[PATCH/RFC 2/2] ALSA: firewire-tascam: Fix integer overflow in midi_port_work()

[Geert Uytterhoeven]

Hi Sakamoto-san,

On Tue, Jan 12, 2021 at 2:43 PM Takashi Sakamoto
<o-takashi at sakamocchi.jp> wrote:
> On Mon, Jan 11, 2021 at 02:02:51PM +0100, Geert Uytterhoeven wrote:
> > As snd_fw_async_midi_port.consume_bytes is unsigned int, and
> > NSEC_PER_SEC is 1000000000L, the second multiplication in
> >
> >     port->consume_bytes * 8 * NSEC_PER_SEC / 31250
> >
> > always overflows on 32-bit platforms, truncating the result.  Fix this
> > by precalculating "NSEC_PER_SEC / 31250", which is an integer constant.
> >
> > Note that this assumes port->consume_bytes <= 16777.
> >
> > Fixes: 531f471834227d03 ("ALSA: firewire-lib/firewire-tascam: localize async midi port")
> > Signed-off-by: Geert Uytterhoeven <geert+renesas at glider.be>
> > ---
> > Compile-tested only.
> >
> > I don't know the maximum transfer length of MIDI, but given it's an old
> > standard, I guess it's rather small.  If it is larger than 16777, the
> > constant "8" should be replaced by "8ULL", to force 64-bit arithmetic.
> > ---
> >  sound/firewire/tascam/tascam-transaction.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
>
> Indeed. The calculation brings integer overflow of 32 bit storage. Thanks
> for your care and proposal of the patch. I agree with the intension of
> patch, however I have a nitpicking that the consume_bytes member is
> defined as 'int', not 'unsigned int' in your commit comment.

Thanks, you're right.
Note that port->consume_bytes being signed halves the limit to
8388 bytes, which is of course still met.

> The member has value returned from the call of 'fill_message()'[1] for the
> length of byte messages in buffer to process, or for error code. The
> error code is checked immediately. The range of value is equal to
> or less than 3 when reaching the calculation, thus it should be less than
> 16777.
>
> Regardless of the type of 'int' or 'unsigned int', this patch can fix
> the issued problem. Feel free to add my tag when you post second version
> with comment fix.
>
> Reviewed-by: Takashi Sakamoto <o-takashi at sakamocchi.jp>

Thanks!

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds