[PATCH 4.4 49/56] atm: zatm: Fix potential Spectre v1

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

4.4-stable review patch.  If anyone has any objections, please let me know.

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

From: Gustavo A. R. Silva <gustavo@embeddedor.com>

commit 2be147f7459db5bbf292e0a6f135037b55e20b39 upstream.

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:1462 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

Cc: stable@vger.kernel.org
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 drivers/atm/zatm.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/drivers/atm/zatm.c
+++ b/drivers/atm/zatm.c
@@ -23,6 +23,7 @@
 #include <linux/bitops.h>
 #include <linux/wait.h>
 #include <linux/slab.h>
+#include <linux/nospec.h>
 #include <asm/byteorder.h>
 #include <asm/string.h>
 #include <asm/io.h>
@@ -1456,6 +1457,8 @@ static int zatm_ioctl(struct atm_dev *de
 					return -EFAULT;
 				if (pool < 0 || pool > ZATM_LAST_POOL)
 					return -EINVAL;
+				pool = array_index_nospec(pool,
+							  ZATM_LAST_POOL + 1);
 				spin_lock_irqsave(&zatm_dev->lock, flags);
 				info = zatm_dev->pool_info[pool];
 				if (cmd == ZATM_GETPOOLZ) {