[PATCH v2] fbdev:modedb: fix a possible UAF in fb_find_mode()

[Tuo Li <islituo@gmail.com>]

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 extending the lifetime of mode_option_buf until the end of the 
function and using scope-based resource management for cleanup.

Signed-off-by: Tuo Li <islituo@gmail.com>
---
v2:
* Use scope-based resource management instead of manual kfree() calls.
  Thanks to Helge Deller for the helpful advice.
---
 drivers/video/fbdev/core/modedb.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

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;
-- 
2.43.0