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

[Dan Carpenter <dan.carpenter@oracle.com>]

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.  ;)

  2550                  }
  2551          }
  2552  
  2553          kvfree(exec_list);

regards,
dan carpenter
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