[RFC PATCH] drm/bridge: panel: Use devm_drm_bridge_add()

Thu Nov 14 13:12:01 UTC 2024

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 at kernel.org> 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 at chromium.org>
> >
> > 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.

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.

It's much more involved than just another helper though :/

Maxime
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