linux-kernel - Re: [PATCH] fbdev: fbcon: Make sure modelist not set on unregistered console

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Message-ID: <041138c8-a1d8-42fb-af4b-4e93a85238af@gmx.de>
Date: Mon, 12 May 2025 09:53:39 +0200
From: Helge Deller <deller@....de>
To: Kees Cook <kees@...nel.org>, Simona Vetter <simona@...ll.ch>
Cc: syzbot+a7d4444e7b6e743572f7@...kaller.appspotmail.com,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 "Jiri Slaby (SUSE)" <jirislaby@...nel.org>,
 Javier Martinez Canillas <javierm@...hat.com>,
 Shixiong Ou <oushixiong@...inos.cn>, linux-fbdev@...r.kernel.org,
 dri-devel@...ts.freedesktop.org, Qianqiang Liu <qianqiang.liu@....com>,
 Jocelyn Falempe <jfalempe@...hat.com>, Zsolt Kajtar <soci@....rulez.org>,
 linux-kernel@...r.kernel.org,
 syzbot+3d613ae53c031502687a@...kaller.appspotmail.com,
 linux-hardening@...r.kernel.org
Subject: Re: [PATCH] fbdev: fbcon: Make sure modelist not set on unregistered
 console

On 5/9/25 22:06, Kees Cook wrote:
> It looks like attempting to write to the "store_modes" sysfs node will
> run afoul of unregistered consoles:
> 
> UBSAN: array-index-out-of-bounds in drivers/video/fbdev/core/fbcon.c:122:28
> index -1 is out of range for type 'fb_info *[32]'
> ...
>   fbcon_info_from_console+0x192/0x1a0 drivers/video/fbdev/core/fbcon.c:122
>   fbcon_new_modelist+0xbf/0x2d0 drivers/video/fbdev/core/fbcon.c:3048
>   fb_new_modelist+0x328/0x440 drivers/video/fbdev/core/fbmem.c:673
>   store_modes+0x1c9/0x3e0 drivers/video/fbdev/core/fbsysfs.c:113
>   dev_attr_store+0x55/0x80 drivers/base/core.c:2439
> 
> static struct fb_info *fbcon_registered_fb[FB_MAX];
> ...
> static signed char con2fb_map[MAX_NR_CONSOLES];
> ...
> static struct fb_info *fbcon_info_from_console(int console)
> ...
>          return fbcon_registered_fb[con2fb_map[console]];
> 
> If con2fb_map contains a -1 things go wrong here. Instead, return NULL,
> as callers of fbcon_info_from_console() are trying to compare against
> existing "info" pointers, so error handling should kick in correctly.
> 
> Reported-by: syzbot+a7d4444e7b6e743572f7@...kaller.appspotmail.com
> Closes: https://lore.kernel.org/all/679d0a8f.050a0220.163cdc.000c.GAE@google.com/
> Signed-off-by: Kees Cook <kees@...nel.org>
> ---
>   drivers/video/fbdev/core/fbcon.c | 7 ++++++-
>   1 file changed, 6 insertions(+), 1 deletion(-)

applied to fbdev for-next git tree.

Thanks!
Helge