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@xxxxxxxxxxxxxx>
> Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
> 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
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