[PATCH 3.16 313/328] IB/ucm: Fix Spectre v1 vulnerability

[Ben Hutchings]

3.16.62-rc1 review patch.  If anyone has any objections, please let me know.

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

From: "Gustavo A. R. Silva" <gustavo@xxxxxxxxxxxxxx>

commit 0295e39595e1146522f2722715dba7f7fba42217 upstream.

hdr.cmd 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/infiniband/core/ucm.c:1127 ib_ucm_write() warn: potential
spectre issue 'ucm_cmd_table' [r] (local cap)

Fix this by sanitizing hdr.cmd before using it to index
ucm_cmd_table.

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

Signed-off-by: Gustavo A. R. Silva <gustavo@xxxxxxxxxxxxxx>
Signed-off-by: Doug Ledford <dledford@xxxxxxxxxx>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 drivers/infiniband/core/ucm.c | 3 +++
 1 file changed, 3 insertions(+)

--- a/drivers/infiniband/core/ucm.c
+++ b/drivers/infiniband/core/ucm.c
@@ -46,6 +46,8 @@
 #include <linux/mutex.h>
 #include <linux/slab.h>
 
+#include <linux/nospec.h>
+
 #include <asm/uaccess.h>
 
 #include <rdma/ib.h>
@@ -1116,6 +1118,7 @@ static ssize_t ib_ucm_write(struct file
 
 	if (hdr.cmd >= ARRAY_SIZE(ucm_cmd_table))
 		return -EINVAL;
+	hdr.cmd = array_index_nospec(hdr.cmd, ARRAY_SIZE(ucm_cmd_table));
 
 	if (hdr.in + sizeof(hdr) > len)
 		return -EINVAL;