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

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Message-ID: <Z2yw_eJwR5ih1Npr@debian.local>
Date: Thu, 26 Dec 2024 01:27:25 +0000
From: Chris Bainbridge <chris.bainbridge@...il.com>
To: Tobias Jakobi <tjakobi@...h.uni-bielefeld.de>
Cc: Mario Limonciello <mario.limonciello@....com>,
	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

On Thu, Dec 26, 2024 at 12:19:02AM +0100, Tobias Jakobi wrote:
> 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()?

Yes, that would work to fix the bad kfree, but there would be a small
memory leak of the acpi_buffer struct. It's not a huge problem since
this code is rarely run, and the Nouveau code has never tried to free
the edid buffer and apparently nobody noticed, but it would be better to
do the correct thing.

One other curiosity is this comment in the code that allocates the
memory:

case ACPI_ALLOCATE_BUFFER:
	/*
	 * Allocate a new buffer. We directectly call acpi_os_allocate here to
	 * purposefully bypass the (optionally enabled) internal allocation
	 * tracking mechanism since we only want to track internal
	 * allocations. Note: The caller should use acpi_os_free to free this
	 * buffer created via ACPI_ALLOCATE_BUFFER.
	 */

Which makes me wonder if all the calls to kfree on acpi_buffer structs
with ACPI_ALLOCATE_BUFFER in acpi_video.c should actually be calls to
acpi_os_free instead? I used kfree just for consistency with the
existing code.