm_device *dev, void *data,
 	out_resp->connector_type = connector->connector_type;
 	out_resp->connector_type_id = connector->connector_type_id;
 
+	is_current_master = drm_is_current_master(file_priv)
+
 	mutex_lock(&dev->mode_config.mutex);
 	if (out_resp->count_modes == 0) {
-		if (drm_is_current_master(file_priv))
+		if (is_current_master)
 			connector->funcs->fill_modes(connector,
 						     dev->mode_config.max_width,
 						     dev->mode_config.max_height);
-- 
2.25.1

[PATCH v4 2/2] drm: protect drm_master pointers in drm_lease.c

[Desmond Cheong Zhi Xi]

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@ffwll.ch>
Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
---
 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
+	 * 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

Re: [PATCH v4 1/2] drm: add a locked version of drm_is_current_master

[Desmond Cheong Zhi Xi]

On 23/6/21 7:37 pm, Desmond Cheong Zhi Xi wrote:
> While checking the master status of the DRM file in
> drm_is_current_master(), the device's master mutex should be
> held. Without the mutex, the pointer fpriv->master may be freed
> concurrently by another process calling drm_setmaster_ioctl(). This
> could lead to use-after-free errors when the pointer is subsequently
> dereferenced in drm_lease_owner().
> 
> The callers of drm_is_current_master() from drm_auth.c hold the
> device's master mutex, but external callers do not. Hence, we implement
> drm_is_current_master_locked() to be used within drm_auth.c, and
> modify drm_is_current_master() to grab the device's master mutex
> before checking the master status.
> 
> Additionally, to avoid a circular lock dependency by introducing
> drm_device.master_mutex into drm_is_current_master(), we move the call
> to drm_is_current_master() in drm_mode_getconnector out from the
> section locked by &dev->mode_config.mutex.
> 
> Failing to avoid this lock dependency produces the following lockdep
> splat:
> 
> ======================================================
> WARNING: possible circular locking dependency detected
> 5.13.0-rc7-CI-CI_DRM_10254+ #1 Not tainted
> ------------------------------------------------------
> kms_frontbuffer/1087 is trying to acquire lock:
> ffff88810dcd01a8 (&dev->master_mutex){+.+.}-{3:3}, at: drm_is_current_master+0x1b/0x40
> but task is already holding lock:
> ffff88810dcd0488 (&dev->mode_config.mutex){+.+.}-{3:3}, at: drm_mode_getconnector+0x1c6/0x4a0
> which lock already depends on the new lock.
> the existing dependency chain (in reverse order) is:
> -> #2 (&dev->mode_config.mutex){+.+.}-{3:3}:
>         __mutex_lock+0xab/0x970
>         drm_client_modeset_probe+0x22e/0xca0
>         __drm_fb_helper_initial_config_and_unlock+0x42/0x540
>         intel_fbdev_initial_config+0xf/0x20 [i915]
>         async_run_entry_fn+0x28/0x130
>         process_one_work+0x26d/0x5c0
>         worker_thread+0x37/0x380
>         kthread+0x144/0x170
>         ret_from_fork+0x1f/0x30
> -> #1 (&client->modeset_mutex){+.+.}-{3:3}:
>         __mutex_lock+0xab/0x970
>         drm_client_modeset_commit_locked+0x1c/0x180
>         drm_client_modeset_commit+0x1c/0x40
>         __drm_fb_helper_restore_fbdev_mode_unlocked+0x88/0xb0
>         drm_fb_helper_set_par+0x34/0x40
>         intel_fbdev_set_par+0x11/0x40 [i915]
>         fbcon_init+0x270/0x4f0
>         visual_init+0xc6/0x130
