ipmi: ssif_bmc: prevent integer overflow on 32bit systems

There are actually two bugs here.  First, we need to ensure that count
is at least sizeof(u32) or msg.len will be uninitialized data.

The "msg.len" variable is a u32 that comes from the user.  On 32bit
systems the "sizeof_field(struct ipmi_ssif_msg, len) + msg.len"
addition can overflow if "msg.len" is greater than U32_MAX - 4.

Valid lengths for "msg.len" are 1-254.  Add a check for that to
prevent the integer overflow.

Fixes: dd2bc5cc ("ipmi: ssif_bmc: Add SSIF BMC driver")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Message-Id: <1431ca2e-4e9c-4520-bfc0-6879313c30e9@moroto.mountain>
Signed-off-by: Corey Minyard <corey@minyard.net>

drivers/char/ipmi/ssif_bmc.c

@@ -177,13 +177,15 @@ static ssize_t ssif_bmc_write(struct file *file, const char __user *buf, size_t
 	unsigned long flags;
 	ssize_t ret;
 
-	if (count > sizeof(struct ipmi_ssif_msg))
+	if (count < sizeof(msg.len) ||
+	    count > sizeof(struct ipmi_ssif_msg))
 		return -EINVAL;
 
 	if (copy_from_user(&msg, buf, count))
 		return -EFAULT;
 
-	if (!msg.len || count < sizeof_field(struct ipmi_ssif_msg, len) + msg.len)
+	if (!msg.len || msg.len > IPMI_SSIF_PAYLOAD_MAX ||
+	    count < sizeof_field(struct ipmi_ssif_msg, len) + msg.len)
 		return -EINVAL;
 
 	spin_lock_irqsave(&ssif_bmc->lock, flags);