Re: [syzbot] KASAN: vmalloc-out-of-bounds Write in imageblit (2)

[Helge Deller]

* Khalid Masum <khalid.masum.92@xxxxxxxxx>:
> On 7/30/22 23:25, Helge Deller wrote:
> > On 7/29/22 08:51, Khalid Masum wrote:
> > > Here is a simplified reproducer for the issue:
> > >
> > > https://gist.githubusercontent.com/Labnann/923d6b9b3a19848fc129637b839b8a55/raw/a68271fcc724569735fe27f80817e561b3ff629a/reproducer.c
> >
> > The reproducer does this:
>
> Thanks for Looking into this. Being to this, so I have some questions.
> > ioctl(3, TIOCLINUX, TIOCL_SETSEL, selection: xs:3  ys:0  xe:0 ye:0 mode:0)  = 0
>
> How did you find out the selection values? From strace and man pages I know
> the third argument is an address.

Right. It's a pointer into userspace.
I simply added printk debug code to see what's happening.
I've attached that patch below.


> > -> sets the text selection area
> > ioctl(4, KDFONTOP)  with op=0 (con_font_set), charcount=512  width=8  height=32, 0x20000000) = 0
>
> Same here, It would be very helpful if you could tell me how.

See patch below.

> > -> changes the font size.
> >
> > It does not crash with current Linus' head (v5.19-rc8).
>
> I tested in 5.19-rc8 in Qemu x86_64 and it crashed for me.

That's strange, since I tested the same. Maybe I did something wrong.
Anyway, the patches I sent applies to all kernel versions.

> > Kernel v5.16, which was used by this KASAN report, hasn't received backports
> > since months, so I tried stable kernel v5.15.58 instead, and this
> > kernel crashed with the reproducer.
> >
> > The reproducer brings up two issues with current code:
> > 1. The reproducer uses ioctl(TIOCLINUX, TIOCL_SETSEL) and hands over (invalid)
> > zero-values for ys and ye for the starting lines.
> > This is wrong, since the API seems to expect a "1" as the very first line for the selection.
> > This can be easily fixed by adding checks for zero-values and return -EINVAL if found.
> >
> > But this bug isn't critical itself and is not the reason for the kernel crash.
> > Without the checks, the ioctl handler simply wraps the coordinate values and converts them
> > from:
> > input selection: xs:3  ys:0  xe:0   ye:0  mode:0    to the new:
> > vc_selection =   xs:2  ys:23 xe:127 ye:23 mode:0
> > which is the current maximum coordinates for the screen.
> >
> > Those higher values now trigger issue #2:
> > After the TIOCL_SETSEL the last line on the screen is now selected. The KDFONTOP ioctl
> > then sets a 8x32 console font, and replaces the former 8x16 console font.
> > With the bigger font the current screen selection is now outside the visible screen
> > and this finally triggeres this backtrace, because vc_do_resize() calls clear_selection()
> > to unhighlight the selection (which starts to render chars outside of the screen):
>
> That makes sense.
>
> >   drm_fb_helper_sys_imageblit drivers/gpu/drm/drm_fb_helper.c:794 [inline]
> >   drm_fbdev_fb_imageblit+0x15c/0x350 drivers/gpu/drm/drm_fb_helper.c:2288
> >   bit_putcs_unaligned drivers/video/fbdev/core/bitblit.c:124 [inline]
> >   bit_putcs+0x6e1/0xd20 drivers/video/fbdev/core/bitblit.c:173
> >   fbcon_putcs+0x353/0x440 drivers/video/fbdev/core/fbcon.c:1277
> >   do_update_region+0x399/0x630 drivers/tty/vt/vt.c:676
> >   invert_screen+0x1d4/0x600 drivers/tty/vt/vt.c:800
> >   highlight drivers/tty/vt/selection.c:57 [inline]
> >   clear_selection drivers/tty/vt/selection.c:84 [inline]
> >   clear_selection+0x55/0x70 drivers/tty/vt/selection.c:80
> >   vc_do_resize+0xe6e/0x1180 drivers/tty/vt/vt.c:1257
> >
> > IMHO the easiest way to prevent this crash is to simply clear the
> > selection before the various con_font_set() console handlers are called.
> > Otherwise every console driver needs to add checks and verify if the current
> > selection still fits with the selected font, which gets tricky because some
> > of those drivers fiddle with the screen width&height before calling vc_do_resize().
> >
> > I'll follow up to this mail with patches for both issues shortly.
>
> I tested the patches. The crash no longer occurs with the reproducer.

Thanks for testing!
Maybe you want to reply to the patches with a Tested-by: tag?

Below is my debug code.

Helge


diff --git a/drivers/tty/vt/selection.c b/drivers/tty/vt/selection.c
index 58692a9b4097..0167b368a70f 100644
--- a/drivers/tty/vt/selection.c
+++ b/drivers/tty/vt/selection.c
@@ -333,6 +333,7 @@ static int vc_selection(struct vc_data *vc, struct tiocl_selection *v,
 	v->ys = min_t(u16, v->ys - 1, vc->vc_rows - 1);
 	v->xe = min_t(u16, v->xe - 1, vc->vc_cols - 1);
 	v->ye = min_t(u16, v->ye - 1, vc->vc_rows - 1);
+	printk("vc_selection =   xs:%u  ys:%u  xe:%u ye:%u mode:%u\n", v->xs, v->ys, v->xe, v->ye, v->sel_mode);

 	if (mouse_reporting() && (v->sel_mode & TIOCL_SELMOUSEREPORT)) {
 		mouse_report(tty, v->sel_mode & TIOCL_SELBUTTONMASK, v->xs,
@@ -357,6 +358,7 @@ int set_selection_kernel(struct tiocl_selection *v, struct tty_struct *tty)
 {
 	int ret;

+	printk("tiocl_selection =   xs:%u  ys:%u  xe:%u ye:%u mode:%u\n", v->xs, v->ys, v->xe, v->ye, v->sel_mode);
 	mutex_lock(&vc_sel.lock);
 	console_lock();
 	ret = vc_selection(vc_cons[fg_console].d, v, tty);
diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index 3f09205185a4..a0b4570c959a 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -3194,6 +3194,8 @@ int tioclinux(struct tty_struct *tty, unsigned long arg)
 		return -EFAULT;
 	ret = 0;

+	printk("tioclinux: type = %d\n", type);  // TIOCL_SETSEL
+
 	switch (type)
 	{
 		case TIOCL_SETSEL:
@@ -4655,6 +4657,8 @@ static int con_font_set(struct vc_data *vc, struct console_font_op *op)
 	if (IS_ERR(font.data))
 		return PTR_ERR(font.data);

+	pr_err("con_font_set   charcount %d   w:%d  h:%d\n", op->charcount, op->width, op->height);
+
 	font.charcount = op->charcount;
 	font.width = op->width;
 	font.height = op->height;
@@ -4709,6 +4713,7 @@ static int con_font_default(struct vc_data *vc, struct console_font_op *op)

 int con_font_op(struct vc_data *vc, struct console_font_op *op)
 {
+	pr_warn("con_font_op  op = %d\n", op->op);
 	switch (op->op) {
 	case KD_FONT_OP_SET:
 		return con_font_set(vc, op);