[PATCH 3.16 314/328] RDMA/ucma: 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 a3671a4f973ee9d9621d60166cc3b037c397d604 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/ucma.c:1686 ucma_write() warn: potential
spectre issue 'ucma_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>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 drivers/infiniband/core/ucma.c | 3 +++
 1 file changed, 3 insertions(+)

--- a/drivers/infiniband/core/ucma.c
+++ b/drivers/infiniband/core/ucma.c
@@ -43,6 +43,8 @@
 #include <linux/sysctl.h>
 #include <linux/module.h>
 
+#include <linux/nospec.h>
+
 #include <rdma/rdma_user_cm.h>
 #include <rdma/ib_marshall.h>
 #include <rdma/rdma_cm.h>
@@ -1540,6 +1542,7 @@ static ssize_t ucma_write(struct file *f
 
 	if (hdr.cmd >= ARRAY_SIZE(ucma_cmd_table))
 		return -EINVAL;
+	hdr.cmd = array_index_nospec(hdr.cmd, ARRAY_SIZE(ucma_cmd_table));
 
 	if (hdr.in + sizeof(hdr) > len)
 		return -EINVAL;