[PATCH v3] fbcon: Fix a NULL pointer dereference issue in fbcon_putcs

[Qianqiang Liu]

syzbot has found a NULL pointer dereference bug in fbcon.
Here is the simplified C reproducer:

struct param {
	uint8_t type;
	struct tiocl_selection ts;
};

int main()
{
	struct fb_con2fbmap con2fb;
	struct param param;

	int fd = open("/dev/fb1", 0, 0);

	con2fb.console = 0x19;
	con2fb.framebuffer = 0;
	ioctl(fd, FBIOPUT_CON2FBMAP, &con2fb);

	param.type = 2;
	param.ts.xs = 0; param.ts.ys = 0;
	param.ts.xe = 0; param.ts.ye = 0;
	param.ts.sel_mode = 0;

	int fd1 = open("/dev/tty1", O_RDWR, 0);
	ioctl(fd1, TIOCLINUX, &param);

	con2fb.console = 1;
	con2fb.framebuffer = 0;
	ioctl(fd, FBIOPUT_CON2FBMAP, &con2fb);

	return 0;
}

After calling ioctl(fd1, TIOCLINUX, &param), the subsequent ioctl(fd, FBIOPUT_CON2FBMAP, &con2fb)
causes the kernel to follow a different execution path:

 set_con2fb_map
  -> con2fb_init_display
   -> fbcon_set_disp
    -> redraw_screen
     -> hide_cursor
      -> clear_selection
       -> highlight
        -> invert_screen
         -> do_update_region
          -> fbcon_putcs
           -> ops->putcs

Since ops->putcs is a NULL pointer, this leads to a kernel panic.
To prevent this, we need to call set_blitting_type() within set_con2fb_map()
to properly initialize ops->putcs.

Reported-by: syzbot+3d613ae53c031502687a at syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=3d613ae53c031502687a
Tested-by: syzbot+3d613ae53c031502687a at syzkaller.appspotmail.com
Signed-off-by: Qianqiang Liu <qianqiang.liu at 163.com>
---
 Changes since v2:
 - Document the commit message in more detail
---
 Changes since v1:
 - Initialize ops->putcs by calling set_blitting_type()
---
 drivers/video/fbdev/core/fbcon.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
index 2e093535884b..d9abae2516d8 100644
--- a/drivers/video/fbdev/core/fbcon.c
+++ b/drivers/video/fbdev/core/fbcon.c
@@ -861,6 +861,8 @@ static int set_con2fb_map(int unit, int newidx, int user)
 			return err;
 
 		fbcon_add_cursor_work(info);
+	} else if (vc) {
+		set_blitting_type(vc, info);
 	}
 
 	con2fb_map[unit] = newidx;
-- 
2.34.1