eeprom: idt_89hpesx: uninitialized data in idt_dbgfs_csr_write()

The simple_write_to_buffer() function will return positive/success if it
is able to write a single byte anywhere within the buffer.  However that
potentially leaves a lot of the buffer uninitialized.

In this code it's better to return 0 if the offset is non-zero.  This
code is not written to support partial writes.  And then return -EFAULT
if the buffer is not completely initialized.

Fixes: cfad642 ("eeprom: Add IDT 89HPESx EEPROM/CSR driver")
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/Ysg1Pu/nzSMe3r1q@kili
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/eeprom/idt_89hpesx.c

@@ -909,14 +909,18 @@ static ssize_t idt_dbgfs_csr_write(struct file *filep, const char __user *ubuf,
 	u32 csraddr, csrval;
 	char *buf;
 
+	if (*offp)
+		return 0;
+
 	/* Copy data from User-space */
 	buf = kmalloc(count + 1, GFP_KERNEL);
 	if (!buf)
 		return -ENOMEM;
 
-	ret = simple_write_to_buffer(buf, count, offp, ubuf, count);
-	if (ret < 0)
+	if (copy_from_user(buf, ubuf, count)) {
+		ret = -EFAULT;
 		goto free_buf;
+	}
 	buf[count] = 0;
 
 	/* Find position of colon in the buffer */