On Fri, Mar 20, 2015 at 02:11:56PM +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 an __int128_t temporary. With
> suitable rearrangement of the calculations, I expect it would be possible
> to avoid the __int128_t. But since we already use __int128_t extensively
> in the memory region code, and this is not a hot path, the super-wide
> integer seems like the simplest approach.
>
> Signed-off-by: David Gibson <da...@gibson.dropbear.id.au>
> ---
> 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..11728af 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 = (__int128_t)get_ticks_per_sec() * timeout / 33000000;
>
> i6300esb_debug("stage %d, timeout %" PRIi64 "\n", d->stage, timeout);
>
> --
> 2.1.0
Reviewed-by: Richard W.M. Jones <rjo...@redhat.com>
Rich.
--
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