Re: [Intel-gfx] [PATCH] drm/i915: Sanity check incoming ioctl data for a NULL pointer

[Ben Widawsky <ben@bwidawsk.net>]

On Thu, Mar 14, 2013 at 12:59:57PM +0000, Chris Wilson wrote:
> In order to prevent a potential NULL deference with hostile userspace,
> we need to check whether the ioctl was passed an invalid args pointer.
> 
> Reported-by: Tommi Rantala <tt.rantala@gmail.com>
> Link: http://lkml.kernel.org/r/CA+ydwtpuBvbwxbt-tdgPUvj1EU7itmCHo_2B3w13HkD5+jWKow@mail.gmail.com
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> ---
>  drivers/gpu/drm/i915/i915_gem_execbuffer.c |   11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> index 365e41a..9f5602e 100644
> --- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> +++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> @@ -1103,7 +1103,11 @@ i915_gem_execbuffer(struct drm_device *dev, void *data,
>  	struct drm_i915_gem_exec_object2 *exec2_list = NULL;
>  	int ret, i;
>  
> -	if (args->buffer_count < 1) {
> +	if (args == NULL)
> +		return -EINVAL;
> +
> +	if (args->buffer_count < 1 ||
> +	    args->buffer_count > INT_MAX / sizeof(*exec2_list)) {
>  		DRM_DEBUG("execbuf with %d buffers\n", args->buffer_count);
>  		return -EINVAL;
>  	}
> @@ -1182,8 +1186,11 @@ i915_gem_execbuffer2(struct drm_device *dev, void *data,
>  	struct drm_i915_gem_exec_object2 *exec2_list = NULL;
>  	int ret;
>  
> +	if (args == NULL)
> +		return -EINVAL;
> +
>  	if (args->buffer_count < 1 ||
> -	    args->buffer_count > UINT_MAX / sizeof(*exec2_list)) {
> +	    args->buffer_count > INT_MAX / sizeof(*exec2_list)) {
>  		DRM_DEBUG("execbuf2 with %d buffers\n", args->buffer_count);
>  		return -EINVAL;
>  	}

Why did you change UINT_MAX to INT_MAX? TBH, I'm confused what we're
trying to achieve, and why we need anything other than:
if (!args->buffer_count)

I'm also not seeing how the NULL checks are needed since at least it
seems to be for execbuffer (IOW) we could never have NULL args.

if (cmd & (IOC_IN | IOC_OUT)) {
	if (asize <= sizeof(stack_kdata)) {
		kdata = stack_kdata;
	} else {
		kdata = kmalloc(asize, GFP_KERNEL);
		if (!kdata) {
			retcode = -ENOMEM;
			goto err_i1;
		}
	}
	if (asize > usize)
		memset(kdata + usize, 0, asize - usize);
}

Sorry if these are stupid questions.

-- 
Ben Widawsky, Intel Open Source Technology Center