Re: [PATCH] drm/connector: hdmi: Handle NULL display mode in state check helper

[Cristian Ciocaltea <cristian.ciocaltea@collabora.com>]

On 1/7/25 5:16 PM, Maxime Ripard wrote:
> On Tue, Jan 07, 2025 at 02:00:35AM +0200, Cristian Ciocaltea wrote:
>> drm_atomic_helper_connector_hdmi_check() helper makes use of
>> connector_state_get_mode() to obtain a drm_display_mode pointer, but it
>> doesn't validate it, which may lead to a NULL pointer dereference in
>> some cases, i.e. unloading a DRM module:
>>
>> [ 1002.910414] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
>> [...]
>> [ 1002.923833] Hardware name: Radxa ROCK 5B (DT)
>> [ 1002.924819] pc : drm_match_cea_mode+0x30/0x280 [drm]
>> [ 1002.925318] lr : hdmi_try_format_bpc+0x7c/0x580 [drm_display_helper]
>> [...]
>> [ 1002.932411] Call trace:
>> [ 1002.932626]  drm_match_cea_mode+0x30/0x280 [drm] (P)
>> [ 1002.933120]  hdmi_try_format_bpc+0x7c/0x580 [drm_display_helper]
>> [ 1002.933662]  drm_atomic_helper_connector_hdmi_check+0x10c/0x478 [drm_display_helper]
>> [ 1002.934355]  drm_bridge_connector_atomic_check+0x20/0x40 [drm_display_helper]
>> [ 1002.934993]  drm_atomic_helper_check_modeset+0x698/0xd28 [drm_kms_helper]
>> [ 1002.935607]  drm_atomic_helper_check+0x28/0xb8 [drm_kms_helper]
>> [ 1002.936143]  drm_atomic_check_only+0x794/0x988 [drm]
>> [ 1002.936635]  drm_atomic_commit+0x60/0xe0 [drm]
>> [ 1002.937082]  drm_atomic_helper_disable_all+0x184/0x218 [drm_kms_helper]
>> [ 1002.937678]  drm_atomic_helper_shutdown+0x90/0x150 [drm_kms_helper]
>> [ 1002.938243]  rockchip_drm_unbind+0x38/0x80 [rockchipdrm]
>> [ 1002.938720]  component_master_del+0xac/0xf8
>> [ 1002.939089]  rockchip_drm_platform_remove+0x34/0x78 [rockchipdrm]
>> [...]
>>
>> Add the missing NULL check before passing the mode pointer further.
>>
>> Fixes: f035f4097f1e ("drm/connector: hdmi: Calculate TMDS character rate")
>> Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
> 
> Looks good to me, but we need to provide a test for that case too

Test cases provided in v2:

https://lore.kernel.org/lkml/20250109-hdmi-conn-null-mode-v2-2-9eeaf1109547@collabora.com/

Thanks,
Cristian