[PATCH 4.14 1/1] i2c: dev: prevent ZERO_SIZE_PTR deref in i2cdev_ioctl_rdwr()

[Harshit Mogalapalli]

From: Alexander Popov <alex.popov@xxxxxxxxx>

commit 23a27722b5292ef0b27403c87a109feea8296a5c upstream.

i2cdev_ioctl_rdwr() allocates i2c_msg.buf using memdup_user(), which
returns ZERO_SIZE_PTR if i2c_msg.len is zero.

Currently i2cdev_ioctl_rdwr() always dereferences the buf pointer in case
of I2C_M_RD | I2C_M_RECV_LEN transfer. That causes a kernel oops in
case of zero len.

Let's check the len against zero before dereferencing buf pointer.

This issue was triggered by syzkaller.

Signed-off-by: Alexander Popov <alex.popov@xxxxxxxxx>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx>
[wsa: use '< 1' instead of '!' for easier readability]
Signed-off-by: Wolfram Sang <wsa@xxxxxxxxxxxxx>
[Harshit: backport to 4.14.y, use rdwr_pa[i].len instead of msgs[i].len
as the 4.14.y  code uses rdwr_pa.]
Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@xxxxxxxxxx>
---
 Conflicts:
	drivers/i2c/i2c-dev.c - use rdwr_pa[i].len instead of
	msgs[i].len
Since a NULL pointer dereference happens on 4.14.y this backport patch
will fix the issue.
---
 drivers/i2c/i2c-dev.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/i2c/i2c-dev.c b/drivers/i2c/i2c-dev.c
index b7f9fb00f695..08aff5ebc99a 100644
--- a/drivers/i2c/i2c-dev.c
+++ b/drivers/i2c/i2c-dev.c
@@ -297,7 +297,7 @@ static noinline int i2cdev_ioctl_rdwr(struct i2c_client *client,
 		 */
 		if (rdwr_pa[i].flags & I2C_M_RECV_LEN) {
 			if (!(rdwr_pa[i].flags & I2C_M_RD) ||
-			    rdwr_pa[i].buf[0] < 1 ||
+			    rdwr_pa[i].len < 1 || rdwr_pa[i].buf[0] < 1 ||
 			    rdwr_pa[i].len < rdwr_pa[i].buf[0] +
 					     I2C_SMBUS_BLOCK_MAX) {
 				i++;
-- 
2.37.1