[PATCH][V2] rtc: tx4939: avoid unintended sign extension on a 24 bit shift

[PATCH][V2] rtc: tx4939: avoid unintended sign extension on a 24 bit shift

[Colin King]

From: Colin Ian King <colin.king@canonical.com>

The shifting of buf[5] by 24 bits to the left will be promoted to
a 32 bit signed int and then sign-extended to an unsigned long. If
the top bit of buf[5] is set then all then all the upper bits sec
end up as also being set because of the sign-extension. Fix this by
casting buf[5] to an unsigned long before the shift.

Detected by CoverityScan, CID#1465292 ("Unintended sign extension")

Fixes: 0e1492330cd2 ("rtc: add rtc-tx4939 driver")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
---
 drivers/rtc/rtc-tx4939.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/rtc/rtc-tx4939.c b/drivers/rtc/rtc-tx4939.c
index feededce3ded..1f351308afdc 100644
--- a/drivers/rtc/rtc-tx4939.c
+++ b/drivers/rtc/rtc-tx4939.c
@@ -109,7 +109,8 @@ static int tx4939_rtc_read_time(struct device *dev, struct rtc_time *tm)
 	for (i = 2; i < 6; i++)
 		buf[i] = __raw_readl(&rtcreg->dat);
 	spin_unlock_irq(&pdata->lock);
-	sec = (buf[5] << 24) | (buf[4] << 16) | (buf[3] << 8) | buf[2];
+	sec = ((unsigned long)buf[5] << 24) | (buf[4] << 16) |
+		(buf[3] << 8) | buf[2];
 	rtc_time_to_tm(sec, tm);
 	return rtc_valid_tm(tm);
 }
@@ -170,7 +171,8 @@ static int tx4939_rtc_read_alarm(struct device *dev, struct rtc_wkalrm *alrm)
 	alrm->enabled = (ctl & TX4939_RTCCTL_ALME) ? 1 : 0;
 	alrm->pending = (ctl & TX4939_RTCCTL_ALMD) ? 1 : 0;
 	spin_unlock_irq(&pdata->lock);
-	sec = (buf[5] << 24) | (buf[4] << 16) | (buf[3] << 8) | buf[2];
+	sec = ((unsigned long)buf[5] << 24) | (buf[4] << 16) |
+		(buf[3] << 8) | buf[2];
 	rtc_time_to_tm(sec, &alrm->time);
 	return rtc_valid_tm(&alrm->time);
 }
-- 
2.15.1

Re: [PATCH][V2] rtc: tx4939: avoid unintended sign extension on a 24 bit shift

[Alexandre Belloni]

On 15/02/2018 at 19:36:14 +0000, Colin King wrote:
> From: Colin Ian King <colin.king@canonical.com>
> 
> The shifting of buf[5] by 24 bits to the left will be promoted to
> a 32 bit signed int and then sign-extended to an unsigned long. If
> the top bit of buf[5] is set then all then all the upper bits sec
> end up as also being set because of the sign-extension. Fix this by
> casting buf[5] to an unsigned long before the shift.
> 

The timing of the discovery of this issue is suspicious. I believe it is
because I just enabled COMPILE_TEST on that driver and now this gets
compiled on a 64bit architecture.

Can I ask on which architecture this is an issue? I don't think (and a
small test program confirms) x86 does the sign extension because both
sec and buf are unsigned.

> Detected by CoverityScan, CID#1465292 ("Unintended sign extension")
> 
> Fixes: 0e1492330cd2 ("rtc: add rtc-tx4939 driver")
> Signed-off-by: Colin Ian King <colin.king@canonical.com>
> ---
>  drivers/rtc/rtc-tx4939.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/rtc/rtc-tx4939.c b/drivers/rtc/rtc-tx4939.c
> index feededce3ded..1f351308afdc 100644
> --- a/drivers/rtc/rtc-tx4939.c
> +++ b/drivers/rtc/rtc-tx4939.c
> @@ -109,7 +109,8 @@ static int tx4939_rtc_read_time(struct device *dev, struct rtc_time *tm)
>  	for (i = 2; i < 6; i++)
>  		buf[i] = __raw_readl(&rtcreg->dat);
>  	spin_unlock_irq(&pdata->lock);
> -	sec = (buf[5] << 24) | (buf[4] << 16) | (buf[3] << 8) | buf[2];
> +	sec = ((unsigned long)buf[5] << 24) | (buf[4] << 16) |
> +		(buf[3] << 8) | buf[2];
>  	rtc_time_to_tm(sec, tm);
>  	return rtc_valid_tm(tm);
>  }
> @@ -170,7 +171,8 @@ static int tx4939_rtc_read_alarm(struct device *dev, struct rtc_wkalrm *alrm)
>  	alrm->enabled = (ctl & TX4939_RTCCTL_ALME) ? 1 : 0;
>  	alrm->pending = (ctl & TX4939_RTCCTL_ALMD) ? 1 : 0;
>  	spin_unlock_irq(&pdata->lock);
> -	sec = (buf[5] << 24) | (buf[4] << 16) | (buf[3] << 8) | buf[2];
> +	sec = ((unsigned long)buf[5] << 24) | (buf[4] << 16) |
> +		(buf[3] << 8) | buf[2];
>  	rtc_time_to_tm(sec, &alrm->time);
>  	return rtc_valid_tm(&alrm->time);
>  }
> -- 
> 2.15.1
> 

