Re: [PATCH v4 1/2] dma-mapping: Add vmap checks to dma_map_single()

[Greg Kroah-Hartman]

On Tue, Oct 29, 2019 at 02:34:22PM -0700, Kees Cook wrote:
> As we've seen from USB and other areas[1], we need to always do runtime
> checks for DMA operating on memory regions that might be remapped. This
> adds vmap checks (similar to those already in USB but missing in other
> places) into dma_map_single() so all callers benefit from the checking.
> 
> [1] https://git.kernel.org/linus/3840c5b78803b2b6cc1ff820100a74a092c40cbb
> 
> Suggested-by: Laura Abbott <labbott@xxxxxxxxxx>
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>  include/linux/dma-mapping.h | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
> index 4a1c4fca475a..54de3c496407 100644
> --- a/include/linux/dma-mapping.h
> +++ b/include/linux/dma-mapping.h
> @@ -583,6 +583,12 @@ static inline unsigned long dma_get_merge_boundary(struct device *dev)
>  static inline dma_addr_t dma_map_single_attrs(struct device *dev, void *ptr,
>  		size_t size, enum dma_data_direction dir, unsigned long attrs)
>  {
> +	/* DMA must never operate on areas that might be remapped. */
> +	if (dev_WARN_ONCE(dev, is_vmalloc_addr(ptr),
> +			  "wanted %zu bytes mapped in vmalloc\n", size)) {
> +		return DMA_MAPPING_ERROR;
> +	}

That's a very odd error string, I know if I saw it for the first time, I
would have no idea what it meant.  The USB message at least gives you a
bit more context as to what went wrong and how to fix it.

How about something like "Memory is not DMA capabable, please fix the
allocation of it to be correct", or "non-dma-able memory was attempted
to be mapped, but this is impossible to to" or something else.

thanks,

greg k-h