Re: [PATCH 0/3] Prevent out-of-bounds access for built-in font data buffers

[Daniel Vetter]

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