[tip: timers/urgent] time: Handle negative seconds correctly in timespec64_to_ns()

[tip: timers/urgent] time: Handle negative seconds correctly in timespec64_to_ns()

[tip-bot2 for Lukas Hannen]

The following commit has been merged into the timers/urgent branch of tip:

Commit-ID:     39ff83f2f6cc5cc1458dfcea9697f96338210beb
Gitweb:        https://git.kernel.org/tip/39ff83f2f6cc5cc1458dfcea9697f96338210beb
Author:        Lukas Hannen <lukas.hannen@opensource.tttech-industrial.com>
AuthorDate:    Wed, 25 Aug 2021 10:12:43 
Committer:     Thomas Gleixner <tglx@linutronix.de>
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@opensource.tttech-industrial.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/AM6PR01MB541637BD6F336B8FFB72AF80EEC69@AM6PR01MB5416.eurprd01.prod.exchangelabs.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;
 }
 

RE: [tip: timers/urgent] time: Handle negative seconds correctly in timespec64_to_ns()

[David Laight]

> Committer:     Thomas Gleixner <tglx@linutronix.de>
> 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@opensource.tttech-industrial.com>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: stable@vger.kernel.org
> Link:
> https://lore.kernel.org/r/AM6PR01MB541637BD6F336B8FFB72AF80EEC69@AM6PR01MB5416.eurprd01.prod.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

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

RE: [tip: timers/urgent] time: Handle negative seconds correctly in timespec64_to_ns()

[Thomas Gleixner]

David,

On Wed, Sep 08 2021 at 16:01, David Laight wrote:
>> +	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.

In kernel timespecs are always normalized:  0 < tv_nsec < 1e9 - 1

Let's do the math:

  KTIME_SEC_MAX = KTIME_MAX / NSEC_PER_SEC

  The overflow prevention does:

    if PSVAL >= KTIME_SEC_MAX:
       return KTIME_MAX

  so the largest positive seconds value which passes the above is:

    PSMAX = KTIME_SEC_MAX - 1

  ergo:

    PSMAX * NSEC_PER_SEC + (NSEC_PER_SEC - 1) < KTIME_SEC_MAX < KTIME_MAX

I leave the proof for negative values as an excercise for the reader.

Thanks,

        tglx
---
"Math is hard, let's go shopping!" - John Stultz