Re: [Qemu-devel] [PATCH 2/2] i6300esb: Fix signed integer overflow

[Paolo Bonzini]

On 20/03/2015 04:11, 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.

We don't use __int128_t, we use the Int128 struct---which however
doesn't have a multiplication function.  __int128_t is not available on
32-bit machines, and is only used under #ifdef CONFIG_INT128.

Instead, you can use muldiv64 which has exactly this purpose.

Paolo

> Signed-off-by: David Gibson <address@hidden>
> ---
>  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);
>  
>