[PATCH] video: fbdev: atmel_lcdfb: fix list_first_entry() on possibly empty modelist in probe

[Helge Deller]

On 10/23/26 09:56, Li Ximo wrote:
> From: Wang Haoran <haoranwangsec at gmail.com>
> 
> list_first_entry() does not return NULL on an empty list; it returns a
> bogus pointer computed by container_of() from the list head.
> atmel_lcdfb_probe() initializes info->modelist with INIT_LIST_HEAD()
> and later takes its first entry with list_first_entry(), assuming
> atmel_lcdfb_of_init() populated it. However, atmel_lcdfb_of_init()
> drops the return value of fb_add_videomode() — unlike every other
> fallible call in the same function — so if fb_add_videomode() fails
> (its internal kmalloc can return -ENOMEM under memory pressure or
> fault injection), the list stays empty while of_init() reports
> success. The probe path then dereferences the bogus pointer in
> fb_videomode_to_var(&info->var, &modelist->mode). Because the bogus
> pointer lands inside the contiguous fb_info allocation, this may not
> fault immediately; instead it silently reads adjacent memory as a
> struct fb_videomode and programs garbage timing parameters into the
> LCD controller. This was found by code audit; we do not have Atmel
> hardware to reproduce it.
> 
> Fix this on two levels: check the return value of fb_add_videomode()
> in atmel_lcdfb_of_init() so the failure is propagated (root cause),
> and use list_first_entry_or_null() in atmel_lcdfb_probe() with a NULL
> check that fails probe gracefully with -EINVAL (defense in depth),
> consistent with the surrounding error handling which bails out to
> stop_clk.
> 
> Reported-by: Wang Haoran <haoranwangsec at gmail.com>
> Reported-by: Li Ximo <lixicaicai at gmail.com>
> Signed-off-by: Wang Haoran <haoranwangsec at gmail.com>
> ---
>   drivers/video/fbdev/atmel_lcdfb.c | 13 ++++++++++---
>   1 file changed, 10 insertions(+), 3 deletions(-)
applied.

Thanks!
Helge