>         do_bind_con_driver+0x1e5/0x2d0
>         do_take_over_console+0x10e/0x180
>         do_fbcon_takeover+0x53/0xb0
>         register_framebuffer+0x22d/0x310
>         __drm_fb_helper_initial_config_and_unlock+0x36c/0x540
>         intel_fbdev_initial_config+0xf/0x20 [i915]
>         async_run_entry_fn+0x28/0x130
>         process_one_work+0x26d/0x5c0
>         worker_thread+0x37/0x380
>         kthread+0x144/0x170
>         ret_from_fork+0x1f/0x30
> -> #0 (&dev->master_mutex){+.+.}-{3:3}:
>         __lock_acquire+0x151e/0x2590
>         lock_acquire+0xd1/0x3d0
>         __mutex_lock+0xab/0x970
>         drm_is_current_master+0x1b/0x40
>         drm_mode_getconnector+0x37e/0x4a0
>         drm_ioctl_kernel+0xa8/0xf0
>         drm_ioctl+0x1e8/0x390
>         __x64_sys_ioctl+0x6a/0xa0
>         do_syscall_64+0x39/0xb0
>         entry_SYSCALL_64_after_hwframe+0x44/0xae
> other info that might help us debug this:
> Chain exists of: &dev->master_mutex --> &client->modeset_mutex --> &dev->mode_config.mutex
>   Possible unsafe locking scenario:
>         CPU0                    CPU1
>         ----                    ----
>    lock(&dev->mode_config.mutex);
>                                 lock(&client->modeset_mutex);
>                                 lock(&dev->mode_config.mutex);
>    lock(&dev->master_mutex);
> *** DEADLOCK ***
> 1 lock held by kms_frontbuffer/1087:
>   #0: ffff88810dcd0488 (&dev->mode_config.mutex){+.+.}-{3:3}, at: drm_mode_getconnector+0x1c6/0x4a0
> stack backtrace:
> CPU: 7 PID: 1087 Comm: kms_frontbuffer Not tainted 5.13.0-rc7-CI-CI_DRM_10254+ #1
> Hardware name: Intel Corporation Ice Lake Client Platform/IceLake U DDR4 SODIMM PD RVP TLC, BIOS ICLSFWR1.R00.3234.A01.1906141750 06/14/2019
> Call Trace:
>   dump_stack+0x7f/0xad
>   check_noncircular+0x12e/0x150
>   __lock_acquire+0x151e/0x2590
>   lock_acquire+0xd1/0x3d0
>   __mutex_lock+0xab/0x970
>   drm_is_current_master+0x1b/0x40
>   drm_mode_getconnector+0x37e/0x4a0
>   drm_ioctl_kernel+0xa8/0xf0
>   drm_ioctl+0x1e8/0x390
>   __x64_sys_ioctl+0x6a/0xa0
>   do_syscall_64+0x39/0xb0
>   entry_SYSCALL_64_after_hwframe+0x44/0xae
> 
> Reported-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
> ---
>   drivers/gpu/drm/drm_auth.c      | 51 +++++++++++++++++++++------------
>   drivers/gpu/drm/drm_connector.c |  5 +++-
>   2 files changed, 36 insertions(+), 20 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
> index f00e5abdbbf4..ab1863c5a5a0 100644
> --- a/drivers/gpu/drm/drm_auth.c
> +++ b/drivers/gpu/drm/drm_auth.c
> @@ -61,6 +61,35 @@
>    * trusted clients.
>    */
>   
> +static bool drm_is_current_master_locked(struct drm_file *fpriv)
> +{
> +	lockdep_assert_held_once(&fpriv->minor->dev->master_mutex);
> +
> +	return fpriv->is_master && drm_lease_owner(fpriv->master) == fpriv->minor->dev->master;
> +}
> +
> +/**
> + * drm_is_current_master - checks whether @priv is the current master
> + * @fpriv: DRM file private
> + *
> + * Checks whether @fpriv is current master on its device. This decides whether a
> + * client is allowed to run DRM_MASTER IOCTLs.
> + *
> + * Most of the modern IOCTL which require DRM_MASTER are for kernel modesetting
> + * - the current master is assumed to own the non-shareable display hardware.
> + */
> +bool drm_is_current_master(struct drm_file *fpriv)
> +{
> +	bool ret;
> +
> +	mutex_lock(&fpriv->minor->dev->master_mutex);
