> Committer: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> CommitterDate: Wed, 08 Sep 2021 17:44:26 +02:00
>
> time: Handle negative seconds correctly in timespec64_to_ns()
>
> timespec64_ns() prevents multiplication overflows by comparing the seconds
> value of the timespec to KTIME_SEC_MAX. If the value is greater or equal it
> returns KTIME_MAX.
>
> But that check casts the signed seconds value to unsigned which makes the
> comparision true for all negative values and therefore return wrongly
> KTIME_MAX.
>
> Negative second values are perfectly valid and required in some places,
> e.g. ptp_clock_adjtime().
>
> Remove the cast and add a check for the negative boundary which is required
> to prevent undefined behaviour due to multiplication underflow.
>
> Fixes: cb47755725da ("time: Prevent undefined behaviour in timespec64_to_ns()")'
> Signed-off-by: Lukas Hannen <lukas.hannen@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Link:
> https://lore.kernel.org/r/AM6PR01MB541637BD6F336B8FFB72AF80EEC69@xxxxxxxxxxxxxxxxxxxxxxxxxxx.exchangel
> abs.com
> ---
> include/linux/time64.h | 9 +++++++--
> 1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/time64.h b/include/linux/time64.h
> index 5117cb5..81b9686 100644
> --- a/include/linux/time64.h
> +++ b/include/linux/time64.h
> @@ -25,7 +25,9 @@ struct itimerspec64 {
> #define TIME64_MIN (-TIME64_MAX - 1)
>
> #define KTIME_MAX ((s64)~((u64)1 << 63))
> +#define KTIME_MIN (-KTIME_MAX - 1)
> #define KTIME_SEC_MAX (KTIME_MAX / NSEC_PER_SEC)
> +#define KTIME_SEC_MIN (KTIME_MIN / NSEC_PER_SEC)
>
> /*
> * Limits for settimeofday():
> @@ -124,10 +126,13 @@ static inline bool timespec64_valid_settod(const struct timespec64 *ts)
> */
> static inline s64 timespec64_to_ns(const struct timespec64 *ts)
> {
> - /* Prevent multiplication overflow */
> - if ((unsigned long long)ts->tv_sec >= KTIME_SEC_MAX)
> + /* Prevent multiplication overflow / underflow */
> + if (ts->tv_sec >= KTIME_SEC_MAX)
> return KTIME_MAX;
>
> + if (ts->tv_sec <= KTIME_SEC_MIN)
> + return KTIME_MIN;
> +
> return ((s64) ts->tv_sec * NSEC_PER_SEC) + ts->tv_nsec;
> }
Adding tv_nsec can still overflow - even if tv_nsec is bounded to +/- 1 second.
This is no more 'garbage in' => 'garbage out' than the code without the
multiply under/overflow check.
David
-
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