Re: [PATCH v2 11/16] block: sed-opal: ioctl for writing to shadow mbr

[David Kozub]

On Sat, 19 Jan 2019, Scott Bauer wrote:

> On Thu, Jan 17, 2019 at 09:31:51PM +0000, David Kozub wrote:
>
> > +static int write_shadow_mbr(struct opal_dev *dev, void *data)
> > +{
> > +	struct opal_shadow_mbr *shadow = data;
> > +	const u8 __user *src;
> > +	u8 *dst;
> > +	size_t off = 0;
> > +	u64 len;
> > +	int err = 0;
> > +
> > +	/* do the actual transmission(s) */
> > +	src = (u8 *) shadow->data;
> > +	while (off < shadow->size) {
> > +		err = cmd_start(dev, opaluid[OPAL_MBR], opalmethod[OPAL_SET]);
> > +		add_token_u8(&err, dev, OPAL_STARTNAME);
> > +		add_token_u8(&err, dev, OPAL_WHERE);
> > +		add_token_u64(&err, dev, shadow->offset + off);
> > +		add_token_u8(&err, dev, OPAL_ENDNAME);
> > +
> > +		add_token_u8(&err, dev, OPAL_STARTNAME);
> > +		add_token_u8(&err, dev, OPAL_VALUES);
> > +
> > +		/*
> > +		 * The bytestring header is either 1 or 2 bytes, so assume 2.
> > +		 * There also needs to be enough space to accommodate the
> > +		 * trailing OPAL_ENDNAME (1 byte) and tokens added by
> > +		 * cmd_finalize.
> > +		 */
> > +		len = min(remaining_size(dev) - (2+1+CMD_FINALIZE_BYTES_NEEDED),
> > +			  (size_t)(shadow->size - off));
>
> What if remaining_size(dev) <  2 + 1 + CMD_FINALIZE_BYTES_NEEDED? If that's possible we
> get min(UINT_MAX(ish) , some size larger than our remaining buffer) and that's not good.

This is only possible for uselessly small values of IO_BUFFER_LENGTH, 
which is a compile-time value. Originally I thought it's OK as nobody 
would set the value so low. But on a second thought, after reading your 
comment, I think that even if IO_BUFFER_LENGTH was set to such a value, 
the code should fail gracefully.

So I will change it into:

while (off < shadow->size) {
	/*
	 * Number of bytes needed in the cmd buffer to terminate the
	 * write shadow mbr command.
	 *
	 * The bytestring header is either 1 or 2 bytes, so assume 2.
	 * There also needs to be enough space to accommodate the
	 * trailing OPAL_ENDNAME (1 byte) and tokens added by
	 * cmd_finalize.
	 */
	const size_t write_shadow_mbr_footer_size =
		2 + 1 + CMD_FINALIZE_BYTES_NEEDED;

	err = cmd_start(dev, opaluid[OPAL_MBR], opalmethod[OPAL_SET]);
	add_token_u8(&err, dev, OPAL_STARTNAME);
	add_token_u8(&err, dev, OPAL_WHERE);
	add_token_u64(&err, dev, shadow->offset + off);
	add_token_u8(&err, dev, OPAL_ENDNAME);

	add_token_u8(&err, dev, OPAL_STARTNAME);
	add_token_u8(&err, dev, OPAL_VALUES);

	if (!can_add(&err, dev, write_shadow_mbr_footer_size))
		break;
	len = min(remaining_size(dev) - write_shadow_mbr_footer_size,
		  (size_t)(shadow->size - off));
	pr_debug("MBR: write bytes %zu+%llu/%llu\n",
		 off, len, shadow->size);

Please let me know if you would prefer a different solution.

Best regards,
David