atm: zatm: Fix potential Spectre v1

pool can be indirectly controlled by user-space, hence leading to a potential exploitation of the Spectre variant 1 vulnerability.

This issue was detected with the help of Smatch:

drivers/atm/zatm.c:1491 zatm_ioctl() warn: potential spectre issue 'zatm_dev->pool_info' (local cap)

Fix this by sanitizing pool before using it to index zatm_dev->pool_info

Notice that given that speculation windows are large, the policy is to kill the speculation on the first load and not worry if it can be completed with a dependent load/store [1].

[1] https://marc.info/?l=linux-kernel&m=152449131114778&w=2

---
 drivers/atm/zatm.c | 2 ++
 1 file changed, 2 insertions(+)

index a8d2eb0..2c288d1 100644
@@ -1483,6 +1483,8 @@ static int zatm_ioctl(struct atm_dev *dev,unsigned int cmd,void __user *arg)
 					return -EFAULT;
 				if (pool < 0 || pool > ZATM_LAST_POOL)
 					return -EINVAL;
+				pool = array_index_nospec(pool,
+							  ZATM_LAST_POOL + 1);
 				if (copy_from_user(&info,
 				    &((struct zatm_pool_req __user *) arg)->info,
 				    sizeof(info))) return -EFAULT;

2.7.4

From: "Gustavo A. R. Silva" <[email protected]>
Date: Fri, 29 Jun 2018 13:28:07 -0500

Applied.