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SubjectRe: [PATCH 0/3] Prevent out-of-bounds access for built-in font data buffers
On Fri, Sep 25, 2020 at 03:25:51PM +0200, Daniel Vetter wrote:
> 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.

Hi Daniel,

It seems that users don't use `console_font` directly, they use
`console_font_op`. Then, in TTY:

(drivers/tty/vt/vt.c)
int con_font_op(struct vc_data *vc, struct console_font_op *op)
{
switch (op->op) {
case KD_FONT_OP_SET:
return con_font_set(vc, op);
case KD_FONT_OP_GET:
return con_font_get(vc, op);
case KD_FONT_OP_SET_DEFAULT:
return con_font_default(vc, op);
case KD_FONT_OP_COPY:
return con_font_copy(vc, op);
}
return -ENOSYS;
}

These 4 functions allocate `console_font`. We can replace them with our
`kernel_console_font`. So, ...

$ vgrep "\.con_font_set"
Index File Line Content
0 drivers/usb/misc/sisusbvga/sisusb_con.c 1294 .con_font_set = sisusbcon_font_set,
1 drivers/usb/misc/sisusbvga/sisusb_con.c 1378 .con_font_set = sisusbdummycon_font_set,
2 drivers/video/console/dummycon.c 162 .con_font_set = dummycon_font_set,
3 drivers/video/console/newport_con.c 693 .con_font_set = newport_font_set,
4 drivers/video/console/vgacon.c 1226 .con_font_set = vgacon_font_set,
5 drivers/video/fbdev/core/fbcon.c 3120 .con_font_set = fbcon_set_font,
$
$ vgrep "\.con_font_get"
Index File Line Content
0 drivers/usb/misc/sisusbvga/sisusb_con.c 1295 .con_font_get = sisusbcon_font_get,
1 drivers/video/console/vgacon.c 1227 .con_font_get = vgacon_font_get,
2 drivers/video/fbdev/core/fbcon.c 3121 .con_font_get = fbcon_get_font,
$
$ vgrep "\.con_font_default"
Index File Line Content
0 drivers/usb/misc/sisusbvga/sisusb_con.c 1379 .con_font_default = sisusbdummycon_font_default,
1 drivers/video/console/dummycon.c 163 .con_font_default = dummycon_font_default,
2 drivers/video/console/newport_con.c 694 .con_font_default = newport_font_default,
3 drivers/video/fbdev/core/fbcon.c 3122 .con_font_default = fbcon_set_def_font,
$
$ vgrep "\.con_font_copy"
Index File Line Content
0 drivers/usb/misc/sisusbvga/sisusb_con.c 1380 .con_font_copy = sisusbdummycon_font_copy,
1 drivers/video/console/dummycon.c 164 .con_font_copy = dummycon_font_copy,
2 drivers/video/fbdev/core/fbcon.c 3123 .con_font_copy = fbcon_copy_font,
$ _

... are these all the callbacks we need to take care of? What about
other console drivers that don't register these callbacks? ...

$ vgrep "\.con_init"
Index File Line Content
[...]
3 drivers/usb/misc/sisusbvga/sisusb_con.c 1285 .con_init = sisusbcon_init,
4 drivers/usb/misc/sisusbvga/sisusb_con.c 1369 .con_init = sisusbdummycon_init,
5 drivers/video/console/dummycon.c 153 .con_init = dummycon_init,
6 drivers/video/console/mdacon.c 544 .con_init = mdacon_init,
7 drivers/video/console/newport_con.c 684 .con_init = newport_init,
8 drivers/video/console/sticon.c 328 .con_init = sticon_init,
9 drivers/video/console/vgacon.c 1217 .con_init = vgacon_init,
10 drivers/video/fbdev/core/fbcon.c 3111 .con_init = fbcon_init,
[...]

... for example, mdacon.c? What font does mdacon.c use? I know that
/lib/fonts/ exports two functions, find_font() and get_default_font(),
but I don't see them being used in mdacon.c.

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`.

> 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.

Currently `struct vc_data` contains a `struct console_font vc_font`, and
I think this is making gradual conversion very hard. As an example, in
fbcon_do_set_font(), we update `vc->vc_font`. We lose all the extra
information we want in `kernel_console_font`, as long as `struct
vc_data` still uses `console_font`...

However, if we let `struct vc_data` use `kernel_console_font` instead,
we'll have to handle a lot of things in one go:

$ vgrep --no-less --no-header ".vc_font" | wc -l
296
$ echo ":("
:(

The good news is, I've tried cleaning up all the macros in fbcon.c in my
playground, and things seem to work. For example, currently we have:

if (userfont)
cnt = FNTCHARCNT(data);
else
cnt = 256;

After introducing `kernel_console_font` (and adding `charcount` to
`struct font_desc` etc.), this should look like:

#define to_font(_data) container_of(_data, struct kernel_console_font, data)
[...]
cnt = to_font(data)->charcount;

No more `if` and `else`, and the framebuffer layer will be able to
support new bulit-in fonts that have more than 256 characters. This
seems really nice, so I'd like to spend some time working on it.

However before I start working on real patches, do you have suggestions
about which console driver I should start with, or how should I split up
the work in general? I couldn't think of how do we clean up subsystems
one by one, while keeping a `console_font` in `struct vc_data`.

Thank you!
Peilin Ye

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