linux-kernel - Re: [PATCH] drm/amdgpu: fix a missing-check bug

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Message-ID: <fbd47fc0-c85c-baad-2e35-83de43d0b405@amd.com>
Date:   Mon, 22 Oct 2018 06:55:04 +0000
From:   "Koenig, Christian" <Christian.Koenig@....com>
To:     Wenwen Wang <wang6495@....edu>
CC:     Kangjie Lu <kjlu@....edu>,
        "Deucher, Alexander" <Alexander.Deucher@....com>,
        "Zhou, David(ChunMing)" <David1.Zhou@....com>,
        David Airlie <airlied@...ux.ie>,
        "Wang, Ken" <Ken.Wang@....com>,
        "open list:RADEON and AMDGPU DRM DRIVERS" 
        <amd-gfx@...ts.freedesktop.org>,
        "open list:DRM DRIVERS" <dri-devel@...ts.freedesktop.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/amdgpu: fix a missing-check bug

Am 20.10.18 um 22:57 schrieb Wenwen Wang:
> In amdgpu_read_bios_from_rom(), the header of the VBIOS is firstly copied
> to 'header' from an IO memory region through
> amdgpu_asic_read_bios_from_rom(). Then the header is checked to see whether
> it is a valid header. If yes, the whole VBIOS, including the header, is
> then copied to 'adev->bios'. The problem here is that no check is enforced
> on the header after the second copy. Given that the device also has the
> permission to access the IO memory region, it is possible for a malicious
> device controlled by an attacker to modify the header between these two
> copies. By doing so, the attacker can supply compromised VBIOS, which can
> cause undefined behavior of the kernel and introduce potential security
> issues.
>
> This patch rewrites the header in 'adev->bios' using the header acquired in
> the first copy.

As already answered previously this is completely superfluous.

We are not mapping VRAM here, but a flash ROM (Read Only Memory). The 
flash rom is not changeable through the mapping.

Additional to that the length check is just to confirm that the rom is 
correctly mapped, it doesn't validate the rom in any way.

Regards,
Christian.

>
> Signed-off-by: Wenwen Wang <wang6495@....edu>
> ---
>   drivers/gpu/drm/amd/amdgpu/amdgpu_bios.c | 2 ++
>   1 file changed, 2 insertions(+)
>
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_bios.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_bios.c
> index a5df80d..ac701f4 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_bios.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_bios.c
> @@ -181,6 +181,8 @@ static bool amdgpu_read_bios_from_rom(struct amdgpu_device *adev)
>   	/* read complete BIOS */
>   	amdgpu_asic_read_bios_from_rom(adev, adev->bios, len);
>   
> +	memcpy(adev->bios, header, AMD_VBIOS_SIGNATURE_END);
> +
>   	if (!check_atom_bios(adev->bios, len)) {
>   		kfree(adev->bios);
>   		return false;