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

[PATCH] 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>
---
 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 2f2daeb..752e399 100644
--- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
@@ -747,7 +747,11 @@ 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 */
+		/*
+		 * Validate memory range. Since we may need to update the
+		 * presumed offsets, use VERIFY_WRITE. (Writable implies
+		 * readable.)
+		 */
 		if (!access_ok(VERIFY_WRITE, ptr, length))
 			return -EFAULT;
 
-- 
1.7.9.5


-- 
Kees Cook
Chrome OS Security

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

[Chris Wilson]

On Mon, Mar 11, 2013 at 12:26:30PM -0700, Kees Cook wrote:
> This clarifies the comment above the access_ok check so a missing
> VERIFY_READ doesn't alarm anyone.

Do we really need to copy the interface documentation?

/**
 * access_ok: - Checks if a user space pointer is valid
 * @type: Type of access: %VERIFY_READ or %VERIFY_WRITE.  Note that
 *        %VERIFY_WRITE is a superset of %VERIFY_READ - if it is safe
 *        to write to a block, it is always safe to read from it.
 * @addr: User space pointer to start of block to check
 * @size: Size of block to check
 */
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre

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

[Kees Cook]

On Mon, Mar 11, 2013 at 1:51 PM, Chris Wilson <chris@chris-wilson.co.uk> wrote:
> On Mon, Mar 11, 2013 at 12:26:30PM -0700, Kees Cook wrote:
>> This clarifies the comment above the access_ok check so a missing
>> VERIFY_READ doesn't alarm anyone.
>
> Do we really need to copy the interface documentation?
>
> /**
>  * access_ok: - Checks if a user space pointer is valid
>  * @type: Type of access: %VERIFY_READ or %VERIFY_WRITE.  Note that
>  *        %VERIFY_WRITE is a superset of %VERIFY_READ - if it is safe
>  *        to write to a block, it is always safe to read from it.
>  * @addr: User space pointer to start of block to check
>  * @size: Size of block to check
>  */
> -Chris

Probably not. It just seemed like the existing comment was
insufficient after the removal of the redundant VERIFY_READ check that
happened recently.

-Kees

-- 
Kees Cook
Chrome OS Security

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

[Chris Wilson]

On Mon, Mar 11, 2013 at 02:04:51PM -0700, Kees Cook wrote:
> Probably not. It just seemed like the existing comment was
> insufficient after the removal of the redundant VERIFY_READ check that
> happened recently.

That's certainly true, the remaining comment is a little cryptic.
Something more like:

/* We need to check that we can read the entire relocation array, but
 * as we may need to update the presumed_offsets upon execution, we
 * need to check now for full write access.
 */
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre