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[213.45.168.79]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4997c939574sm37352315e9.1.2026.08.12.03.53.24 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 12 Aug 2026 03:53:25 -0700 (PDT) From: Nicola Fiorillo To: linux-media@vger.kernel.org Cc: mchehab@kernel.org, sakari.ailus@linux.intel.com, bingbu.cao@intel.com, tian.shu.qiu@intel.com, linux-kernel@vger.kernel.org, Nicola Fiorillo Subject: [PATCH 2/3] media: v4l2-subdev: Check v4l2_dev before dereferencing it in open() Date: Wed, 12 Aug 2026 12:53:04 +0200 Message-ID: <20260812105305.32447-3-nicfio@gmail.com> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20260812105305.32447-1-nicfio@gmail.com> References: <20260812105305.32447-1-nicfio@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Unbinding a sensor driver while something opens its /dev/v4l-subdevN node oopses the kernel: BUG: kernel NULL pointer dereference, address: 0000000000000008 RIP: 0010:subdev_open+0x8a/0x190 [videodev] Call Trace: v4l2_open+0xa9/0x100 [videodev] chrdev_open+0xb2/0x230 do_dentry_open+0x14c/0x440 vfs_open+0x2e/0xe0 path_openat+0x82e/0x12d0 do_filp_open+0xc4/0x170 do_sys_openat2+0xae/0xe0 __x64_sys_openat+0x55/0xa0 v4l2_device_unregister_subdev() clears sd->v4l2_dev, then unregisters the media entity, and only then unregisters the device node. Until the node is gone userspace can still open it, and subdev_open() dereferences sd->v4l2_dev unconditionally. The faulting address is the offset of the mdev member in struct v4l2_device. The window is not a narrow one: media_device_unregister_entity() sleeps, and the first oops seen here was not provoked at all, it was hit by v4l_id, run by udev on the very node that was appearing and disappearing. The same window leaves sd->entity.graph_obj.mdev NULL while sd->v4l2_dev->mdev is not, and the second dereference on that line goes through it. That one was found by reading the teardown path, not by crashing on it; it arrived later, with commit 218bf10e39ed ("media: v4l2-subdev: handle module refcounting here"). Unregistering the device node before clearing the pointers would narrow the window but not close it, because v4l2_open() drops videodev_lock before it calls fops->open() and the whole of v4l2_device_unregister_subdev() can run in between. Check the pointers in subdev_open() instead. Reproduced on a CHUWI Hi10 X1 (Alder Lake-N, IPU6) running 6.12.86, at cycle 7 of a loop unbinding and rebinding a sensor while four processes opened every /dev/v4l-subdev*. The code is unchanged in 7.2-rc7. Fixes: 61f5db549dde ("[media] v4l: Make v4l2_subdev inherit from media_entity") Signed-off-by: Nicola Fiorillo --- drivers/media/v4l2-core/v4l2-subdev.c | 31 +++++++++++++++++++++------ 1 file changed, 25 insertions(+), 6 deletions(-) diff --git a/drivers/media/v4l2-core/v4l2-subdev.c b/drivers/media/v4l2-core/v4l2-subdev.c index e9f81b9be..2a47b9730 100644 --- a/drivers/media/v4l2-core/v4l2-subdev.c +++ b/drivers/media/v4l2-core/v4l2-subdev.c @@ -97,8 +97,19 @@ static int subdev_open(struct file *file) struct video_device *vdev = video_devdata(file); struct v4l2_subdev *sd = vdev_to_v4l2_subdev(vdev); struct v4l2_subdev_fh *subdev_fh; + struct v4l2_device *v4l2_dev; int ret; + /* + * v4l2_device_unregister_subdev() clears sd->v4l2_dev and unregisters + * the entity before it unregisters the device node, so an open() that + * races with the sub-device going away lands here with those pointers + * already gone. + */ + v4l2_dev = READ_ONCE(sd->v4l2_dev); + if (!v4l2_dev) + return -ENODEV; + subdev_fh = kzalloc_obj(*subdev_fh); if (subdev_fh == NULL) return -ENOMEM; @@ -112,15 +123,23 @@ static int subdev_open(struct file *file) v4l2_fh_init(&subdev_fh->vfh, vdev); v4l2_fh_add(&subdev_fh->vfh, file); - if (sd->v4l2_dev->mdev && sd->entity.graph_obj.mdev->dev) { - struct module *owner; + if (v4l2_dev->mdev) { + struct media_device *mdev = READ_ONCE(sd->entity.graph_obj.mdev); - owner = sd->entity.graph_obj.mdev->dev->driver->owner; - if (!try_module_get(owner)) { - ret = -EBUSY; + if (!mdev) { + ret = -ENODEV; goto err; } - subdev_fh->owner = owner; + + if (mdev->dev) { + struct module *owner = mdev->dev->driver->owner; + + if (!try_module_get(owner)) { + ret = -EBUSY; + goto err; + } + subdev_fh->owner = owner; + } } if (sd->internal_ops && sd->internal_ops->open) { -- 2.47.3