[RFT 2/2] i2c: s3c2410: fix possible NULL pointer deref on read message after write

[Krzysztof Kozlowski]

Interrupt handler processes multiple message write requests one after
another, till the driver message queue is drained.  However if driver
encounters a read message without preceding START, it stops the I2C
transfer as it is an invalid condition for the controller.  At least the
comment describes a requirement "the controller forces us to send a new
START when we change direction".  This stop results in clearing the
message queue (i2c->msg = NULL).

The code however immediately jumped back to label "retry_write" which
dereferenced the "i2c->msg" making it a possible NULL pointer
dereference.

The Coverity analysis:
1. Condition !is_msgend(i2c), taking false branch.
   if (!is_msgend(i2c)) {

2. Condition !is_lastmsg(i2c), taking true branch.
   } else if (!is_lastmsg(i2c)) {

3. Condition i2c->msg->flags & 1, taking true branch.
   if (i2c->msg->flags & I2C_M_RD) {

4. write_zero_model: Passing i2c to s3c24xx_i2c_stop, which sets i2c->msg to NULL.
   s3c24xx_i2c_stop(i2c, -EINVAL);

5. Jumping to label retry_write.
   goto retry_write;

All previous calls to s3c24xx_i2c_stop() in this interrupt service
routine are followed by jumping to end of function (acknowledging
the interrupt and returning).  This seems a reasonable choice also here
since message buffer was entirely emptied.

Addresses-Coverity: Explicit null dereferenced
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@xxxxxxxxxxxxx>
---
 drivers/i2c/busses/i2c-s3c2410.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/i2c/busses/i2c-s3c2410.c b/drivers/i2c/busses/i2c-s3c2410.c
index ab928613afba..4d82761e1585 100644
--- a/drivers/i2c/busses/i2c-s3c2410.c
+++ b/drivers/i2c/busses/i2c-s3c2410.c
@@ -480,7 +480,10 @@ static int i2c_s3c_irq_nextbyte(struct s3c24xx_i2c *i2c, unsigned long iicstat)
 					 * forces us to send a new START
 					 * when we change direction
 					 */
+					dev_dbg(i2c->dev,
+						"missing START before write->read\n");
 					s3c24xx_i2c_stop(i2c, -EINVAL);
+					break;
 				}
 
 				goto retry_write;
-- 
2.25.1