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

[Weiming Shi <bestswngs@gmail.com>]

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(-)

diff --git a/drivers/video/fbdev/vfb.c b/drivers/video/fbdev/vfb.c
index 5b7965f36c5ed..de137b2bdaedc 100644
--- a/drivers/video/fbdev/vfb.c
+++ b/drivers/video/fbdev/vfb.c
@@ -78,6 +78,13 @@ static int vfb_pan_display(struct fb_var_screeninfo *var,
 static int vfb_mmap(struct fb_info *info,
 		    struct vm_area_struct *vma);
 
+static void vfb_destroy(struct fb_info *info)
+{
+	vfree(info->screen_buffer);
+	fb_dealloc_cmap(&info->cmap);
+	framebuffer_release(info);
+}
+
 static const struct fb_ops vfb_ops = {
 	.owner		= THIS_MODULE,
 	__FB_DEFAULT_SYSMEM_OPS_RDWR,
@@ -87,6 +94,7 @@ static const struct fb_ops vfb_ops = {
 	.fb_pan_display	= vfb_pan_display,
 	__FB_DEFAULT_SYSMEM_OPS_DRAW,
 	.fb_mmap	= vfb_mmap,
+	.fb_destroy	= vfb_destroy,
 };
 
     /*
@@ -485,9 +493,6 @@ static void vfb_remove(struct platform_device *dev)
 
 	if (info) {
 		unregister_framebuffer(info);
-		vfree(videomemory);
-		fb_dealloc_cmap(&info->cmap);
-		framebuffer_release(info);
 	}
 }
 
-- 
2.55.0