On Fri, Dec 8, 2017 at 2:17 AM, David Laight David.Laight@aculab.com wrote:

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'.

If it really makes a difference, okay, but access_ok() checks are fast. :P

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.)

Well, the good news there is that it's using sizeof(fence), so no hardening check is done (it's not a size that changes at runtime). What I didn't like is that the access_ok() doesn't use sizeof(fence) (it is currently correct: 2 u32s == sizeof(fence)) but that kind of fragility keeps me up at night. ;)

So, fixing either would be fine, but if we're going to touch it, it seems best to just do away with the __copy_*() usage instead.

-Kees

    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