[PATCH v2] drm/i915: clarify reasoning for the access_ok call

[PATCH v2] drm/i915: clarify reasoning for the access_ok call

[Kees Cook]

This clarifies the comment above the access_ok check so a missing
VERIFY_READ doesn't alarm anyone.

Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
---
v2:
 - rewrote comment, thanks to Chris Wilson
---
 drivers/gpu/drm/i915/i915_gem_execbuffer.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
index bf7ceca..89c4039 100644
--- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
@@ -751,7 +751,11 @@ validate_exec_list(struct drm_i915_gem_exec_object2 *exec,
 
 		length = exec[i].relocation_count *
 			sizeof(struct drm_i915_gem_relocation_entry);
-		/* we may also need to update the presumed offsets */
+		/*
+		 * We must check that the entire relocation array is safe
+		 * to read, but since we may need to update the presumed
+		 * offsets during execution, check for full write access.
+		 */
 		if (!access_ok(VERIFY_WRITE, ptr, length))
 			return -EFAULT;
 
-- 
1.7.9.5


-- 
Kees Cook
Chrome OS Security

Re: [PATCH v2] drm/i915: clarify reasoning for the access_ok call

[Daniel Vetter]

On Mon, Mar 11, 2013 at 02:37:35PM -0700, Kees Cook wrote:
> This clarifies the comment above the access_ok check so a missing
> VERIFY_READ doesn't alarm anyone.
> 
> Signed-off-by: Kees Cook <keescook@chromium.org>
> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> ---
> v2:
>  - rewrote comment, thanks to Chris Wilson

Queued for -next, thanks for the patch. Fyi I prefer the patch changelog
in the actual commit message so that it gets recorded in git. Usually it's
not that interesting, but sometimes knowning the history of a patch is
really important. I've fixed this while applying.
-Daniel
> ---
>  drivers/gpu/drm/i915/i915_gem_execbuffer.c |    6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> index bf7ceca..89c4039 100644
> --- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> +++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> @@ -751,7 +751,11 @@ validate_exec_list(struct drm_i915_gem_exec_object2 *exec,
>  
>  		length = exec[i].relocation_count *
>  			sizeof(struct drm_i915_gem_relocation_entry);
> -		/* we may also need to update the presumed offsets */
> +		/*
> +		 * We must check that the entire relocation array is safe
> +		 * to read, but since we may need to update the presumed
> +		 * offsets during execution, check for full write access.
> +		 */
>  		if (!access_ok(VERIFY_WRITE, ptr, length))
>  			return -EFAULT;
>  
> -- 
> 1.7.9.5
> 
> 
> -- 
> Kees Cook
> Chrome OS Security

-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch