ACPI: APEI: EINJ: prevent memory corruption in error_type_set()

The "einj_buf" buffer is 32 chars.  If "count" is larger than that it
results in memory corruption.  Cap it at 31 so that we leave the last
character as a NUL terminator.  By the way, the highest reasonable value
for "count" is 24.

Fixes: 0c6176e ("ACPI: APEI: EINJ: Enable the discovery of EINJv2 capabilities")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Link: https://patch.msgid.link/ae6286cf-4d73-4b97-8c0f-0782a65b8f51@sabinyo.mountain
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Author: Dan Carpenter

drivers/acpi/apei/einj-core.c

@@ -826,6 +826,10 @@ static ssize_t error_type_set(struct file *file, const char __user *buf,
 	int rc;
 	u64 val;
 
+	/* Leave the last character for the NUL terminator */
+	if (count > sizeof(einj_buf) - 1)
+		return -EINVAL;
+
 	memset(einj_buf, 0, sizeof(einj_buf));
 	if (copy_from_user(einj_buf, buf, count))
 		return -EFAULT;