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Message-ID: <20200715151220.GE2571@kadam>
Date: Wed, 15 Jul 2020 18:12:20 +0300
From: Dan Carpenter <dan.carpenter@...cle.com>
To: Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Jiri Slaby <jslaby@...e.com>,
Dmitry Vyukov <dvyukov@...gle.com>,
George Kennedy <george.kennedy@...cle.com>,
dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org,
linux-kernel@...r.kernel.org,
syzbot <syzbot+e5fd3e65515b48c02a30@...kaller.appspotmail.com>
Subject: Re: [PATCH v2] fbdev: Detect integer underflow at "struct
fbcon_ops"->clear_margins.
On Wed, Jul 15, 2020 at 11:02:58PM +0900, Tetsuo Handa wrote:
> On 2020/07/15 20:17, Tetsuo Handa wrote:
> > On 2020/07/15 18:48, Dan Carpenter wrote:
> >>> @@ -216,7 +216,7 @@ static void bit_clear_margins(struct vc_data *vc, struct fb_info *info,
> >>> region.color = color;
> >>> region.rop = ROP_COPY;
> >>>
> >>> - if (rw && !bottom_only) {
> >>> + if ((int) rw > 0 && !bottom_only) {
> >>> region.dx = info->var.xoffset + rs;
> >> ^^^^^^^^^^^^^^^^^^^^^^
> >>
> >> If you choose a very high positive "rw" then this addition can overflow.
> >> info->var.xoffset comes from the user and I don't think it's checked...
> >
> > Well, I think it would be checked by "struct fb_ops"->check_var hook.
> > For example, vmw_fb_check_var() has
> >
> > if ((var->xoffset + var->xres) > par->max_width ||
> > (var->yoffset + var->yres) > par->max_height) {
> > DRM_ERROR("Requested geom can not fit in framebuffer\n");
> > return -EINVAL;
> > }
> >
> > check. Of course, there might be integer overflow in that check...
> > Having sanity check at caller of "struct fb_ops"->check_var might be nice.
> >
>
> Well, while
>
> const int fd = open("/dev/fb0", O_ACCMODE);
> struct fb_var_screeninfo var = { };
> ioctl(fd, FBIOGET_VSCREENINFO, &var);
> var.xres = var.yres = 4;
> var.xoffset = 4294967292U;
> ioctl(fd, FBIOPUT_VSCREENINFO, &var);
>
> bypassed
>
> (var->xoffset + var->xres) > par->max_width
>
> check in vmw_fb_check_var(),
>
> ----------
> --- a/drivers/video/fbdev/core/bitblit.c
> +++ b/drivers/video/fbdev/core/bitblit.c
> @@ -216,6 +216,7 @@ static void bit_clear_margins(struct vc_data *vc, struct fb_info *info,
> region.color = color;
> region.rop = ROP_COPY;
>
> + printk(KERN_INFO "%s info->var.xoffset=%u rs=%u info->var.yoffset=%u bs=%u\n", __func__, info->var.xoffset, rs, info->var.yoffset, bs);
> if ((int) rw > 0 && !bottom_only) {
> region.dx = info->var.xoffset + rs;
> region.dy = 0;
> ----------
>
> says that info->var.xoffset does not come from the user.
>
> ----------
> bit_clear_margins info->var.xoffset=0 rs=1024 info->var.yoffset=0 bs=800
> ----------
In fb_set_var() we do:
drivers/video/fbdev/core/fbmem.c
1055 ret = info->fbops->fb_check_var(var, info);
1056
1057 if (ret)
1058 return ret;
1059
1060 if ((var->activate & FB_ACTIVATE_MASK) != FB_ACTIVATE_NOW)
1061 return 0;
1062
1063 if (!basic_checks(var))
1064 return -EINVAL;
1065
1066 if (info->fbops->fb_get_caps) {
1067 ret = fb_check_caps(info, var, var->activate);
1068
1069 if (ret)
1070 return ret;
1071 }
1072
1073 old_var = info->var;
1074 info->var = *var;
^^^^^^^^^^^^^^^^
This should set "info->var.offset".
1075
1076 if (info->fbops->fb_set_par) {
1077 ret = info->fbops->fb_set_par(info);
1078
1079 if (ret) {
1080 info->var = old_var;
1081 printk(KERN_WARNING "detected "
I've complained about integer overflows in fbdev for a long time...
What I'd like to see is something like the following maybe. I don't
know how to get the vc_data in fbmem.c so it doesn't include your checks
for negative.
diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
index caf817bcb05c..5c74181fea5d 100644
--- a/drivers/video/fbdev/core/fbmem.c
+++ b/drivers/video/fbdev/core/fbmem.c
@@ -934,6 +934,54 @@ fb_pan_display(struct fb_info *info, struct fb_var_screeninfo *var)
}
EXPORT_SYMBOL(fb_pan_display);
+static bool basic_checks(struct fb_var_screeninfo *var)
+{
+ unsigned int v_margins, h_margins;
+
+ /* I think var->height and var->width == UINT_MAX means something. */
+
+ if (var->xres > INT_MAX ||
+ var->yres > INT_MAX ||
+ var->xres_virtual > INT_MAX ||
+ var->yres_virtual > INT_MAX ||
+ var->xoffset > INT_MAX ||
+ var->yoffset > INT_MAX ||
+ var->left_margin > INT_MAX ||
+ var->right_margin > INT_MAX ||
+ var->upper_margin > INT_MAX ||
+ var->lower_margin > INT_MAX ||
+ var->hsync_len > INT_MAX ||
+ var->vsync_len > INT_MAX)
+ return false;
+
+ if (var->bits_per_pixel > 128)
+ return false;
+ if (var->rotate > FB_ROTATE_CCW)
+ return false;
+
+ if (var->xoffset > INT_MAX - var->xres)
+ return false;
+ if (var->yoffset > INT_MAX - var->yres)
+ return false;
+
+ if (var->left_margin > INT_MAX - var->right_margin ||
+ var->upper_margin > INT_MAX - var->lower_margin)
+ return false;
+
+ v_margins = var->left_margin + var->right_margin;
+ h_margins = var->upper_margin + var->lower_margin;
+
+ if (var->xres > INT_MAX - var->hsync_len ||
+ var->yres > INT_MAX - var->vsync_len)
+ return false;
+
+ if (v_margins > INT_MAX - var->hsync_len - var->xres ||
+ h_margins > INT_MAX - var->vsync_len - var->yres)
+ return false;
+
+ return true;
+}
+
static int fb_check_caps(struct fb_info *info, struct fb_var_screeninfo *var,
u32 activate)
{
@@ -1012,6 +1060,9 @@ fb_set_var(struct fb_info *info, struct fb_var_screeninfo *var)
if ((var->activate & FB_ACTIVATE_MASK) != FB_ACTIVATE_NOW)
return 0;
+ if (!basic_checks(var))
+ return -EINVAL;
+
if (info->fbops->fb_get_caps) {
ret = fb_check_caps(info, var, var->activate);
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