applicom: Fix potential Spectre v1 vulnerabilities

IndexCard 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/applicom.c:418 ac_write() warn: potential spectre issue 'apbs' [r]
drivers/char/applicom.c:728 ac_ioctl() warn: potential spectre issue 'apbs' [r] (local cap)

Fix this by sanitizing IndexCard before using it to index apbs.

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://lore.kernel.org/lkml/[e-mail]/

Cc: [e-mail]
Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/char/applicom.c

···
 #include <linux/wait.h>
 #include <linux/init.h>
 #include <linux/fs.h>
+#include <linux/nospec.h>
 
 #include <asm/io.h>
 #include <linux/uaccess.h>
···
 	TicCard = st_loc.tic_des_from_pc;	/* tic number to send            */
 	IndexCard = NumCard - 1;
 
-	if((NumCard < 1) || (NumCard > MAX_BOARD) || !apbs[IndexCard].RamIO)
+	if (IndexCard >= MAX_BOARD)
+		return -EINVAL;
+	IndexCard = array_index_nospec(IndexCard, MAX_BOARD);
+
+	if (!apbs[IndexCard].RamIO)
 		return -EINVAL;
 
 #ifdef DEBUG
···
 	unsigned char IndexCard;
 	void __iomem *pmem;
 	int ret = 0;
+	static int warncount = 10;
 	volatile unsigned char byte_reset_it;
 	struct st_ram_io *adgl;
 	void __user *argp = (void __user *)arg;
···
 	mutex_lock(&ac_mutex);	
 	IndexCard = adgl->num_card-1;
 	 
-	if(cmd != 6 && ((IndexCard >= MAX_BOARD) || !apbs[IndexCard].RamIO)) {
-		static int warncount = 10;
-		if (warncount) {
-			printk( KERN_WARNING "APPLICOM driver IOCTL, bad board number %d\n",(int)IndexCard+1);
-			warncount--;
-		}
-		kfree(adgl);
-		mutex_unlock(&ac_mutex);
-		return -EINVAL;
-	}
+	if (cmd != 6 && IndexCard >= MAX_BOARD)
+		goto err;
+	IndexCard = array_index_nospec(IndexCard, MAX_BOARD);
+
+	if (cmd != 6 && !apbs[IndexCard].RamIO)
+		goto err;
 
 	switch (cmd) {
 		
···
 	kfree(adgl);
 	mutex_unlock(&ac_mutex);
 	return 0;
+
+err:
+	if (warncount) {
+		pr_warn("APPLICOM driver IOCTL, bad board number %d\n",
+			(int)IndexCard + 1);
+		warncount--;
+	}
+	kfree(adgl);
+	mutex_unlock(&ac_mutex);
+	return -EINVAL;
+
 }