On Sat, 17 Feb 2024 at 17:03, Johan Hovold <johan+lin...@kernel.org> wrote:
>
> 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
> fbc35b45f9f6 ("Add documentation on meaning of -EPROBE_DEFER")).
>
> Fixes: 080b4e24852b ("soc: qcom: pmic_glink: Introduce altmode support")
> Fixes: 2bcca96abfbf ("soc: qcom: pmic-glink: switch to DRM_AUX_HPD_BRIDGE")
> Cc: sta...@vger.kernel.org # 6.3
> Cc: Bjorn Andersson <anders...@kernel.org>
> Cc: Dmitry Baryshkov <dmitry.barysh...@linaro.org>
> Signed-off-by: Johan Hovold <johan+lin...@kernel.org>
> ---
> drivers/soc/qcom/pmic_glink_altmode.c | 16 +++++++++++++---
> 1 file changed, 13 insertions(+), 3 deletions(-)
>
Reviewed-by: Dmitry Baryshkov <dmitry.barysh...@linaro.org>
--
With best wishes
Dmitry
