This was found by Alexandra Kossovsky, who noted this traceback from
kmemleak:

quoted

unreferenced object 0xffff880216fcfe00 (size 512):
  comm "swapper/0", pid 1, jiffies 4294895429 (age 1415.320s)
  hex dump (first 32 bytes):
    00 00 00 00 00 00 00 00 aa aa aa aa aa aa aa aa  ................
    55 55 55 55 55 55 55 55 ff ff ff ff ff ff ff ff  UUUUUUUU........
  backtrace:
    [<ffffffff813e415c>] kmemleak_alloc+0x21/0x3e
    [<ffffffff8111c17f>]
    kmemleak_alloc_recursive.constprop.57+0x16/0x18
    [<ffffffff8111e63b>] __kmalloc+0xf9/0x144
    [<ffffffff8123d9cf>] fb_alloc_cmap_gfp+0x47/0xe1
    [<ffffffff8123da77>] fb_alloc_cmap+0xe/0x10
    [<ffffffff81aff40a>] efifb_probe+0x3e9/0x48f
    [<ffffffff812c566f>] platform_drv_probe+0x34/0x5e
    [<ffffffff812c3e6d>] driver_probe_device+0x98/0x1b4
    [<ffffffff812c3fd7>] __driver_attach+0x4e/0x6f
    [<ffffffff812c25bf>] bus_for_each_dev+0x57/0x8a
    [<ffffffff812c3984>] driver_attach+0x19/0x1b
    [<ffffffff812c362b>] bus_add_driver+0xde/0x201
    [<ffffffff812c453f>] driver_register+0x8c/0x110
    [<ffffffff812c510d>] platform_driver_register+0x41/0x43
    [<ffffffff812c5127>] platform_driver_probe+0x18/0x8a
    [<ffffffff81aff002>] efifb_init+0x276/0x295

---
 drivers/video/efifb.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/video/efifb.c b/drivers/video/efifb.c
index 390b61b..1f3eab3 100644
--- a/drivers/video/efifb.c
+++ b/drivers/video/efifb.c

@@ -289,6 +289,7 @@ static void efifb_destroy(struct fb_info *info)
 	if (request_mem_succeeded)
 		release_mem_region(info->apertures->ranges[0].base,
 				   info->apertures->ranges[0].size);
+	fb_dealloc_cmap(&info->cmap);
 	framebuffer_release(info);
 }