[PATCH 3.16 33/83] staging: comedi: dt3000: Fix signed integer overflow 'divider * base'

[Ben Hutchings]

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

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

From: Ian Abbott <abbotti@xxxxxxxxx>

commit b4d98bc3fc93ec3a58459948a2c0e0c9b501cd88 upstream.

In `dt3k_ns_to_timer()` the following lines near the end of the function
result in a signed integer overflow:

	prescale = 15;
	base = timer_base * (1 << prescale);
	divider = 65535;
	*nanosec = divider * base;

(`divider`, `base` and `prescale` are type `int`, `timer_base` and
`*nanosec` are type `unsigned int`.  The value of `timer_base` will be
either 50 or 100.)

The main reason for the overflow is that the calculation for `base` is
completely wrong.  It should be:

	base = timer_base * (prescale + 1);

which matches an earlier instance of this calculation in the same
function.

Reported-by: David Binderman <dcb314@xxxxxxxxxxx>
Signed-off-by: Ian Abbott <abbotti@xxxxxxxxx>
Link: https://lore.kernel.org/r/20190812111517.26803-1-abbotti@xxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 drivers/staging/comedi/drivers/dt3000.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/drivers/staging/comedi/drivers/dt3000.c
+++ b/drivers/staging/comedi/drivers/dt3000.c
@@ -405,7 +405,7 @@ static int dt3k_ns_to_timer(unsigned int
 	}
 
 	prescale = 15;
-	base = timer_base * (1 << prescale);
+	base = timer_base * (prescale + 1);
 	divider = 65535;
 	*nanosec = divider * base;
 	return (prescale << 16) | (divider);