Re: [PATCH] fbdev: vfb: defer cleanup until the last reference

[Helge Deller <deller@gmx.de>]

On 9/9/26 21:10, Weiming Shi wrote:
> 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
> Closes: https://lore.kernel.org/linux-fbdev/f2Kf9GYn1lKR5S1dbvGVtykMxK1RlgP5z8sW@bugs.sh/
> Assisted-by: Codex:gpt-5
> Signed-off-by: Weiming Shi <bestswngs@gmail.com>
> ---
>   drivers/video/fbdev/vfb.c | 11 ++++++++---
>   1 file changed, 8 insertions(+), 3 deletions(-)
applied.

Thanks!
Helge