linux-kernel - [PATCH 3.16 026/366] rtc: tx4939: avoid unintended sign extension on a 24 bit shift

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Message-ID: <lsq.1539530740.981795071@decadent.org.uk>
Date:   Sun, 14 Oct 2018 16:25:40 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org,
        "Colin Ian King" <colin.king@...onical.com>,
        "Alexandre Belloni" <alexandre.belloni@...tlin.com>
Subject: [PATCH 3.16 026/366] rtc: tx4939: avoid unintended sign extension
 on a 24 bit shift

3.16.60-rc1 review patch.  If anyone has any objections, please let me know.

------------------

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

commit 347876ad47b9923ce26e686173bbf46581802ffa upstream.

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@...onical.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@...tlin.com>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/rtc/rtc-tx4939.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

--- a/drivers/rtc/rtc-tx4939.c
+++ b/drivers/rtc/rtc-tx4939.c
@@ -86,7 +86,8 @@ static int tx4939_rtc_read_time(struct d
 	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);
 }
@@ -147,7 +148,8 @@ static int tx4939_rtc_read_alarm(struct
 	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);
 }