-- 
Alexandre Belloni, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
http://bootlin.com

Re: [PATCH][V2] rtc: tx4939: avoid unintended sign extension on a 24 bit shift

[Alexandre Belloni]

On 15/02/2018 at 21:44:53 +0100, Alexandre Belloni wrote:
> On 15/02/2018 at 19:36:14 +0000, Colin King wrote:
> > From: Colin Ian King <colin.king@canonical.com>
> > 
> > The shifting of buf[5] by 24 bits to the left will be promoted to
> > a 32 bit signed int and then sign-extended to an unsigned long. If
> > the top bit of buf[5] is set then all then all the upper bits sec
> > end up as also being set because of the sign-extension. Fix this by
> > casting buf[5] to an unsigned long before the shift.
> > 
> 
> The timing of the discovery of this issue is suspicious. I believe it is
> because I just enabled COMPILE_TEST on that driver and now this gets
> compiled on a 64bit architecture.
> 
> Can I ask on which architecture this is an issue? I don't think (and a
> small test program confirms) x86 does the sign extension because both
> sec and buf are unsigned.
> 

Actually, my test program was wrong and you are right.

-- 
Alexandre Belloni, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
http://bootlin.com

Re: [PATCH][V2] rtc: tx4939: avoid unintended sign extension on a 24 bit shift

[Colin Ian King]

On 16/02/18 15:24, Alexandre Belloni wrote:
> On 15/02/2018 at 21:44:53 +0100, Alexandre Belloni wrote:
>> On 15/02/2018 at 19:36:14 +0000, Colin King wrote:
>>> From: Colin Ian King <colin.king@canonical.com>
>>>
>>> The shifting of buf[5] by 24 bits to the left will be promoted to
>>> a 32 bit signed int and then sign-extended to an unsigned long. If
>>> the top bit of buf[5] is set then all then all the upper bits sec
>>> end up as also being set because of the sign-extension. Fix this by
>>> casting buf[5] to an unsigned long before the shift.
>>>
>>
>> The timing of the discovery of this issue is suspicious. I believe it is
>> because I just enabled COMPILE_TEST on that driver and now this gets
>> compiled on a 64bit architecture.
>>
>> Can I ask on which architecture this is an issue? I don't think (and a
>> small test program confirms) x86 does the sign extension because both
>> sec and buf are unsigned.
>>
> 
> Actually, my test program was wrong and you are right.
> Kudos to CoverityScan static analysis for finding it. It's not obvious
for sure

Colin

Re: [PATCH][V2] rtc: tx4939: avoid unintended sign extension on a 24 bit shift

[Alexandre Belloni]

On 15/02/2018 at 19:36:14 +0000, Colin King wrote:
> From: Colin Ian King <colin.king@canonical.com>
> 
> The shifting of buf[5] by 24 bits to the left will be promoted to
> a 32 bit signed int and then sign-extended to an unsigned long. If
> the top bit of buf[5] is set then all then all the upper bits sec
> end up as also being set because of the sign-extension. Fix this by
> casting buf[5] to an unsigned long before the shift.
> 
> Detected by CoverityScan, CID#1465292 ("Unintended sign extension")
> 
> Fixes: 0e1492330cd2 ("rtc: add rtc-tx4939 driver")
> Signed-off-by: Colin Ian King <colin.king@canonical.com>
> ---
>  drivers/rtc/rtc-tx4939.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
Applied, thanks.

-- 
Alexandre Belloni, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
http://bootlin.com