Re: [PATCH] media: venus: fix possible buffer overlow casued bad DMA value in venus_sfr_print()

[Mauro Carvalho Chehab <mchehab+huawei@kernel.org>]

Em Sat, 30 May 2020 10:41:17 +0800
Jia-Ju Bai <baijiaju1990@gmail.com> escreveu:

> The value hdev->sfr.kva is stored in DMA memory, and it is assigned to
> sfr, so sfr->buf_size can be modified at anytime by malicious hardware. 
> In this case, a buffer overflow may happen when the code 
> "sfr->data[sfr->buf_size - 1]" is executed.
> 
> To fix this possible bug, sfr->buf_size is assigned to a local variable,
> and then this variable is checked before being used.
> 
> Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
> ---
>  drivers/media/platform/qcom/venus/hfi_venus.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/media/platform/qcom/venus/hfi_venus.c b/drivers/media/platform/qcom/venus/hfi_venus.c
> index 0d8855014ab3..4251a9e47a1b 100644
> --- a/drivers/media/platform/qcom/venus/hfi_venus.c
> +++ b/drivers/media/platform/qcom/venus/hfi_venus.c
> @@ -960,18 +960,23 @@ static void venus_sfr_print(struct venus_hfi_device *hdev)
>  {
>  	struct device *dev = hdev->core->dev;
>  	struct hfi_sfr *sfr = hdev->sfr.kva;
> +	u32 buf_size;
>  	void *p;
>  
>  	if (!sfr)
>  		return;
>  
> -	p = memchr(sfr->data, '\0', sfr->buf_size);
> +	buf_size = sfr->buf_size;
> +	if (buf_size > 1)

That seems plain wrong to me... I suspect you wanted to do,
instead:

	if (buf_size < 1)

or even:
	if (buf_size < 1 || buf_size >= maximum_size_of_data)


> +		return;
> +
> +	p = memchr(sfr->data, '\0', buf_size);
>  	/*
>  	 * SFR isn't guaranteed to be NULL terminated since SYS_ERROR indicates
>  	 * that Venus is in the process of crashing.
>  	 */
>  	if (!p)
> -		sfr->data[sfr->buf_size - 1] = '\0';
> +		sfr->data[buf_size - 1] = '\0';

Well, a malicious hardware with DMA access could simply write 0 to
some random address, without needing to rely on the value
of sfr->buf_size. I can't see how a change like that would prevent
that.

A check like that only makes sense if the driver can ever
call this function with an invalid value for sfr->buf_size.


>  
>  	dev_err_ratelimited(dev, "SFR message from FW: %s\n", sfr->data);
>  }



Thanks,
Mauro