fbcon: Make sure modelist not set on unregistered console

Overview

In the Linux kernel, the following vulnerability has been resolved:

fbcon: Make sure modelist not set on unregistered console

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.

Affected

• Linux >= 409d6c95f9c68bb7046410a3502e26e454a1e636 < b3237d451bf3a4490cb1a76f3b7c91d9888f1c4b
• Linux >= 409d6c95f9c68bb7046410a3502e26e454a1e636 < 519ba75728ee8cd561dce25fc52a2ec5c47171dc
• Linux >= 409d6c95f9c68bb7046410a3502e26e454a1e636 < f28f1f578cd810779d01999c60618cda14c281dd
• Linux >= 409d6c95f9c68bb7046410a3502e26e454a1e636 < 54b28f7c567dd659e5f9562f518e4d7f3f6a367b
• Linux >= 409d6c95f9c68bb7046410a3502e26e454a1e636 < cedc1b63394a866bf8663a3e40f4546f1d28c8d8
• Linux 5.19

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.