timekeeping: Prevent 32bit truncation in scale64_check_overflow()

[ Upstream commit 4cbbc3a0 ]

While unlikely the divisor in scale64_check_overflow() could be >= 32bit in
scale64_check_overflow(). do_div() truncates the divisor to 32bit at least
on 32bit platforms.

Use div64_u64() instead to avoid the truncation to 32-bit.

[ tglx: Massaged changelog ]
Signed-off-by: Wen Yang <wenyang@linux.alibaba.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20200120100523.45656-1-wenyang@linux.alibaba.comSigned-off-by: Sasha Levin <sashal@kernel.org>

kernel/time/timekeeping.c

@@ -950,9 +950,8 @@ static int scale64_check_overflow(u64 mult, u64 div, u64 *base)
 	    ((int)sizeof(u64)*8 - fls64(mult) < fls64(rem)))
 		return -EOVERFLOW;
 	tmp *= mult;
-	rem *= mult;
 
-	do_div(rem, div);
+	rem = div64_u64(rem * mult, div);
 	*base = tmp + rem;
 	return 0;
 }