linux-kernel - Re: [PATCH] drm/bridge: megachips: Fix a null pointer dereference bug

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Date:   Tue, 19 Jul 2022 09:38:16 +0100
From:   Martyn Welch <martyn.welch@...labora.co.uk>
To:     Zheyu Ma <zheyuma97@...il.com>, peter.senna@...il.com,
        martin.donnelly@...com, andrzej.hajda@...el.com,
        narmstrong@...libre.com, robert.foss@...aro.org,
        Laurent.pinchart@...asonboard.com, jonas@...boo.se,
        jernej.skrabec@...il.com, airlied@...ux.ie, daniel@...ll.ch
Cc:     dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/bridge: megachips: Fix a null pointer dereference
 bug

On Sat, 2022-07-16 at 16:13 +0800, Zheyu Ma wrote:
> When removing the module we will get the following warning:
> 
> [   31.911505] i2c-core: driver [stdp2690-ge-b850v3-fw] unregistered
> [   31.912484] general protection fault, probably for non-canonical
> address 0xdffffc0000000001: 0000 [#1] PREEMPT SMP KASAN PTI
> [   31.913338] KASAN: null-ptr-deref in range [0x0000000000000008-
> 0x000000000000000f]
> [   31.915280] RIP: 0010:drm_bridge_remove+0x97/0x130
> [   31.921825] Call Trace:
> [   31.922533]  stdp4028_ge_b850v3_fw_remove+0x34/0x60
> [megachips_stdpxxxx_ge_b850v3_fw]
> [   31.923139]  i2c_device_remove+0x181/0x1f0
> 
> The two bridges (stdp2690, stdp4028) do not probe at the same time,
> so
> the driver does not call ge_b850v3_resgiter() when probing, causing
> the
> driver to try to remove the object that has not been initialized.
> 
> Fix this by checking whether both the bridges are probed.
> 
> Signed-off-by: Zheyu Ma <zheyuma97@...il.com>

Good catch!

Acked-by: Martyn Welch <martyn.welch@...labora.com>

Would be worth adding:

Fixes: 11632d4aa2b3 ("drm/bridge: megachips: Ensure both bridges are
probed before registration")

> ---
>  drivers/gpu/drm/bridge/megachips-stdpxxxx-ge-b850v3-fw.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/bridge/megachips-stdpxxxx-ge-b850v3-fw.c
> b/drivers/gpu/drm/bridge/megachips-stdpxxxx-ge-b850v3-fw.c
> index cce98bf2a4e7..c68a4cdf4625 100644
> --- a/drivers/gpu/drm/bridge/megachips-stdpxxxx-ge-b850v3-fw.c
> +++ b/drivers/gpu/drm/bridge/megachips-stdpxxxx-ge-b850v3-fw.c
> @@ -296,7 +296,9 @@ static void ge_b850v3_lvds_remove(void)
>          * This check is to avoid both the drivers
>          * removing the bridge in their remove() function
>          */
> -       if (!ge_b850v3_lvds_ptr)
> +       if (!ge_b850v3_lvds_ptr ||
> +               !ge_b850v3_lvds_ptr->stdp2690_i2c ||
> +               !ge_b850v3_lvds_ptr->stdp4028_i2c)
>                 goto out;
>  
>         drm_bridge_remove(&ge_b850v3_lvds_ptr->bridge);