[RFC PATCH] drm/bridge: panel: Use devm_drm_bridge_add()
Chen-Yu Tsai
wenst at chromium.org
Fri Nov 29 04:52:54 UTC 2024
On Thu, Nov 28, 2024 at 2:46 AM Sui Jingfeng <sui.jingfeng at linux.dev> wrote:
>
> Hi,
>
> On 2024/11/27 17:58, Chen-Yu Tsai wrote:
> > 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 at kernel.org> 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 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.
> > 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.
>
> Well, I think this is more of probe issue of multiple kernel modules.
>
> The root issue is that the global bridge list still stores the
> pointer to *old* the bridge instance which has been freed after
> the first '-EPROBE_DEFER' happened. The next time the
> 'drm_bridge_add(&panel_bridge->bridge);' is called, we will deference
> the *old* NULL pointer. Because it will touch the 'struct drm_bridge::list'
> field, which's backing memory has been freed.
Yes. That is what is causing the crash.
> > 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.
>
> When the devm_drm_panel_bridge_add_typed() is passed a pointer of
> DSI host device, we essentially tie the lifetime of the freshly
> created drm bridge instance to the DSI host device. But, the
> 'struct panel_bridge' clearly hint that the bridge instance has
> the same lifetime with the backing panel, after all, it's the
> underlying panel baking the bridge.
Exactly.
> >> 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?
>
> I think, align the lifetime of the bridge with 'panel->dev' probably helps.
> Modifying the devm_drm_of_get_bridge() function like the following pattern:
>
>
> ```
>
> struct drm_bridge *devm_drm_of_get_bridge(struct device_node *np, u32
> port, u32 endpoint)
> {
> struct drm_bridge *bridge;
> struct drm_panel *panel;
> int ret;
>
> ret = drm_of_find_panel_or_bridge(np, port, endpoint, &panel, &bridge);
> if (ret)
> return ERR_PTR(ret);
>
> if (panel)
> bridge = devm_drm_panel_bridge_add(panel->dev, panel);
>
> return bridge;
> }
That's one possible solution I thought of, but then the devm_ prefix
no longer makes sense. Also the panel_bridge is still implicitly created,
and we might as well move that to the panel side.
> ```
>
>
> Or alternatively, inline this to drm/mediatek,
> rename it as mtk_drm_of_get_bridge().
I would prefer to not do that, since that only fixes the issue for
MediaTek, while we have some 30 odd users of devm_drm_of_get_bridge().
> Or alternatively, manage the bridge's lifetime manually.
> Remove it from the global bridge list if errors happen.
That's also one way; it's just messy.
Thanks
ChenYu
> >
> > Thanks
> > ChenYu
> >
> >> It's much more involved than just another helper though :/
> >>
> >> Maxime
>
> --
> Best regards,
> Sui
>
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