Re: [PATCH] intel_th: Fix a missing-check bug

[Wenwen Wang]

Hello,

Can anyone confirm this bug? Thanks!

Wenwen

On Fri, Oct 19, 2018 at 8:47 AM Wenwen Wang <wang6495@xxxxxxx> wrote:
>
> In msc_data_sz(), the 'valid_dw' field of the msc block descriptor 'bdesc'
> is firstly checked to see whether the descriptor has a valid data width. If
> yes, i.e., 'bdesc->valid_dw' is not 0, the data size of this descriptor
> will be returned. It is worth noting that the data size is calculated from
> 'bdesc->valid_dw'. The problem here is that 'bdesc' actually points to a
> consistent DMA region, which is allocated through dma_alloc_coherent() in
> msc_buffer_win_alloc(). Given that the device also has the permission to
> access this DMA region, it is possible that a malicious device controlled
> by an attacker can modify the 'valid_dw' field after the if statement but
> before the return statement in msc_data_sz(). Through this way, the device
> can bypass the check and supply unexpected data width.
>
> This patch copies the 'valid_dw' field to a local variable and then
> performs the check and the calculation on the local variable to avoid the
> above issue.
>
> Signed-off-by: Wenwen Wang <wang6495@xxxxxxx>
> ---
>  drivers/hwtracing/intel_th/msu.h | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/hwtracing/intel_th/msu.h b/drivers/hwtracing/intel_th/msu.h
> index 9cc8ace..b7d846e 100644
> --- a/drivers/hwtracing/intel_th/msu.h
> +++ b/drivers/hwtracing/intel_th/msu.h
> @@ -79,10 +79,12 @@ struct msc_block_desc {
>
>  static inline unsigned long msc_data_sz(struct msc_block_desc *bdesc)
>  {
> -       if (!bdesc->valid_dw)
> +       u32 valid_dw = bdesc->valid_dw;
> +
> +       if (!valid_dw)
>                 return 0;
>
> -       return bdesc->valid_dw * 4 - MSC_BDESC;
> +       return valid_dw * 4 - MSC_BDESC;
>  }
>
>  static inline bool msc_block_wrapped(struct msc_block_desc *bdesc)
> --
> 2.7.4
>