linux-kernel - Re: [PATCH 3/6] soc: qcom: pmic_glink_altmode: fix drm bridge use-after-free

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Message-ID: <CAA8EJpoPaknqPUEg8p37Nh1MV62Cr8fH+MxE-1b+T-8h3BmO9Q@mail.gmail.com>
Date: Thu, 22 Feb 2024 23:10:11 +0200
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Johan Hovold <johan+linaro@...nel.org>
Cc: Bjorn Andersson <andersson@...nel.org>, Andrzej Hajda <andrzej.hajda@...el.com>, 
	Neil Armstrong <neil.armstrong@...aro.org>, Robert Foss <rfoss@...nel.org>, 
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>, 
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>, 
	Vinod Koul <vkoul@...nel.org>, Jonas Karlman <jonas@...boo.se>, 
	Laurent Pinchart <Laurent.pinchart@...asonboard.com>, 
	Jernej Skrabec <jernej.skrabec@...il.com>, Konrad Dybcio <konrad.dybcio@...aro.org>, 
	Kishon Vijay Abraham I <kishon@...nel.org>, Rob Clark <robdclark@...il.com>, 
	Abhinav Kumar <quic_abhinavk@...cinc.com>, Kuogee Hsieh <quic_khsieh@...cinc.com>, 
	freedreno@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org, 
	linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org, 
	linux-phy@...ts.infradead.org, stable@...r.kernel.org
Subject: Re: [PATCH 3/6] soc: qcom: pmic_glink_altmode: fix drm bridge use-after-free

On Sat, 17 Feb 2024 at 17:03, Johan Hovold <johan+linaro@...nel.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@...hy@...b000 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: stable@...r.kernel.org      # 6.3
> Cc: Bjorn Andersson <andersson@...nel.org>
> Cc: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
> Signed-off-by: Johan Hovold <johan+linaro@...nel.org>
> ---
>  drivers/soc/qcom/pmic_glink_altmode.c | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)
>

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>


-- 
With best wishes
Dmitry