On Fri, Mar 08, 2024 at 10:03:57AM +0100, Johan Hovold wrote:
commit b979f2d50a099f3402418d7ff5f26c3952fb08bb upstream.
A recent DRM series purporting to simplify support for "transparent bridges" and handling of probe deferrals ironically exposed a use-after-free issue on pmic_glink_altmode probe deferral.
This has manifested itself as the display subsystem occasionally failing to initialise and NULL-pointer dereferences during boot of machines like the Lenovo ThinkPad X13s.
Specifically, the dp-hpd bridge is currently registered before all resources have been acquired which means that it can also be deregistered on probe deferrals.
In the meantime there is a race window where the new aux bridge driver (or PHY driver previously) may have looked up the dp-hpd bridge and stored a (non-reference-counted) pointer to the bridge which is about to be deallocated.
When the display controller is later initialised, this triggers a use-after-free when attaching the bridges:
dp -> aux -> dp-hpd (freed)
which may, for example, result in the freed bridge failing to attach:
[drm:drm_bridge_attach [drm]] *ERROR* failed to attach bridge /soc@0/phy@88eb000 to encoder TMDS-31: -16
or a NULL-pointer dereference:
Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000 ... Call trace: drm_bridge_attach+0x70/0x1a8 [drm] drm_aux_bridge_attach+0x24/0x38 [aux_bridge] drm_bridge_attach+0x80/0x1a8 [drm] dp_bridge_init+0xa8/0x15c [msm] msm_dp_modeset_init+0x28/0xc4 [msm]
The DRM bridge implementation is clearly fragile and implicitly built on the assumption that bridges may never go away. In this case, the fix is to move the bridge registration in the pmic_glink_altmode driver to after all resources have been looked up.
Incidentally, with the new dp-hpd bridge implementation, which registers child devices, this is also a requirement due to a long-standing issue in driver core that can otherwise lead to a probe deferral loop (see commit fbc35b45f9f6 ("Add documentation on meaning of -EPROBE_DEFER")).
[DB: slightly fixed commit message by adding the word 'commit'] Fixes: 080b4e24852b ("soc: qcom: pmic_glink: Introduce altmode support") Fixes: 2bcca96abfbf ("soc: qcom: pmic-glink: switch to DRM_AUX_HPD_BRIDGE") Cc: stable@vger.kernel.org # 6.3 Cc: Bjorn Andersson andersson@kernel.org Cc: Dmitry Baryshkov dmitry.baryshkov@linaro.org Signed-off-by: Johan Hovold johan+linaro@kernel.org Reviewed-by: Bjorn Andersson andersson@kernel.org Reviewed-by: Dmitry Baryshkov dmitry.baryshkov@linaro.org Signed-off-by: Dmitry Baryshkov dmitry.baryshkov@linaro.org Link: https://patchwork.freedesktop.org/patch/msgid/20240217150228.5788-4-johan+li... [ johan: backport to 6.7 which does not have DRM aux bridge ] Signed-off-by: Johan Hovold johan+linaro@kernel.org
Queued up, thanks!