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

[Daniel Vetter]

On Wed, Sep 30, 2020 at 12:56 PM Peilin Ye <yepeilin.cs at gmail.com> wrote:
>
> On Wed, Sep 30, 2020 at 11:53:17AM +0200, Daniel Vetter wrote:
> > On Wed, Sep 30, 2020 at 03:11:51AM -0400, Peilin Ye wrote:
> > > On Tue, Sep 29, 2020 at 04:38:49PM +0200, Daniel Vetter wrote:
> > > > On Tue, Sep 29, 2020 at 2:34 PM Peilin Ye <yepeilin.cs at gmail.com> wrote:
> > > > > Ah, and speaking of built-in fonts, see fbcon_startup():
> > > > >
> > > > >         /* Setup default font */
> > > > >                 [...]
> > > > >                 vc->vc_font.charcount = 256; /* FIXME  Need to support more fonts */
> > > > >                             ^^^^^^^^^^^^^^^
> > > > >
> > > > > This is because find_font() and get_default_font() return a `struct
> > > > > font_desc *`, but `struct font_desc` doesn't contain `charcount`. I
> > > > > think we also need to add a `charcount` field to `struct font_desc`.
> > > >
> > > > Hm yeah ... I guess maybe struct font_desc should be the starting
> > > > point for the kernel internal font structure. It's at least there
> > > > already ...
> > >
> > > I see, that will also make handling built-in fonts much easier!
> >
> > I think the only downside with starting with font_desc as the internal
> > font represenation is that there's a few fields we don't need/have for
> > userspace fonts (like the id/name stuff). So any helpers to e.g. print out
> > font information need to make sure they don't trip over that
> >
> > But otherwise I don't see a problem with this, I think.
>
> Yes, and built-in fonts don't use refcount. Or maybe we can let
> find_font() and get_default_font() kmalloc() a copy of built-in font
> data, then keep track of refcount for both user and built-in fonts, but
> that will waste a few K of memory for each built-in font we use...

A possible trick for this would be to make sure built-in fonts start
out with a refcount of 1. So never get freed. Plus maybe a check that
if the name is set, then it's a built-in font and if we ever underflow
the refcount we just WARN, but don't free anything.

Another trick would be kern_font_get/put wrappers (we'd want those
anyway if the userspace fonts are refcounted) and if kern_font->name
!= NULL (i.e. built-in font with name) then we simply don't call
kref_get/put.
-Daniel

> > > > I think for vc_date->vc_font we might need a multi-step approach:
> > > > - first add a new helper function which sets the font for a vc using
> > > > an uapi console_font struct (and probably hard-coded assumes cnt ==
> > > > 256.
> > >
> > > But user fonts may have a charcount different to 256... But yes I'll try
> > > to figure out how.
> >
> > Hm yeah, maybe we need a helper to give us the charcount then, which by
> > default is using the magic negative offset.
>
> Ah, I see! :)
>
> > Then once we've converted everything over to explicitly passing charcount
> > around, we can switch that helper. So something like
> >
> > int kern_font_charcount(struct kern_font *font);
> >
> > Feel free to bikeshed the struct name however you see fit :-)
>
> I think both `kern_font` and `font_desc` makes sense, naming is so
> hard...
>
> > > > For first steps I'd start with demidlayering some of the internal
> > > > users of uapi structs, like the console_font_op really shouldn't be
> > > > used anywhere in any function, except in the ioctl handler that
> > > > converts it into the right function call. You'll probably discover a
> > > > few other places like this on the go.
> > >
> > > Sure, I'll start from this, then cleaning up these dummy functions, then
> > > `vc_data`. Thank you for the insights!
> >
> > Please don't take this rough plan as fixed, it's just where I'd start from
> > browsing the code and your analysis a bit. We'll probably have to adapt as
> > we go and more nasty things turn up ...
>
> Sure, I'll first give it a try and see. Thank you!
>
> Peilin Ye
>


-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch