Re: [PATCH] drm/radeon: fix a missing-check bug

["Koenig, Christian" <Christian.Koenig@amd.com>]

Am 18.10.18 um 19:13 schrieb Wenwen Wang:
> In radeon_read_bios(), the bios rom is firstly mapped to the IO memory
> region 'bios' through pci_map_rom(). Then the first two bytes of 'bios' are
> copied to 'val1' and 'val2' respectively through readb(). After that,
> 'val1' and 'val2' are checked to see whether they have expected values,
> i.e., 0x55 and 0xaa, respectively. If yes, the whole data in 'bios' is then
> copied to 'rdev->bios' through memcpy_fromio(). Obviously, the first two
> bytes in 'bios' are copied twice. More importantly, no check is enforced on
> the first two bytes of 'rdev->bios' after memcpy_fromio(). Given that the
> IO memory region can also be accessed by the device, it is possible that a
> malicious device can race to modify these two bytes between the two copies
> and thus after memcpy_fromio(), the first two bytes in 'rdev->bios' can
> have unexpected values.  This can cause undefined behavior of the kernel
> and introduce potential security risk, if the device can be controlled by
> attackers.
>
> This patch rewrites the first two bytes of 'rdev->bios' after
> memcpy_fromio() with expected values. Through this way, the above issue can
> be avoided.

Well NAK, that doesn't make any sense to me.

First of all we don't map VRAM, but rather the ROM which is a read only 
flash. Writing to the flash is not supported through the BAR as far as I 
know.

Then we check the first two bytes to make sure that the ROM is correctly 
mapped and not to prevent any malicious attacks.

Regards,
Christian.

>
> Signed-off-by: Wenwen Wang <wang6495@umn.edu>
> ---
>   drivers/gpu/drm/radeon/radeon_bios.c | 2 ++
>   1 file changed, 2 insertions(+)
>
> diff --git a/drivers/gpu/drm/radeon/radeon_bios.c b/drivers/gpu/drm/radeon/radeon_bios.c
> index 04c0ed4..f336719 100644
> --- a/drivers/gpu/drm/radeon/radeon_bios.c
> +++ b/drivers/gpu/drm/radeon/radeon_bios.c
> @@ -98,6 +98,8 @@ static bool radeon_read_bios(struct radeon_device *rdev)
>   		return false;
>   	}
>   	memcpy_fromio(rdev->bios, bios, size);
> +	rdev->bios[0] = val1;
> +	rdev->bios[1] = val2;
>   	pci_unmap_rom(rdev->pdev, bios);
>   	return true;
>   }