> +	ret = drm_is_current_master_locked(fpriv);
> +	mutex_unlock(&fpriv->minor->dev->master_mutex);
> +
> +	return ret;
> +}
> +EXPORT_SYMBOL(drm_is_current_master);
> +
>   int drm_getmagic(struct drm_device *dev, void *data, struct drm_file *file_priv)
>   {
>   	struct drm_auth *auth = data;
> @@ -223,7 +252,7 @@ int drm_setmaster_ioctl(struct drm_device *dev, void *data,
>   	if (ret)
>   		goto out_unlock;
>   
> -	if (drm_is_current_master(file_priv))
> +	if (drm_is_current_master_locked(file_priv))
>   		goto out_unlock;
>   
>   	if (dev->master) {
> @@ -272,7 +301,7 @@ int drm_dropmaster_ioctl(struct drm_device *dev, void *data,
>   	if (ret)
>   		goto out_unlock;
>   
> -	if (!drm_is_current_master(file_priv)) {
> +	if (!drm_is_current_master_locked(file_priv)) {
>   		ret = -EINVAL;
>   		goto out_unlock;
>   	}
> @@ -321,7 +350,7 @@ void drm_master_release(struct drm_file *file_priv)
>   	if (file_priv->magic)
>   		idr_remove(&file_priv->master->magic_map, file_priv->magic);
>   
> -	if (!drm_is_current_master(file_priv))
> +	if (!drm_is_current_master_locked(file_priv))
>   		goto out;
>   
>   	drm_legacy_lock_master_cleanup(dev, master);
> @@ -342,22 +371,6 @@ void drm_master_release(struct drm_file *file_priv)
>   	mutex_unlock(&dev->master_mutex);
>   }
>   
> -/**
> - * drm_is_current_master - checks whether @priv is the current master
> - * @fpriv: DRM file private
> - *
> - * Checks whether @fpriv is current master on its device. This decides whether a
> - * client is allowed to run DRM_MASTER IOCTLs.
> - *
> - * Most of the modern IOCTL which require DRM_MASTER are for kernel modesetting
> - * - the current master is assumed to own the non-shareable display hardware.
> - */
> -bool drm_is_current_master(struct drm_file *fpriv)
> -{
> -	return fpriv->is_master && drm_lease_owner(fpriv->master) == fpriv->minor->dev->master;
> -}
> -EXPORT_SYMBOL(drm_is_current_master);
> -
>   /**
>    * drm_master_get - reference a master pointer
>    * @master: &struct drm_master
> diff --git a/drivers/gpu/drm/drm_connector.c b/drivers/gpu/drm/drm_connector.c
> index da39e7ff6965..a5f63bad3893 100644
> --- a/drivers/gpu/drm/drm_connector.c
> +++ b/drivers/gpu/drm/drm_connector.c
> @@ -2414,6 +2414,7 @@ int drm_mode_getconnector(struct drm_device *dev, void *data,
>   	struct drm_mode_modeinfo u_mode;
>   	struct drm_mode_modeinfo __user *mode_ptr;
>   	uint32_t __user *encoder_ptr;
> +	bool is_current_master;
>   
>   	if (!drm_core_check_feature(dev, DRIVER_MODESET))
>   		return -EOPNOTSUPP;
> @@ -2444,9 +2445,11 @@ int drm_mode_getconnector(struct drm_device *dev, void *data,
>   	out_resp->connector_type = connector->connector_type;
>   	out_resp->connector_type_id = connector->connector_type_id;
>   
> +	is_current_master = drm_is_current_master(file_priv)
> +
>   	mutex_lock(&dev->mode_config.mutex);
>   	if (out_resp->count_modes == 0) {
> -		if (drm_is_current_master(file_priv))
> +		if (is_current_master)
>   			connector->funcs->fill_modes(connector,
>   						     dev->mode_config.max_width,
>   						     dev->mode_config.max_height);
> 

Hi Daniel,

My bad, I just realized that this patch won't apply to stable because of 
the changes to drm_connector.c.

Will the intel-gfx CI apply the patches in order if I break up this 
patch and move the changes to drm_connector.c earlier in the series?

Best wishes,
Desmond