IB/ucm: Fix Spectre v1 vulnerability

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

---
 drivers/infiniband/core/ucm.c | 3 +++
 1 file changed, 3 insertions(+)

index faa9e61..73332b9 100644
@@ -46,6 +46,8 @@
 #include <linux/mutex.h>
 #include <linux/slab.h>
 
+#include <linux/nospec.h>
+
 #include <linux/uaccess.h>
 
 #include <rdma/ib.h>

@@ -1120,6 +1122,7 @@ static ssize_t ib_ucm_write(struct file *filp, const char __user *buf,
 
 	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;

2.7.4

Thanks, applied to for-rc.

Doug Ledford <[email protected]>
    GPG KeyID: B826A3330E572FDD
    Key fingerprint = AE6B 1BDA 122B 23B4 265B  1274 B826 A333 0E57 2FDD