On Mon, Mar 23, 2015 at 12:51:48PM +1100, David Gibson wrote: > If the guest programs a sufficiently large timeout value an integer > overflow can occur in i6300esb_restart_timer(). e.g. if the maximum > possible timer preload value of 0xfffff is programmed then we end up with > the calculation: > > timeout = get_ticks_per_sec() * (0xfffff << 15) / 33000000; > > get_ticks_per_sec() returns 1000000000 (10^9) giving: > > 10^9 * (0xfffff * 2^15) == 0x1dcd632329b000000 (65 bits) > > Obviously the division by 33MHz brings it back under 64-bits, but the > overflow has already occurred. > > Since signed integer overflow has undefined behaviour in C, in theory this > could be arbitrarily bad. In practice, the overflowed value wraps around > to something negative, causing the watchdog to immediately expire, killing > the guest, which is still fairly bad. > > The bug can be triggered by running a Linux guest, loading the i6300esb > driver with parameter "heartbeat=2046" and opening /dev/watchdog. The > watchdog will trigger as soon as the device is opened. > > This patch corrects the problem by using muldiv64(), which effectively > allows a 128-bit intermediate value between the multiplication and > division. > > Signed-off-by: David Gibson <[email protected]> > --- > hw/watchdog/wdt_i6300esb.c | 10 ++++++++-- > 1 file changed, 8 insertions(+), 2 deletions(-) > > diff --git a/hw/watchdog/wdt_i6300esb.c b/hw/watchdog/wdt_i6300esb.c > index e694fa9..c7316f5 100644 > --- a/hw/watchdog/wdt_i6300esb.c > +++ b/hw/watchdog/wdt_i6300esb.c > @@ -125,8 +125,14 @@ static void i6300esb_restart_timer(I6300State *d, int > stage) > else > timeout <<= 5; > > - /* Get the timeout in units of ticks_per_sec. */ > - timeout = get_ticks_per_sec() * timeout / 33000000; > + /* Get the timeout in units of ticks_per_sec. > + * > + * ticks_per_sec is typically 10^9 == 0x3B9ACA00 (30 bits), with > + * 20 bits of user supplied preload, and 15 bits of scale, the > + * multiply here can exceed 64-bits, before we divide by 33MHz, so > + * we use a 128-bit temporary > + */ > + timeout = muldiv64(get_ticks_per_sec(), timeout, 33000000);
ACK. Reviewed-by: Richard W.M. Jones <[email protected]> Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-top is 'top' for virtual machines. Tiny program with many powerful monitoring features, net stats, disk stats, logging, etc. http://people.redhat.com/~rjones/virt-top
