From: Peilin Ye <yepeilin.cs@gmail.com>
To: David Laight <David.Laight@ACULAB.COM>
Cc: linux-fbdev@vger.kernel.org,
Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
Daniel Vetter <daniel.vetter@ffwll.ch>,
syzkaller-bugs@googlegroups.com, linux-kernel@vger.kernel.org,
dri-devel@lists.freedesktop.org,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Jiri Slaby <jirislaby@kernel.org>,
linux-kernel-mentees@lists.linuxfoundation.org
Subject: Re: [PATCH 0/3] Prevent out-of-bounds access for built-in font data buffers
Date: Thu, 24 Sep 2020 15:30:35 +0000 [thread overview]
Message-ID: <20200924153035.GA879703@PWN> (raw)
In-Reply-To: <394733ab6fae47488d078cb22f22a85b@AcuMS.aculab.com>
Hi!
On Thu, Sep 24, 2020 at 02:42:18PM +0000, David Laight wrote:
> > On Thu, Sep 24, 2020 at 09:38:22AM -0400, Peilin Ye wrote:
> > > Hi all,
> > >
> > > syzbot has reported [1] a global out-of-bounds read issue in
> > > fbcon_get_font(). A malicious user may resize `vc_font.height` to a large
> > > value in vt_ioctl(), causing fbcon_get_font() to overflow our built-in
> > > font data buffers, declared in lib/fonts/font_*.c:
> ...
> > > (drivers/video/fbdev/core/fbcon.c)
> > > if (font->width <= 8) {
> > > j = vc->vc_font.height;
> > > + if (font->charcount * j > FNTSIZE(fontdata))
> > > + return -EINVAL;
>
> Can that still go wrong because the multiply wraps?
Thank you for bringing this up!
The resizing of `vc_font.height` happened in vt_resizex():
(drivers/tty/vt/vt_ioctl.c)
if (v.v_clin > 32)
return -EINVAL;
[...]
for (i = 0; i < MAX_NR_CONSOLES; i++) {
[...]
if (v.v_clin)
vcp->vc_font.height = v.v_clin;
^^^^^^^^^^^^^^
It does check if `v.v_clin` is greater than 32. And, currently, all
built-in fonts have a `charcount` of 256.
Therefore, for built-in fonts and resizing happened in vt_resizex(), it
cannot cause an interger overflow.
However I am not very sure about user-provided fonts, and if there are
other functions that can resize `height` or even `charcount` to a really
huge value, but I will do more investigation and think about it.
Thank you,
Peilin Ye
next prev parent reply other threads:[~2020-09-24 15:30 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-12-10 4:35 KASAN: global-out-of-bounds Read in fbcon_get_font syzbot
2020-01-01 17:40 ` syzbot
2020-09-24 13:38 ` [PATCH 0/3] Prevent out-of-bounds access for built-in font data buffers Peilin Ye
2020-09-24 13:40 ` [PATCH 1/3] fbdev, newport_con: Move FONT_EXTRA_WORDS macros into linux/font.h Peilin Ye
2020-09-24 13:42 ` [PATCH 2/3] Fonts: Support FONT_EXTRA_WORDS macros for built-in fonts Peilin Ye
2020-09-24 13:43 ` [PATCH 3/3] fbcon: Fix global-out-of-bounds read in fbcon_get_font() Peilin Ye
2020-09-24 14:09 ` [PATCH 0/3] Prevent out-of-bounds access for built-in font data buffers Greg Kroah-Hartman
2020-09-24 14:25 ` Peilin Ye
2020-09-24 14:42 ` David Laight
2020-09-24 15:30 ` Peilin Ye [this message]
2020-09-24 15:45 ` Dan Carpenter
2020-09-24 16:59 ` Peilin Ye
2020-09-25 8:38 ` Daniel Vetter
2020-09-25 6:46 ` Jiri Slaby
2020-09-25 10:13 ` Peilin Ye
2020-09-25 13:25 ` Daniel Vetter
2020-09-25 15:35 ` Peilin Ye
2020-09-29 9:09 ` Daniel Vetter
2020-09-29 9:44 ` Peilin Ye
2020-09-29 12:34 ` Peilin Ye
2020-09-29 14:38 ` Daniel Vetter
2020-09-30 7:11 ` Peilin Ye
2020-09-30 9:53 ` Daniel Vetter
2020-09-30 10:55 ` Peilin Ye
2020-09-30 11:25 ` Daniel Vetter
2020-09-30 11:52 ` Greg Kroah-Hartman
2020-09-30 12:58 ` Peilin Ye
2020-09-30 5:26 ` Jiri Slaby
2020-09-30 7:16 ` Peilin Ye
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