linux-kernel - Re: [PATCH] fbcon: fix buffer overflow in fbcon_set

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Message-ID: <b76b8168-bca1-4374-9626-7d7d155d79ae@suse.de>
Date: Tue, 23 Sep 2025 17:21:36 +0200
From: Thomas Zimmermann <tzimmermann@...e.de>
To: Simon Richter <Simon.Richter@...yros.de>, linux-fbdev@...r.kernel.org,
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Cc: stable <stable@...r.kernel.org>
Subject: Re: [PATCH] fbcon: fix buffer overflow in fbcon_set_font

Hi,

thanks for the patch. The fix at [1] has meanwhile been merged into the 
DRM tree.

[1] https://patchwork.freedesktop.org/series/154848/

Best regards
Thomas

Am 23.09.25 um 17:06 schrieb Simon Richter:
> Commit 1a194e6c8e1ee745e914b0b7f50fa86c89ed13fe introduced overflow
> checking for the font allocation size calculation, but in doing so moved
> the addition of the size for font housekeeping data out of the kmalloc
> call.
>
> As a result, the calculated size now includes those extra bytes, which
> marks the same number of bytes beyond the allocation as valid font data.
>
> The crc32() call and the later memcmp() in fbcon_set_font() already perform
> an out-of-bounds read, the latter is flagged on ppc64el:
>
>      memcmp: detected buffer overflow: 4112 byte read of buffer size 4096
>
> when loading Lat15-Fixed16.psf.gz.
>
> Since the addition of the extra size should only go into the kmalloc()
> call, calculate this size in a separate variable.
>
> Signed-off-by: Simon Richter <Simon.Richter@...yros.de>
> Fixes: 1a194e6c8e1e ("fbcon: fix integer overflow in fbcon_do_set_font")
> Cc: stable <stable@...r.kernel.org> #v5.9+
> ---
>   drivers/video/fbdev/core/fbcon.c | 6 +++---
>   1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
> index 5fade44931b8..a3fbf42c57d9 100644
> --- a/drivers/video/fbdev/core/fbcon.c
> +++ b/drivers/video/fbdev/core/fbcon.c
> @@ -2518,7 +2518,7 @@ static int fbcon_set_font(struct vc_data *vc, const struct console_font *font,
>   	unsigned charcount = font->charcount;
>   	int w = font->width;
>   	int h = font->height;
> -	int size;
> +	int size, allocsize;
>   	int i, csum;
>   	u8 *new_data, *data = font->data;
>   	int pitch = PITCH(font->width);
> @@ -2551,10 +2551,10 @@ static int fbcon_set_font(struct vc_data *vc, const struct console_font *font,
>   		return -EINVAL;
>   
>   	/* Check for overflow in allocation size calculation */
> -	if (check_add_overflow(FONT_EXTRA_WORDS * sizeof(int), size, &size))
> +	if (check_add_overflow(FONT_EXTRA_WORDS * sizeof(int), size, &allocsize))
>   		return -EINVAL;
>   
> -	new_data = kmalloc(size, GFP_USER);
> +	new_data = kmalloc(allocsize, GFP_USER);
>   
>   	if (!new_data)
>   		return -ENOMEM;

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)