From: Daniel Vetter <daniel@ffwll.ch>
To: Peilin Ye <yepeilin.cs@gmail.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,
Jiri Slaby <jirislaby@kernel.org>,
linux-kernel-mentees@lists.linuxfoundation.org
Subject: Re: [Linux-kernel-mentees] [PATCH 0/3] Prevent out-of-bounds access for built-in font data buffers
Date: Fri, 25 Sep 2020 15:25:51 +0200 [thread overview]
Message-ID: <20200925132551.GF438822@phenom.ffwll.local> (raw)
In-Reply-To: <20200925101300.GA890211@PWN>
On Fri, Sep 25, 2020 at 06:13:00AM -0400, Peilin Ye wrote:
> Hi all!
>
> On Fri, Sep 25, 2020 at 08:46:04AM +0200, Jiri Slaby wrote:
> > > In order to perform a reliable range check, fbcon_get_font() needs to know
> > > `FONTDATAMAX` for each built-in font under lib/fonts/. Unfortunately, we
> > > do not keep that information in our font descriptor,
> > > `struct console_font`:
> > >
> > > (include/uapi/linux/kd.h)
> > > struct console_font {
> > > unsigned int width, height; /* font size */
> > > unsigned int charcount;
> > > unsigned char *data; /* font data with height fixed to 32 */
> > > };
> > >
> > > To make things worse, `struct console_font` is part of the UAPI, so we
> > > cannot add a new field to keep track of `FONTDATAMAX`.
> >
> > Hi,
> >
> > but you still can define struct kernel_console_font containing struct
> > console_font and the 4 more members you need in the kernel. See below.
> >
> > > Fortunately, the framebuffer layer itself gives us a hint of how to
> > > resolve this issue without changing UAPI. When allocating a buffer for a
> > > user-provided font, fbcon_set_font() reserves four "extra words" at the
> > > beginning of the buffer:
> > >
> > > (drivers/video/fbdev/core/fbcon.c)
> > > new_data = kmalloc(FONT_EXTRA_WORDS * sizeof(int) + size, GFP_USER);
> >
> > I might be missing something (like coffee in the morning), but why don't
> > you just:
> > 1) declare struct font_data as
> > {
> > unsigned sum, char_count, size, refcnt;
> > const unsigned char data[];
> > }
> >
> > Or maybe "struct console_font font" instead of "const unsigned char
> > data[]", if need be.
> >
> > 2) allocate by:
> > kmalloc(struct_size(struct font_data, data, size));
> >
> > 3) use container_of wherever needed
> >
> > That is you name the data on negative indexes using struct as you
> > already have to define one.
> >
> > Then you don't need the ugly macros with negative indexes. And you can
> > pass this structure down e.g. to fbcon_do_set_font, avoiding potential
> > mistakes in accessing data[-1] and similar.
>
> Sorry that I didn't mention it in the cover letter, but yes, I've tried
> this - a new `kernel_console_font` would be much cleaner than negative
> array indexing.
>
> The reason I ended up giving it up was, frankly speaking, these macros
> are being used at about 30 places, and I am not familiar enough with the
> framebuffer and newport_con code, so I wasn't confident how to clean
> them up and plug in `kernel_console_font` properly...
>
> Another reason was that, functions like fbcon_get_font() handle both user
> fonts and built-in fonts, so I wanted a single solution for both of
> them. I think we can't really introduce `kernel_console_font` while
> keeping these macros, that would make the error handling logics etc.
> very messy.
>
> I'm not very sure what to do now. Should I give it another try cleaning
> up all the macros?
>
> And thank you for reviewing this!
I think the only way to make this work is that we have one place which
takes in the userspace uapi struct, and then converts it once into a
kernel_console_font. With all the error checking.
Then all internal code deals in terms of kernel_console_font, with
properly typed and named struct members and helper functions and
everything. And we might need a gradual conversion for this, so that first
we can convert over invidual console drivers, then subsystems, until at
the end we've pushed the conversion from uapi array to kernel_console_font
all the way to the ioctl entry points.
But that's indeed a huge pile of work, and fair warning: fbcon is
semi-orphaned, so by doing this you'll pretty much volunteer for
maintainership :-)
But I'd be very happy to help get this done and throw some maintainership
credentials at you in the proces ...
Cheers, Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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next prev parent reply other threads:[~2020-09-25 13:26 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <0000000000006b9e8d059952095e@google.com>
2020-09-24 13:38 ` [Linux-kernel-mentees] [PATCH 0/3] Prevent out-of-bounds access for built-in font data buffers Peilin Ye
2020-09-24 13:40 ` [Linux-kernel-mentees] [PATCH 1/3] fbdev, newport_con: Move FONT_EXTRA_WORDS macros into linux/font.h Peilin Ye
2020-09-24 13:42 ` [Linux-kernel-mentees] [PATCH 2/3] Fonts: Support FONT_EXTRA_WORDS macros for built-in fonts Peilin Ye
2020-09-24 13:43 ` [Linux-kernel-mentees] [PATCH 3/3] fbcon: Fix global-out-of-bounds read in fbcon_get_font() Peilin Ye
2020-09-24 14:09 ` [Linux-kernel-mentees] [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
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 [this message]
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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