fbcon: Fix a NULL pointer dereference issue in fbcon_putcs

In the Linux kernel, the following vulnerability has been resolved:

fbcon: Fix a NULL pointer dereference issue in fbcon_putcs

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.