Revisiting this thread since I just stepped on the same problem on a different device. On Thu, Nov 14, 2024 at 9:12 PM Maxime Ripard <mripard@xxxxxxxxxx> wrote: > > On Tue, Oct 29, 2024 at 10:53:49PM +0800, Fei Shao wrote: > > On Thu, Oct 24, 2024 at 8:36 PM Maxime Ripard <mripard@xxxxxxxxxx> wrote: > > > > > > On Wed, Oct 09, 2024 at 01:23:31PM +0800, Fei Shao wrote: > > > > In the mtk_dsi driver, its DSI host attach callback calls > > > > devm_drm_of_get_bridge() to get the next bridge. If that next bridge is > > > > a panel bridge, a panel_bridge object is allocated and managed by the > > > > panel device. > > > > > > > > Later, if the attach callback fails with -EPROBE_DEFER from subsequent > > > > component_add(), the panel device invoking the callback at probe time > > > > also fails, and all device-managed resources are freed accordingly. > > > > > > > > This exposes a drm_bridge bridge_list corruption due to the unbalanced > > > > lifecycle between the DSI host and the panel devices: the panel_bridge > > > > object managed by panel device is freed, while drm_bridge_remove() is > > > > bound to DSI host device and never gets called. > > > > The next drm_bridge_add() will trigger UAF against the freed bridge list > > > > object and result in kernel panic. > > > > > > > > This bug is observed on a MediaTek MT8188-based Chromebook with MIPI DSI > > > > outputting to a DSI panel (DT is WIP for upstream). > > > > > > > > As a fix, using devm_drm_bridge_add() with the panel device in the panel > > > > path seems reasonable. This also implies a chain of potential cleanup > > > > actions: > > > > > > > > 1. Removing drm_bridge_remove() means devm_drm_panel_bridge_release() > > > > becomes hollow and can be removed. > > > > > > > > 2. devm_drm_panel_bridge_add_typed() is almost emptied except for the > > > > `bridge->pre_enable_prev_first` line. Itself can be also removed if > > > > we move the line into drm_panel_bridge_add_typed(). (maybe?) > > > > > > > > 3. drm_panel_bridge_add_typed() now calls all the needed devm_* calls, > > > > so it's essentially the new devm_drm_panel_bridge_add_typed(). > > > > > > > > 4. drmm_panel_bridge_add() needs to be updated accordingly since it > > > > calls drm_panel_bridge_add_typed(). But now there's only one bridge > > > > object to be freed, and it's already being managed by panel device. > > > > I wonder if we still need both drmm_ and devm_ version in this case. > > > > (maybe yes from DRM PoV, I don't know much about the context) > > > > > > > > This is a RFC patch since I'm not sure if my understanding is correct > > > > (for both the fix and the cleanup). It fixes the issue I encountered, > > > > but I don't expect it to be picked up directly due to the redundant > > > > commit message and the dangling devm_drm_panel_bridge_release(). > > > > I plan to resend the official patch(es) once I know what I supposed to > > > > do next. > > > > > > > > For reference, here's the KASAN report from the device: > > > > ================================================================== > > > > BUG: KASAN: slab-use-after-free in drm_bridge_add+0x98/0x230 > > > > Read of size 8 at addr ffffff80c4e9e100 by task kworker/u32:1/69 > > > > > > > > CPU: 1 UID: 0 PID: 69 Comm: kworker/u32:1 Not tainted 6.12.0-rc1-next-20241004-kasan-00030-g062135fa4046 #1 > > > > Hardware name: Google Ciri sku0/unprovisioned board (DT) > > > > Workqueue: events_unbound deferred_probe_work_func > > > > Call trace: > > > > dump_backtrace+0xfc/0x140 > > > > show_stack+0x24/0x38 > > > > dump_stack_lvl+0x40/0xc8 > > > > print_report+0x140/0x700 > > > > kasan_report+0xcc/0x130 > > > > __asan_report_load8_noabort+0x20/0x30 > > > > drm_bridge_add+0x98/0x230 > > > > devm_drm_panel_bridge_add_typed+0x174/0x298 > > > > devm_drm_of_get_bridge+0xe8/0x190 > > > > mtk_dsi_host_attach+0x130/0x2b0 > > > > mipi_dsi_attach+0x8c/0xe8 > > > > hx83102_probe+0x1a8/0x368 > > > > mipi_dsi_drv_probe+0x6c/0x88 > > > > really_probe+0x1c4/0x698 > > > > __driver_probe_device+0x160/0x298 > > > > driver_probe_device+0x7c/0x2a8 > > > > __device_attach_driver+0x2a0/0x398 > > > > bus_for_each_drv+0x198/0x200 > > > > __device_attach+0x1c0/0x308 > > > > device_initial_probe+0x20/0x38 > > > > bus_probe_device+0x11c/0x1f8 > > > > deferred_probe_work_func+0x80/0x250 > > > > worker_thread+0x9b4/0x2780 > > > > kthread+0x274/0x350 > > > > ret_from_fork+0x10/0x20 > > > > > > > > Allocated by task 69: > > > > kasan_save_track+0x40/0x78 > > > > kasan_save_alloc_info+0x44/0x58 > > > > __kasan_kmalloc+0x84/0xa0 > > > > __kmalloc_node_track_caller_noprof+0x228/0x450 > > > > devm_kmalloc+0x6c/0x288 > > > > devm_drm_panel_bridge_add_typed+0xa0/0x298 > > > > devm_drm_of_get_bridge+0xe8/0x190 > > > > mtk_dsi_host_attach+0x130/0x2b0 > > > > mipi_dsi_attach+0x8c/0xe8 > > > > hx83102_probe+0x1a8/0x368 > > > > mipi_dsi_drv_probe+0x6c/0x88 > > > > really_probe+0x1c4/0x698 > > > > __driver_probe_device+0x160/0x298 > > > > driver_probe_device+0x7c/0x2a8 > > > > __device_attach_driver+0x2a0/0x398 > > > > bus_for_each_drv+0x198/0x200 > > > > __device_attach+0x1c0/0x308 > > > > device_initial_probe+0x20/0x38 > > > > bus_probe_device+0x11c/0x1f8 > > > > deferred_probe_work_func+0x80/0x250 > > > > worker_thread+0x9b4/0x2780 > > > > kthread+0x274/0x350 > > > > ret_from_fork+0x10/0x20 > > > > > > > > Freed by task 69: > > > > kasan_save_track+0x40/0x78 > > > > kasan_save_free_info+0x58/0x78 > > > > __kasan_slab_free+0x48/0x68 > > > > kfree+0xd4/0x750 > > > > devres_release_all+0x144/0x1e8 > > > > really_probe+0x48c/0x698 > > > > __driver_probe_device+0x160/0x298 > > > > driver_probe_device+0x7c/0x2a8 > > > > __device_attach_driver+0x2a0/0x398 > > > > bus_for_each_drv+0x198/0x200 > > > > __device_attach+0x1c0/0x308 > > > > device_initial_probe+0x20/0x38 > > > > bus_probe_device+0x11c/0x1f8 > > > > deferred_probe_work_func+0x80/0x250 > > > > worker_thread+0x9b4/0x2780 > > > > kthread+0x274/0x350 > > > > ret_from_fork+0x10/0x20 > > > > > > > > The buggy address belongs to the object at ffffff80c4e9e000 > > > > which belongs to the cache kmalloc-4k of size 4096 > > > > The buggy address is located 256 bytes inside of > > > > freed 4096-byte region [ffffff80c4e9e000, ffffff80c4e9f000) > > > > > > > > The buggy address belongs to the physical page: > > > > head: order:3 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0 > > > > flags: 0x8000000000000040(head|zone=2) > > > > page_type: f5(slab) > > > > page: refcount:1 mapcount:0 mapping:0000000000000000 > > > > index:0x0 pfn:0x104e98 > > > > raw: 8000000000000040 ffffff80c0003040 dead000000000122 0000000000000000 > > > > raw: 0000000000000000 0000000000040004 00000001f5000000 0000000000000000 > > > > head: 8000000000000040 ffffff80c0003040 dead000000000122 0000000000000000 > > > > head: 0000000000000000 0000000000040004 00000001f5000000 0000000000000000 > > > > head: 8000000000000003 fffffffec313a601 ffffffffffffffff 0000000000000000 > > > > head: 0000000000000008 0000000000000000 00000000ffffffff 0000000000000000 > > > > page dumped because: kasan: bad access detected > > > > > > > > Memory state around the buggy address: > > > > ffffff80c4e9e000: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb > > > > ffffff80c4e9e080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb > > > > >ffffff80c4e9e100: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb > > > > ^ > > > > ffffff80c4e9e180: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb > > > > ffffff80c4e9e200: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb > > > > =================================================================== > > > > > > > > Signed-off-by: Fei Shao <fshao@xxxxxxxxxxxx> > > > > > > I was looking at the driver to try to follow your (awesome btw, thanks) > > > commit log, and it does have a quite different structure compared to > > > what we recommend. > > > > > > Would following > > > https://docs.kernel.org/gpu/drm-kms-helpers.html#special-care-with-mipi-dsi-bridges > > > help? > > > > Hi Maxime, > > > > Thank you for the pointer. > > I read the suggested pattern in the doc and compared it with the > > drivers. If I understand correctly, both the MIPI-DSI host and panel > > drivers follow the instructions: > > > > 1. The MIPI-DSI host driver must run mipi_dsi_host_register() in its probe hook. > > >> drm/mediatek/mtk_dsi.c runs mipi_dsi_host_register() in the probe hook. > > 2. In its probe hook, the bridge driver must try to find its MIPI-DSI > > host, register as a MIPI-DSI device and attach the MIPI-DSI device to > > its host. > > >> drm/panel/panel-himax-hx83102.c follows and runs > > mipi_dsi_attach() at the end of probe hook. > > 3. In its struct mipi_dsi_host_ops.attach hook, the MIPI-DSI host can > > now add its component. > > >> drm/mediatek/mtk_dsi.c calls component_add() in the attach callback. > > > > Could you elaborate on the "different structures" you mentioned? > > Yeah, you're right, sorry. > > > To clarify my point: the issue is that component_add() may return > > -EPROBE_DEFER if the component (e.g. DSI encoder) is not ready, > > causing the panel bridge to be removed. However, drm_bridge_remove() > > is bound to MIPI-DSI host instead of panel bridge, which owns the > > actual list_head object. > > > > This might be reproducible with other MIPI-DSI host + panel > > combinations by forcibly returning -EPROBE_DEFER in the host attach > > hook (verification with another device is needed), so the fix may be > > required in drm/bridge/panel.c. > Yeah, I think you're just hitting another bridge lifetime issue, and > it's not the only one unfortunately. Tying the bridge structure lifetime > itself to the device is wrong, it should be tied to the DRM device > lifetime instead. I think the more immediate issue is that the bridge object's lifetime and drm_bridge_add/remove are inconsistent when devm_drm_of_get_bridge() or drmm_of_get_bridge() are used. These helpers tie the bridge add/removal to the device or drm_device passed in, but internally they call down to drm_panel_bridge_add_typed() which allocates the bridge object tied to the panel device. > But then, the discussion becomes that bridges typically probe outside of > the "main" DRM device probe path, so you don't have access to the DRM > device structure until attach at best. > > That's why I'm a bit skeptical about your patch. It might workaround > your issue, but it doesn't actually solve the problem. I guess the best > way about it would be to convert bridges to reference counting, with the > device taking a reference at probe time when it allocates the structure > (and giving it back at remove time), and the DRM device taking one when > it's attached and one when it's detached. Without going as far, it's probably better to align the lifecycle of the two parts. Most other bridge drivers in the kernel have |drm_bridge| lifecycle tied to their underlying |device|, either with explicit drm_bridge_{add,remove}() calls in their probe/bind and remove/unbind callbacks respectively, or with devm_drm_bridge_add in the probe/bind path. The only ones with a narrower lifecycle are the DSI hosts, which add the bridge in during host attach and remove it during host detach. I'm thinking about fixing the panel_bridge lifecycle such that it is tied to the panel itself. Maybe that would involve making devm_drm_of_get_bridge() correctly return bridges even if a panel was found, and then making the panels create and add panel bridges directly, possibly within drm_panel_add(). Would that make sense? Thanks ChenYu > It's much more involved than just another helper though :/ > > Maxime
