RE: [PATCH] drm/i915: Use copy_from_user() in fence copying

[David Laight <David.Laight@ACULAB.COM>]

From: Kees Cook
> Sent: 06 December 2017 20:29
>
> There's no good reason to separate the access_ok() from the copy,
> especially since the access_ok() size is hard-coded instead of using
> sizeof(). Instead, just use copy_from_user() directly.

Looks like an optimisation to save doing the access_ok() check
for every 'fence'.

OTOH 'user copy hardening' probably makes a much larger performance
degradation on this kind of code.
(Might be ok here because &fence probably refers to something in the
current stack frame.)

	David

> Fixes: cf6e7bac6357 ("drm/i915: Add support for drm syncobjs")
> Cc: Jason Ekstrand <jason@jlekstrand.net>
> Cc: Chris Wilson <chris@chris-wilson.co.uk>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  drivers/gpu/drm/i915/i915_gem_execbuffer.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> index 435ed95df144..1da703213b17 100644
> --- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> +++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> @@ -2087,8 +2087,6 @@ get_fence_array(struct drm_i915_gem_execbuffer2 *args,
>  		return ERR_PTR(-EINVAL);
> 
>  	user = u64_to_user_ptr(args->cliprects_ptr);
> -	if (!access_ok(VERIFY_READ, user, nfences * 2 * sizeof(u32)))
> -		return ERR_PTR(-EFAULT);
> 
>  	fences = kvmalloc_array(args->num_cliprects, sizeof(*fences),
>  				__GFP_NOWARN | GFP_KERNEL);
> @@ -2099,7 +2097,7 @@ get_fence_array(struct drm_i915_gem_execbuffer2 *args,
>  		struct drm_i915_gem_exec_fence fence;
>  		struct drm_syncobj *syncobj;
> 
> -		if (__copy_from_user(&fence, user++, sizeof(fence))) {
> +		if (copy_from_user(&fence, user++, sizeof(fence))) {
>  			err = -EFAULT;
>  			goto err;
>  		}
> --
> 2.7.4
> 
> 
> --
> Kees Cook
> Pixel Security