ipmi: ssif_bmc: fix missing check for copy_to_user() partial failure

copy_to_user() returns the number of bytes that could not be copied,
with a non-zero value indicating a partial or complete failure. The
current code only checks for negative return values and treats all
non-negative results as success.

Treating any positive return value from copy_to_user() as
an error and returning -EFAULT.

Fixes: dd2bc5c ("ipmi: ssif_bmc: Add SSIF BMC driver")
Signed-off-by: Jian Zhang <zhangjian.3032@bytedance.com>
Message-ID: <20260403090603.3988423-2-zhangjian.3032@bytedance.com>
Signed-off-by: Corey Minyard <corey@minyard.net>

Author: zhangjian3032

drivers/char/ipmi/ssif_bmc.c

@@ -163,6 +163,8 @@ static ssize_t ssif_bmc_read(struct file *file, char __user *buf, size_t count,
 		spin_unlock_irqrestore(&ssif_bmc->lock, flags);
 
 		ret = copy_to_user(buf, &msg, count);
+		if (ret > 0)
+			ret = -EFAULT;
 	}
 
 	return (ret < 0) ? ret : count;