Re: [PATCH v2] 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¥65882df74fa76f10d3a6fec4be31098dbb37c6

Reported-and-tested-by: syzbot <redacted>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
---
 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(-)

diff --git a/drivers/video/fbdev/core/bitblit.c b/drivers/video/fbdev/core/bitblit.c
index ca935c09a261..35ebeeccde4d 100644
--- a/drivers/video/fbdev/core/bitblit.c
+++ b/drivers/video/fbdev/core/bitblit.c

@@ -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;