linux-kernel - Re: [PATCH] drm/amd: Fix random crashes due to bad kfree

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Message-ID: <b98f2fa5-fbe8-4958-bf20-fa5d28c5a38b@math.uni-bielefeld.de>
Date: Thu, 26 Dec 2024 00:19:02 +0100
From: Tobias Jakobi <tjakobi@...h.uni-bielefeld.de>
To: Chris Bainbridge <chris.bainbridge@...il.com>,
 Mario Limonciello <mario.limonciello@....com>
Cc: amd-gfx@...ts.freedesktop.org, alex.hung@....com,
 regressions@...ts.linux.dev, rafael@...nel.org, lenb@...nel.org,
 linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/amd: Fix random crashes due to bad kfree

Hi Chris!

On 12/26/24 00:09, Chris Bainbridge wrote:

> Commit c6a837088bed ("drm/amd/display: Fetch the EDID from _DDC if
> available for eDP") added function dm_helpers_probe_acpi_edid, which
> fetches the EDID from the BIOS by calling acpi_video_get_edid.
> acpi_video_get_edid returns a pointer to the EDID, but this pointer does
> not originate from kmalloc - it is actually the internal "pointer" field
> from an acpi_buffer struct (which did come from kmalloc).
> dm_helpers_probe_acpi_edid then attempts to kfree the EDID pointer,
> resulting in memory corruption which leads to random, intermittent
> crashes (e.g. 4% of boots will fail with some Oops).
>
> Fix this by allocating a new array (which can be safely freed) for the
> EDID data in acpi_video_get_edid, and correctly freeing the acpi_buffer.

Hmm, maybe I'm missing something here. But shouldn't it suffice to just 
remove the kfree call in dm_helpers_probe_acpi_edid()?

With best wishes,
Tobias

>
> The only other caller of acpi_video_get_edid is nouveau_acpi_edid:
> remove the extraneous kmemdup here as the EDID data is now copied in
> acpi_video_get_edid.
>
> Signed-off-by: Chris Bainbridge <chris.bainbridge@...il.com>
> Fixes: c6a837088bed ("drm/amd/display: Fetch the EDID from _DDC if available for eDP")
> ---
>   drivers/acpi/acpi_video.c              | 3 ++-
>   drivers/gpu/drm/nouveau/nouveau_acpi.c | 2 +-
>   2 files changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/acpi/acpi_video.c b/drivers/acpi/acpi_video.c
> index 8274a17872ed..151d1d534264 100644
> --- a/drivers/acpi/acpi_video.c
> +++ b/drivers/acpi/acpi_video.c
> @@ -1485,7 +1485,8 @@ int acpi_video_get_edid(struct acpi_device *device, int type, int device_id,
>   		if (!length)
>   			continue;
>   
> -		*edid = buffer->buffer.pointer;
> +		*edid = kmemdup(buffer->buffer.pointer, length, GFP_KERNEL);
> +		kfree(buffer);
>   		return length;
>   	}
>   
> diff --git a/drivers/gpu/drm/nouveau/nouveau_acpi.c b/drivers/gpu/drm/nouveau/nouveau_acpi.c
> index 8f0c69aad248..21b56cc7605c 100644
> --- a/drivers/gpu/drm/nouveau/nouveau_acpi.c
> +++ b/drivers/gpu/drm/nouveau/nouveau_acpi.c
> @@ -384,7 +384,7 @@ nouveau_acpi_edid(struct drm_device *dev, struct drm_connector *connector)
>   	if (ret < 0)
>   		return NULL;
>   
> -	return kmemdup(edid, EDID_LENGTH, GFP_KERNEL);
> +	return edid;
>   }
>   
>   bool nouveau_acpi_video_backlight_use_native(void)