drm/i915/gem: Disallow creating contexts with too many engines

There's no sense in allowing userspace to create more engines than it
can possibly access via execbuf.
Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210708154835.528166-13-jason@jlekstrand.net

drivers/gpu/drm/i915/gem/i915_gem_context.c

@@ -1639,11 +1639,11 @@ set_engines(struct i915_gem_context *ctx,
 		return -EINVAL;
 	}
 
-	/*
-	 * Note that I915_EXEC_RING_MASK limits execbuf to only using the
-	 * first 64 engines defined here.
-	 */
 	num_engines = (args->size - sizeof(*user)) / sizeof(*user->engines);
+	/* RING_MASK has no shift so we can use it directly here */
+	if (num_engines > I915_EXEC_RING_MASK + 1)
+		return -EINVAL;
+
 	set.engines = alloc_engines(num_engines);
 	if (!set.engines)
 		return -ENOMEM;