char/mwave: fix potential Spectre v1 vulnerability

BugLink: https://bugs.launchpad.net/bugs/1818803

commit 701956d4 upstream.

ipcnum 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/char/mwave/mwavedd.c:299 mwave_ioctl() warn: potential spectre issue 'pDrvData->IPCs' [w] (local cap)

Fix this by sanitizing ipcnum before using it to index pDrvData->IPCs.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

drivers/char/mwave/mwavedd.c

@@ -59,6 +59,7 @@
 #include <linux/mutex.h>
 #include <linux/delay.h>
 #include <linux/serial_8250.h>
+#include <linux/nospec.h>
 #include "smapi.h"
 #include "mwavedd.h"
 #include "3780i.h"
@@ -289,6 +290,8 @@ static long mwave_ioctl(struct file *file, unsigned int iocmd,
 						ipcnum);
 				return -EINVAL;
 			}
+			ipcnum = array_index_nospec(ipcnum,
+						    ARRAY_SIZE(pDrvData->IPCs));
 			PRINTK_3(TRACE_MWAVE,
 				"mwavedd::mwave_ioctl IOCTL_MW_REGISTER_IPC"
 				" ipcnum %x entry usIntCount %x\n",
@@ -317,6 +320,8 @@ static long mwave_ioctl(struct file *file, unsigned int iocmd,
 						" Invalid ipcnum %x\n", ipcnum);
 				return -EINVAL;
 			}
+			ipcnum = array_index_nospec(ipcnum,
+						    ARRAY_SIZE(pDrvData->IPCs));
 			PRINTK_3(TRACE_MWAVE,
 				"mwavedd::mwave_ioctl IOCTL_MW_GET_IPC"
 				" ipcnum %x, usIntCount %x\n",
@@ -383,6 +388,8 @@ static long mwave_ioctl(struct file *file, unsigned int iocmd,
 						ipcnum);
 				return -EINVAL;
 			}
+			ipcnum = array_index_nospec(ipcnum,
+						    ARRAY_SIZE(pDrvData->IPCs));
 			mutex_lock(&mwave_mutex);
 			if (pDrvData->IPCs[ipcnum].bIsEnabled == TRUE) {
 				pDrvData->IPCs[ipcnum].bIsEnabled = FALSE;