[PATCH] switchtec: Fix potential Spectre v1

[Gustavo A. R. Silva]

p.port can is 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/pci/switch/switchtec.c:912 ioctl_port_to_pff() warn: potential
spectre issue 'pcfg->dsp_pff_inst_id' [r]

Fix this by sanitizing p.port before using it to index
pcfg->dsp_pff_inst_id

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@xxxxxxxxxxxxxxx
Signed-off-by: Gustavo A. R. Silva <gustavo@xxxxxxxxxxxxxx>
---
 drivers/pci/switch/switchtec.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/pci/switch/switchtec.c b/drivers/pci/switch/switchtec.c
index 9940cc7..54a8b30 100644
--- a/drivers/pci/switch/switchtec.c
+++ b/drivers/pci/switch/switchtec.c
@@ -14,6 +14,8 @@
 #include <linux/poll.h>
 #include <linux/wait.h>
 
+#include <linux/nospec.h>
+
 MODULE_DESCRIPTION("Microsemi Switchtec(tm) PCIe Management Driver");
 MODULE_VERSION("0.1");
 MODULE_LICENSE("GPL");
@@ -909,6 +911,8 @@ static int ioctl_port_to_pff(struct switchtec_dev *stdev,
 	default:
 		if (p.port > ARRAY_SIZE(pcfg->dsp_pff_inst_id))
 			return -EINVAL;
+		p.port = array_index_nospec(p.port,
+					ARRAY_SIZE(pcfg->dsp_pff_inst_id) + 1);
 		p.pff = ioread32(&pcfg->dsp_pff_inst_id[p.port - 1]);
 		break;
 	}
-- 
2.7.4