linux-kernel - Re: tty: kmalloc size WARNING in vc_do

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Message-ID: <20160125161201.5bf22992@lxorguk.ukuu.org.uk>
Date:	Mon, 25 Jan 2016 16:12:01 +0000
From:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
To:	Dmitry Vyukov <dvyukov@...gle.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.com>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	David Herrmann <dh.herrmann@...il.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Imre Deak <imre.deak@...el.com>,
	"Lad, Prabhakar" <prabhakar.csengg@...il.com>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Nicholas Mc Guire <hofrat@...dl.org>,
	Scot Doyle <lkml14@...tdoyle.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Takashi Iwai <tiwai@...e.de>,
	LKML <linux-kernel@...r.kernel.org>,
	syzkaller <syzkaller@...glegroups.com>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>
Subject: Re: tty: kmalloc size WARNING in vc_do_resize

> I think that either the kmalloc should use __GFP_NOWARN or
> vc_do_resize should do stricter size check.

vc_do_resize doesn't know enough to do a stricter size check. There are
not many methods it calls but those are:

vgacon_resize doesn't error when stupid things happen because of a silly
hack for legacy svgatextmode tools.

sisusbcon_resize does the right checks but we can get in a situation
where we unplug during a resize so attempting to resize and size back
might fail.

fbcon_resize does the right thing, but again can error if we go
size/resize/size back.

That means we really have to guess or would have to add 'max size' info
the structures. Right now it's clamped to 1Gbyte which is a 32767 x 32767
character display and probably ludicrous. Clamping to 4048 x 1024 chars
would cover displays up to 16K x 6K even with the 4x6 font (which is
insane on a 4K display, let alone 4 of them).

Alan