[PATCH 4.4 31/31] tty: vt: always invoke vc->vc_sw->con_resize callback

Greg Kroah-Hartman
2021-05-24T15:35:23Z

From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>

commit ffb324e6f874121f7dce5bdae5e05d02baae7269 upstream.

syzbot is reporting OOB write at vga16fb_imageblit() [1], for
resize_screen() from ioctl(VT_RESIZE) returns 0 without checking whether
requested rows/columns fit the amount of memory reserved for the graphical
screen if current mode is KD_GRAPHICS.

----------
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <linux/kd.h>
#include <linux/vt.h>

int main(int argc, char *argv[])
{
const int fd = open("/dev/char/4:1", O_RDWR);
struct vt_sizes vt = { 0x4100, 2 };

ioctl(fd, KDSETMODE, KD_GRAPHICS);
ioctl(fd, VT_RESIZE, &vt);
ioctl(fd, KDSETMODE, KD_TEXT);
return 0;
}
----------

Allow framebuffer drivers to return -EINVAL, by moving vc->vc_mode !=
KD_GRAPHICS check from resize_screen() to fbcon_resize().

Link: https://syzkaller.appspot.com/bug?extid=1f29e126cf461c4de3b3 [1]
Reported-by: syzbot <syzbot+1f29e126cf461c4de3b3@syzkaller.appspotmail.com>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Tested-by: syzbot <syzbot+1f29e126cf461c4de3b3@syzkaller.appspotmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/tty/vt/vt.c | 2 +-
drivers/video/console/fbcon.c | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)

--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -826,7 +826,7 @@ static inline int resize_screen(struct v
/* Resizes the resolution of the display adapater */
int err = 0;

- if (vc->vc_mode != KD_GRAPHICS && vc->vc_sw->con_resize)
+ if (vc->vc_sw->con_resize)
err = vc->vc_sw->con_resize(vc, width, height, user);

return err;
--- a/drivers/video/console/fbcon.c
+++ b/drivers/video/console/fbcon.c
@@ -1986,7 +1986,7 @@ static int fbcon_resize(struct vc_data *
return -EINVAL;

DPRINTK("resize now %ix%i\n", var.xres, var.yres);
- if (CON_IS_VISIBLE(vc)) {
+ if (CON_IS_VISIBLE(vc) && vc->vc_mode == KD_TEXT) {
var.activate = FB_ACTIVATE_NOW |
FB_ACTIVATE_FORCE;
fb_set_var(info, &var);