Re: [PATCH] fbdev:modedb: fix a possible UAF in fb_find_mode()

[Helge Deller <deller@gmx.de>]

On 5/26/26 11:15, Tuo Li wrote:
> If mode_option is NULL, it is assigned from mode_option_buf:
> 
>    if (!mode_option) {
>      fb_get_options(NULL, &mode_option_buf);
>      mode_option = mode_option_buf;
>    }
> 
> Later, name is assigned from mode_option:
> 
>    const char *name = mode_option;
> 
> However, mode_option_buf is freed before name is no longer used:
> 
>    kfree(mode_option_buf);
> 
> while name is still accessed by:
> 
>    if ((name_matches(db[i], name, namelen) ||
> 
> Since name aliases mode_option_buf, this may result in a
> use-after-free.
> 
> Fix this by moving the kfree(mode_option_buf) call behind the access, and
> add corresponding cleanup before early returns.

I wonder if this isn't a typical good use-case for the new "Scope-based
resource management for the kernel" [1] feature.

Instead of adding kfree() at various places, we could do:

diff --git a/drivers/video/fbdev/core/modedb.c b/drivers/video/fbdev/core/modedb.c
index 703d0b7aec32..b6926764a99c 100644
--- a/drivers/video/fbdev/core/modedb.c
+++ b/drivers/video/fbdev/core/modedb.c
@@ -626,7 +626,7 @@ int fb_find_mode(struct fb_var_screeninfo *var,
                  const struct fb_videomode *default_mode,
                  unsigned int default_bpp)
  {
-       char *mode_option_buf = NULL;
+       char *mode_option_buf __free(kfree) = NULL;
         int i;
  
         /* Set up defaults */
@@ -724,7 +724,6 @@ int fb_find_mode(struct fb_var_screeninfo *var,
                         res_specified = 1;
                 }
  done:
-               kfree(mode_option_buf);
                 if (cvt) {
                         struct fb_videomode cvt_mode;
                         int ret;


[1] https://lwn.net/Articles/934679/


Do you want to check if that's correct, and if yes resend a patch?

Helge