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);

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>`.