linux-kernel - [PATCH] fbcon: Check font dimension limits

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Message-ID: <20230126004921.616264824@ens-lyon.org>
Date:   Thu, 26 Jan 2023 01:49:12 +0100
From:   Samuel Thibault <samuel.thibault@...-lyon.org>
To:     gregkh@...uxfoundation.org, Daniel Vetter <daniel@...ll.ch>,
        Helge Deller <deller@....de>
Cc:     linux-fbdev@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Sanan Hasanov <sanan.hasanov@...ghts.ucf.edu>,
        Samuel Thibault <samuel.thibault@...-lyon.org>
Subject: [PATCH] fbcon: Check font dimension limits

blit_x and blit_y are uint32_t, so fbcon currently cannot support fonts
larger than 32x32.

The 32x32 case also needs shifting an unsigned int, to properly set bit
31, otherwise we get "UBSAN: shift-out-of-bounds in fbcon_set_font",
as reported on

http://lore.kernel.org/all/IA1PR07MB98308653E259A6F2CE94A4AFABCE9@IA1PR07MB9830.namprd07.prod.outlook.com
Kernel Branch: 6.2.0-rc5-next-20230124
Kernel config: https://drive.google.com/file/d/1F-LszDAizEEH0ZX0HcSR06v5q8FPl2Uv/view?usp=sharing
Reproducer: https://drive.google.com/file/d/1mP1jcLBY7vWCNM60OMf-ogw-urQRjNrm/view?usp=sharing

Reported-by: Sanan Hasanov <sanan.hasanov@...ghts.ucf.edu>
Signed-off-by: Samuel Thibault <samuel.thibault@...-lyon.org>

Index: linux-6.0/drivers/video/fbdev/core/fbcon.c
===================================================================
--- linux-6.0.orig/drivers/video/fbdev/core/fbcon.c
+++ linux-6.0/drivers/video/fbdev/core/fbcon.c
@@ -2489,9 +2489,12 @@ static int fbcon_set_font(struct vc_data
 	    h > FBCON_SWAP(info->var.rotate, info->var.yres, info->var.xres))
 		return -EINVAL;
 
+	if (font->width > 32 || font->height > 32)
+		return -EINVAL;
+
 	/* Make sure drawing engine can handle the font */
-	if (!(info->pixmap.blit_x & (1 << (font->width - 1))) ||
-	    !(info->pixmap.blit_y & (1 << (font->height - 1))))
+	if (!(info->pixmap.blit_x & (1U << (font->width - 1))) ||
+	    !(info->pixmap.blit_y & (1U << (font->height - 1))))
 		return -EINVAL;
 
 	/* Make sure driver can handle the font length */