[Intel-gfx] [bug report] drm/i915: Eliminate lots of iterations over the execobjects array

[Dan Carpenter]

On Thu, Mar 15, 2018 at 01:17:23PM +0000, Chris Wilson wrote:
> Quoting Dan Carpenter (2018-03-15 13:10:30)
> > Hello Chris Wilson,
> > 
> > The patch 2889caa92321: "drm/i915: Eliminate lots of iterations over
> > the execobjects array" from Jun 16, 2017, leads to the following
> > static checker warning:
> > 
> >         drivers/gpu/drm/i915/i915_gem_execbuffer.c:2546 i915_gem_execbuffer_ioctl()
> >         warn: calling '__copy_to_user()' without access_ok()
> > 
> > drivers/gpu/drm/i915/i915_gem_execbuffer.c
> >   2510          err = copy_from_user(exec_list,
> >   2511                               u64_to_user_ptr(args->buffers_ptr),
> >   2512                               sizeof(*exec_list) * count);
> >   2513          if (err) {
> >   2514                  DRM_DEBUG("copy %d exec entries failed %d\n",
> >   2515                            args->buffer_count, err);
> >   2516                  kvfree(exec_list);
> >   2517                  kvfree(exec2_list);
> >   2518                  return -EFAULT;
> >   2519          }
> >   2520  
> >   2521          for (i = 0; i < args->buffer_count; i++) {
> >   2522                  exec2_list[i].handle = exec_list[i].handle;
> >   2523                  exec2_list[i].relocation_count = exec_list[i].relocation_count;
> >   2524                  exec2_list[i].relocs_ptr = exec_list[i].relocs_ptr;
> >   2525                  exec2_list[i].alignment = exec_list[i].alignment;
> >   2526                  exec2_list[i].offset = exec_list[i].offset;
> >   2527                  if (INTEL_GEN(to_i915(dev)) < 4)
> >   2528                          exec2_list[i].flags = EXEC_OBJECT_NEEDS_FENCE;
> >   2529                  else
> >   2530                          exec2_list[i].flags = 0;
> >   2531          }
> >   2532  
> >   2533          err = i915_gem_do_execbuffer(dev, file, &exec2, exec2_list, NULL);
> >   2534          if (exec2.flags & __EXEC_HAS_RELOC) {
> >   2535                  struct drm_i915_gem_exec_object __user *user_exec_list =
> >   2536                          u64_to_user_ptr(args->buffers_ptr);
> >   2537  
> >   2538                  /* Copy the new buffer offsets back to the user's exec list. */
> >   2539                  for (i = 0; i < args->buffer_count; i++) {
> >   2540                          if (!(exec2_list[i].offset & UPDATE))
> >   2541                                  continue;
> >   2542  
> >   2543                          exec2_list[i].offset =
> >   2544                                  gen8_canonical_addr(exec2_list[i].offset & PIN_OFFSET_MASK);
> >   2545                          exec2_list[i].offset &= PIN_OFFSET_MASK;
> >   2546                          if (__copy_to_user(&user_exec_list[i].offset,
> >   2547                                             &exec2_list[i].offset,
> >                                                     ^^^^^^^^^^^^^^^^^^^^
> >   2548                                             sizeof(user_exec_list[i].offset)))
> >   2549                                  break;
> > 
> > The story of this warning is that one day Linus was grumpy about
> > security issues and said something like, "We should make it a rule that
> > code which uses __copy_to_user() should call access_ok() in that exact
> > same function or it becomes too hard to audit and error prone.  Can
> > someone write a static checker for this?"  And so I did.  But up to now
> > I've always just looked at the code and either figured out where the
> > access_ok() is or just assumed that "Probably it's there if I looked
> > harder".
> > 
> > But today I've drawn a line in the sand!  No more!
> > 
> > Also the error code is wrong, we should return -EFAULT if the copy
> > fails.  ;)
> 
> There is an access_ok on that address earlier

Which function?

regards,
dan carpenter