patch "fbdev: Detect integer underflow at "struct fbcon_ops"->clear_margins." added to tty-linus

This is a note to let you know that I've just added the patch titled

    fbdev: Detect integer underflow at "struct fbcon_ops"->clear_margins.

to my tty git tree which can be found at
git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git
in the tty-linus branch.

The patch will show up in the next release of the linux-next tree (usually sometime within the next 24 hours during the week.)

The patch will hopefully also be merged in Linus's tree for the next -rc kernel release.

If you have any questions about this process, please let me know.

From 033724d6864245a11f8e04c066002e6ad22b3fd0 Mon Sep 17 00:00:00 2001
From: Tetsuo Handa <[email protected]>
Date: Wed, 15 Jul 2020 10:51:02 +0900
Subject: fbdev: Detect integer underflow at "struct fbcon_ops"->clear_margins.

syzbot is reporting general protection fault in bitfill_aligned() [1] caused by integer underflow in bit_clear_margins(). The cause of this problem is when and how do_vc_resize() updates vc->vc_{cols,rows}.

If vc_do_resize() fails (e.g. kzalloc() fails) when var.xres or var.yres is going to shrink, vc->vc_{cols,rows} will not be updated. This allows bit_clear_margins() to see info->var.xres < (vc->vc_cols * cw) or info->var.yres < (vc->vc_rows * ch). Unexpectedly large rw or bh will try to overrun the __iomem region and causes general protection fault.

Also, vc_resize(vc, 0, 0) does not set vc->vc_{cols,rows} = 0 due to

  new_cols = (cols ? cols : vc->vc_cols);
  new_rows = (lines ? lines : vc->vc_rows);

exception. Since cols and lines are calculated as

  cols = FBCON_SWAP(ops->rotate, info->var.xres, info->var.yres);
  rows = FBCON_SWAP(ops->rotate, info->var.yres, info->var.xres);
  cols /= vc->vc_font.width;
  rows /= vc->vc_font.height;
  vc_resize(vc, cols, rows);

in fbcon_modechanged(), var.xres < vc->vc_font.width makes cols = 0 and var.yres < vc->vc_font.height makes rows = 0. This means that

  const int fd = open("/dev/fb0", O_ACCMODE);
  struct fb_var_screeninfo var = { };
  ioctl(fd, FBIOGET_VSCREENINFO, &var);
  var.xres = var.yres = 1;
  ioctl(fd, FBIOPUT_VSCREENINFO, &var);

easily reproduces integer underflow bug explained above.

Of course, callers of vc_resize() are not handling vc_do_resize() failure is bad. But we can't avoid vc_resize(vc, 0, 0) which returns 0. Therefore, as a band-aid workaround, this patch checks integer underflow in "struct fbcon_ops"->clear_margins call, assuming that vc->vc_cols * vc->vc_font.width and vc->vc_rows * vc->vc_font.heigh do not cause integer overflow.

[1] https://syzkaller.appspot.com/bug?id=a565882df74fa76f10d3a6fec4be31098dbb37c6

---
 drivers/video/fbdev/core/bitblit.c   | 4 ++--
 drivers/video/fbdev/core/fbcon_ccw.c | 4 ++--
 drivers/video/fbdev/core/fbcon_cw.c  | 4 ++--
 drivers/video/fbdev/core/fbcon_ud.c  | 4 ++--
 4 files changed, 8 insertions(+), 8 deletions(-)

index ca935c09a261..35ebeeccde4d 100644
@@ -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;
 		region.dy = 0;
 		region.width = rw;

@@ -224,7 +224,7 @@ static void bit_clear_margins(struct vc_data *vc, struct fb_info *info,
 		info->fbops->fb_fillrect(info, &region);
 	}
 
-	if (bh) {
+	if ((int) bh > 0) {
 		region.dx = info->var.xoffset;
 		region.dy = info->var.yoffset + bs;
 		region.width = rs;

index dfa9a8aa4509..78f3a5621478 100644
@@ -201,7 +201,7 @@ static void ccw_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 = 0;
 		region.dy = info->var.yoffset;
 		region.height = rw;

@@ -209,7 +209,7 @@ static void ccw_clear_margins(struct vc_data *vc, struct fb_info *info,
 		info->fbops->fb_fillrect(info, &region);
 	}
 
-	if (bh) {
+	if ((int) bh > 0) {
 		region.dx = info->var.xoffset + bs;
 		region.dy = 0;
                 region.height = info->var.yres_virtual;

index ce08251bfd38..fd098ff17574 100644
@@ -184,7 +184,7 @@ static void cw_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 = 0;
 		region.dy = info->var.yoffset + rs;
 		region.height = rw;

@@ -192,7 +192,7 @@ static void cw_clear_margins(struct vc_data *vc, struct fb_info *info,
 		info->fbops->fb_fillrect(info, &region);
 	}
 
-	if (bh) {
+	if ((int) bh > 0) {
 		region.dx = info->var.xoffset;
 		region.dy = info->var.yoffset;
                 region.height = info->var.yres;

index 1936afc78fec..e165a3fad29a 100644
@@ -231,7 +231,7 @@ static void ud_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.dy = 0;
 		region.dx = info->var.xoffset;
 		region.width  = rw;

@@ -239,7 +239,7 @@ static void ud_clear_margins(struct vc_data *vc, struct fb_info *info,
 		info->fbops->fb_fillrect(info, &region);
 	}
 
-	if (bh) {
+	if ((int) bh > 0) {
 		region.dy = info->var.yoffset;
 		region.dx = info->var.xoffset;
                 region.height  = bh;

2.27.0