fbdev: vfb: defer cleanup until the last reference

FBIOGETCMAP takes a shallow snapshot of info->cmap and performs the
usercopy after dropping info->lock. vfb_remove() frees the colormap
immediately after unregistering the framebuffer, even when an open file
still holds a reference to fb_info. A concurrent driver unbind can
therefore free the colormap while the ioctl copies it to userspace.

KASAN reports:

  BUG: KASAN: slab-use-after-free in _copy_to_user
  Read of size 512 by task poc/125

   _copy_to_user (./include/linux/instrumented.h:129 ./include/linux/uaccess.h:201 lib/usercopy.c:24)
   fb_cmap_to_user (./include/linux/uaccess.h:230 drivers/video/fbdev/core/fbcmap.c:211)
   do_fb_ioctl (drivers/video/fbdev/core/fb_chrdev.c:114)

   Allocated by task 1:
   fb_alloc_cmap_gfp (./include/linux/slab.h:973 ./include/linux/slab.h:1290 drivers/video/fbdev/core/fbcmap.c:108)
   vfb_probe (drivers/video/fbdev/vfb.c:459)

   Freed by task 124:
   fb_dealloc_cmap (drivers/video/fbdev/core/fbcmap.c:151)
   vfb_remove (drivers/video/fbdev/vfb.c:489)

unregister_framebuffer() drops the registration reference, and fbdev calls
fb_destroy after the last put_fb_info(). Move the registered framebuffer's
cleanup into an fb_destroy callback so its colormap and screen buffer stay
alive until all file references have been released.

Fixes: 5e266e2e0e19 ("vfb: fix memory leaks in removal path")
Reported-by: co+c25629c98ba36ebe@bugs.sh
Cc: stable@kernel.org
Closes: https://lore.kernel.org/linux-fbdev/f2Kf9GYn1lKR5S1dbvGVtykMxK1RlgP5z8sW@bugs.sh/
Assisted-by: Codex:gpt-5
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
Link: https://lore.kernel.org/linux-fbdev/f2Kf9GYn1lKR5S1dbvGVtykMxK1RlgP5z8sW@bugs.sh/
Signed-off-by: Helge Deller <deller@gmx.de>

0 files changed, 0 insertions, 0 deletions