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

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