On 2021/05/15 1:19, Tetsuo Handa wrote:
> Even if it turns out to be safe to always call this
> callback, we will need to involve another callback via "struct fb_ops" for
> checking the upper limits from fbcon_resize(). As a result, we will need
> to modify
>
> drivers/tty/vt/vt.c
> drivers/video/fbdev/core/fbcon.c
> drivers/video/fbdev/vga16fb.c
> include/linux/fb.h
>
> files only for checking rows/columns values passed to ioctl(VT_RESIZE)
> request.
I was by error assuming that fbcon_resize() cannot reject bogus rows/columns
and thus we need to add another callback via "struct fb_ops" for that purpose.
But fbcon_resize() does reject bogus rows/columns; it was simply because
resize_screen() did not call fbcon_resize() if vc->vc_mode == KD_GRAPHICS.
Thus, removing vc->vc_mode check alone is sufficient.
On 2021/05/15 6:10, Linus Torvalds wrote:
> So I think just removing the "vc->vc_mode != KD_GRAPHICS" test from
> resize_screen() might be the way to go. That way, the low-level data
> structures actually are in sync with the resize, and the "out of
> bounds" bug should never happen.
>
> Would you mind testing that?
OK. Your suggested changes passed the test by me and by syzbot.
>From e5e326c90c5b919c6aba30072d665a00b18715a5 Mon Sep 17 00:00:00 2001
From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Date: Sat, 15 May 2021 03:00:37 +0000
Subject: [PATCH v2] tty: vt: always invoke vc->vc_sw->con_resize callback
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().
[1] https://syzkaller.appspot.com/bug?extid=1f29e126cf461c4de3b3
Reported-by: syzbot <syzbot+1f29e126cf461c4de3b3@xxxxxxxxxxxxxxxxxxxxxxxxx>
Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Tested-by: syzbot <syzbot+1f29e126cf461c4de3b3@xxxxxxxxxxxxxxxxxxxxxxxxx>
---
drivers/tty/vt/vt.c | 2 +-
drivers/video/fbdev/core/fbcon.c | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)
diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index 01645e87b3d5..fa1548d4f94b 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -1171,7 +1171,7 @@ static inline int resize_screen(struct vc_data *vc, int width, int height,
/* 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;
diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
index 3406067985b1..22bb3892f6bd 100644
--- a/drivers/video/fbdev/core/fbcon.c
+++ b/drivers/video/fbdev/core/fbcon.c
@@ -2019,7 +2019,7 @@ static int fbcon_resize(struct vc_data *vc, unsigned int width,
return -EINVAL;
pr_debug("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);
--
2.25.1
