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Date: Mon, 8 Jun 2026 10:11:42 +0200
From: Lukas Wunner <lukas@wunner.de>
To: w15303746062@163.com
Cc: airlied@redhat.com, dri-devel@lists.freedesktop.org,
	linux-kernel@vger.kernel.org, x86@kernel.org,
	linux-pci@vger.kernel.org,
	Mingyu Wang <25181214217@stu.xidian.edu.cn>
Subject: Re: [PATCH v2] char: agp: amd64 - fix broken error propagation in
 agp_amd64_probe()
Message-ID: <aiZ5PomNR3ZoRzbM@wunner.de>
References: <20260504074823.99377-1-w15303746062@163.com>
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In-Reply-To: <20260504074823.99377-1-w15303746062@163.com>

On Mon, May 04, 2026 at 03:48:23PM +0800, w15303746062@163.com wrote:
> A NULL pointer dereference was observed in the AMD64 AGP driver when
> running in a virtualized environment (e.g., QEMU/KVM) without a physical
> AMD Northbridge. The crash occurs in amd64_fetch_size() when attempting
> to dereference the pointer returned by node_to_amd_nb(0).
> 
> The root cause of this crash is broken error propagation in
> agp_amd64_probe(). When no AMD Northbridges are found, cache_nbs()
> correctly returns -ENODEV. However, the probe function erroneously
> checked the return value against exactly -1, rather than < 0.
> 
> As a result, the hardware absence error was masked, allowing the driver
> to improperly proceed with initialization. It eventually called
> agp_add_bridge(), which invokes amd64_fetch_size(). Since the hardware
> does not exist, node_to_amd_nb(0) returns NULL, leading to a General
> Protection Fault (GPF) when accessing its ->misc member.
> 
> Fix the issue by correcting the error check in agp_amd64_probe() to
> abort properly when cache_nbs() returns any negative error code. This
> prevents the driver from erroneously proceeding without hardware, thereby
> resolving the subsequent NULL pointer dereference at its source.
> 
> Signed-off-by: Mingyu Wang <25181214217@stu.xidian.edu.cn>

Applied to drm-misc-next-fixes for v7.2, thank you for the patch.