[RESEND PATCH] i2c: s3c2410: fix possible NULL pointer deref on read message after write

Krzysztof Kozlowski
2021-05-26T12:39:52Z

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;

6. var_deref_model: Passing i2c to is_msgend, which dereferences null i2c->msg.
if (!is_msgend(i2c)) {"

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@canonical.com>

---

Reason for resend - the patch did not get necessary testing however I
think it is correct, therefore resending without "RFT".
---
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;
--
2.27.0