dlfb_setup_modes() frees info->modelist with fb_destroy_modelist() and rebuilds it, but fbcon holds raw pointers into that list in fb_display[i].mode and nothing un-publishes them the way do_unregister_framebuffer() and store_modes() do with fbcon_delete_modelist(). The function does refuse to install a new mode while the framebuffer has users (dlfb->fb_count != 0), but only after the list is already destroyed. Reached from the 0666 "edid" sysfs attribute, an unprivileged write to /sys/class/graphics/fb0/edid therefore fails with -EINVAL and still leaves every fb_display[i].mode dangling; the next console switch reads the freed fb_videomode in fb_videomode_to_var().
Test fb_count before touching the list. fbcon takes a reference through fbcon_open(), so that condition covers exactly the states in which fb_display[] points into the list, and the errno returned to userspace is unchanged. BUG: KASAN: slab-use-after-free in fb_videomode_to_var (drivers/video/fbdev/core/modedb.c:905) Read of size 4 at addr ffff8880107b449c by task kworker/1:1/47 Workqueue: events console_callback Call Trace: fb_videomode_to_var (drivers/video/fbdev/core/modedb.c:905) display_to_var (drivers/video/fbdev/core/fbcon.c:998) fbcon_switch (drivers/video/fbdev/core/fbcon.c:2182) redraw_screen (drivers/tty/vt/vt.c:994) complete_change_console (drivers/tty/vt/vt_ioctl.c:1141) console_callback (drivers/tty/vt/vt.c:3358) process_one_work (kernel/workqueue.c:3396) worker_thread (kernel/workqueue.c:3479 kernel/workqueue.c:3560) kthread (kernel/kthread.c:436) ret_from_fork (arch/x86/kernel/process.c:158) ret_from_fork_asm (arch/x86/entry/entry_64.S:245) ... The buggy address is located 28 bytes inside of freed 96-byte region [ffff8880107b4480, ffff8880107b44e0) Cc: [email protected] Fixes: 7d9485e2c53c ("Staging: udlfb: Add functions to expose sysfs metrics and controls") Reported-by: [email protected] Closes: https://lore.kernel.org/all/1jSCSNaDKmuUG7h40rTsSl1rMaSwdlJef4rp%40bugs.sh/ Assisted-by: Claude:claude-opus-5 Signed-off-by: Xiang Mei <[email protected]> --- drivers/video/fbdev/udlfb.c | 5 +++++ 1 file changed, 5 insertions(+) diff --git a/drivers/video/fbdev/udlfb.c b/drivers/video/fbdev/udlfb.c index e78d6f95c9c5..92b352bc7e96 100644 --- a/drivers/video/fbdev/udlfb.c +++ b/drivers/video/fbdev/udlfb.c @@ -1293,6 +1293,11 @@ static int dlfb_setup_modes(struct dlfb_data *dlfb, goto error; } + if (dlfb->fb_count) { + result = -EINVAL; + goto error; + } + fb_destroy_modelist(&info->modelist); memset(&info->monspecs, 0, sizeof(info->monspecs)); -- 2.43.0
