On Sun, Jun 26, 2022 at 12:27:51AM +0200, Daniel Vetter wrote:

On Sat, Jun 25, 2022 at 04:53:25PM +0200, Helge Deller wrote:

quoted

On 6/25/22 14:45, Daniel Vetter wrote:

quoted

On Sat, Jun 25, 2022 at 02:24:59PM +0200, Helge Deller wrote:

quoted

Prevent that users set a font size which is bigger than the physical screen.
It's unlikely this may happen (because screens are usually much larger than the
fonts and each font char is limited to 32x32 pixels), but it may happen on
smaller screens/LCD displays.

Signed-off-by: Helge Deller <deller@gmx.de>
Cc: stable@vger.kernel.org # v4.14+
---
 drivers/video/fbdev/core/fbcon.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
index c4e91715ef00..e162d5e753e5 100644
--- a/drivers/video/fbdev/core/fbcon.c
+++ b/drivers/video/fbdev/core/fbcon.c

@@ -2469,6 +2469,11 @@ static int fbcon_set_font(struct vc_data *vc, struct console_font *font,
 	if (charcount != 256 && charcount != 512)
 		return -EINVAL;

+	/* font bigger than screen resolution ? */
+	if (font->width  > FBCON_SWAP(info->var.rotate, info->var.xres, info->var.yres) ||
+	    font->height > FBCON_SWAP(info->var.rotate, info->var.yres, info->var.xres))
+		return -EINVAL;

Reviewed-by: Daniel Vetter <redacted>

Thanks!

quoted

Maybe as a safety follow up patch, we have a few copies of the below:

	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;

Might be good to extract that into a helper and also add

	WARN_ON(!cols);
	WARN_ON(!rows);

That's not needed then.
The checks I added above will ensure that cols and rows are both greater than 0.

Yeah I reviewed it too, but I don't trust review all over the place.
Especially with something like fbcon with entry points from everywhere.

Also the other motivation is that a bit of common code extraction refactor
for non-trivial math like the above is simply the right thing to do.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch