ptp: fix Spectre v1 vulnerability

pin_index 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/ptp/ptp_chardev.c:253 ptp_ioctl() warn: potential spectre issue
'ops->pin_config' [r] (local cap)

Fix this by sanitizing pin_index before using it to index
ops->pin_config, and before passing it as an argument to
function ptp_set_pinfunc(), in which it is used to index
info->pin_config.

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>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/ptp/ptp_chardev.c

@@ -24,6 +24,8 @@
 #include <linux/slab.h>
 #include <linux/timekeeping.h>
 
+#include <linux/nospec.h>
+
 #include "ptp_private.h"
 
 static int ptp_disable_pinfunc(struct ptp_clock_info *ops,
@@ -248,6 +250,7 @@ long ptp_ioctl(struct posix_clock *pc, unsigned int cmd, unsigned long arg)
 			err = -EINVAL;
 			break;
 		}
+		pin_index = array_index_nospec(pin_index, ops->n_pins);
 		if (mutex_lock_interruptible(&ptp->pincfg_mux))
 			return -ERESTARTSYS;
 		pd = ops->pin_config[pin_index];
@@ -266,6 +269,7 @@ long ptp_ioctl(struct posix_clock *pc, unsigned int cmd, unsigned long arg)
 			err = -EINVAL;
 			break;
 		}
+		pin_index = array_index_nospec(pin_index, ops->n_pins);
 		if (mutex_lock_interruptible(&ptp->pincfg_mux))
 			return -ERESTARTSYS;
 		err = ptp_set_pinfunc(ptp, pin_index, pd.func, pd.chan);