drm/i915: Force the slow path after a user-write error

If we fail to write the user relocation back when it is changed, force
ourselves to take the slow relocation path where we can handle faults in
the write path. There is still an element of dubiousness as having
patched up the batch to use the correct offset, it no longer matches the
presumed_offset in the relocation, so a second pass may miss any changes
in layout.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180903083337.13134-3-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem_execbuffer.c

@@ -1491,8 +1491,10 @@ static int eb_relocate_vma(struct i915_execbuffer *eb, struct i915_vma *vma)
 				 * can read from this userspace address.
 				 */
 				offset = gen8_canonical_addr(offset & ~UPDATE);
-				__put_user(offset,
-					   &urelocs[r-stack].presumed_offset);
+				if (unlikely(__put_user(offset, &urelocs[r-stack].presumed_offset))) {
+					remain = -EFAULT;
+					goto out;
+				}
 			}
 		} while (r++, --count);
 		urelocs += ARRAY_SIZE(stack);
@@ -1577,7 +1579,6 @@ static int eb_copy_relocations(const struct i915_execbuffer *eb)
 
 		relocs = kvmalloc_array(size, 1, GFP_KERNEL);
 		if (!relocs) {
-			kvfree(relocs);
 			err = -ENOMEM;
 			goto err;
 		}
@@ -1591,6 +1592,7 @@ static int eb_copy_relocations(const struct i915_execbuffer *eb)
 			if (__copy_from_user((char *)relocs + copied,
 					     (char __user *)urelocs + copied,
 					     len)) {
+end_user:
 				kvfree(relocs);
 				err = -EFAULT;
 				goto err;
@@ -1614,7 +1616,6 @@ static int eb_copy_relocations(const struct i915_execbuffer *eb)
 			unsafe_put_user(-1,
 					&urelocs[copied].presumed_offset,
 					end_user);
-end_user:
 		user_access_end();
 
 		eb->exec[i].relocs_ptr = (uintptr_t)relocs;