linux-kernel - Re: [PATCH v5 2/2] KEYS: Avoid false positive ENOMEM error on key read

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Date:   Fri, 20 Mar 2020 09:27:03 -0400
From:   Waiman Long <longman@...hat.com>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Cc:     David Howells <dhowells@...hat.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, keyrings@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-integrity@...r.kernel.org, netdev@...r.kernel.org,
        linux-afs@...ts.infradead.org, Sumit Garg <sumit.garg@...aro.org>,
        Jerry Snitselaar <jsnitsel@...hat.com>,
        Roberto Sassu <roberto.sassu@...wei.com>,
        Eric Biggers <ebiggers@...gle.com>,
        Chris von Recklinghausen <crecklin@...hat.com>
Subject: Re: [PATCH v5 2/2] KEYS: Avoid false positive ENOMEM error on key
 read

On 3/19/20 10:07 PM, Jarkko Sakkinen wrote:
> On Thu, Mar 19, 2020 at 08:07:55PM -0400, Waiman Long wrote:
>> On 3/19/20 3:46 PM, Jarkko Sakkinen wrote:
>>> On Wed, Mar 18, 2020 at 06:14:57PM -0400, Waiman Long wrote:
>>>> +			 * It is possible, though unlikely, that the key
>>>> +			 * changes in between the up_read->down_read period.
>>>> +			 * If the key becomes longer, we will have to
>>>> +			 * allocate a larger buffer and redo the key read
>>>> +			 * again.
>>>> +			 */
>>>> +			if (!tmpbuf || unlikely(ret > tmpbuflen)) {
>>> Shouldn't you check that tmpbuflen stays below buflen (why else
>>> you had made copy of buflen otherwise)?
>> The check above this thunk:
>>
>> if ((ret > 0) && (ret <= buflen)) {
>>
>> will make sure that ret will not be larger than buflen. So tmpbuflen
>> will never be bigger than buflen.
> Ah right, of course, thanks.
>
> What would go wrong if the condition was instead
> ((ret > 0) && (ret <= tmpbuflen))?

That if statement is a check to see if the actual key length is longer
than the user-supplied buffer (buflen). If that is the case, it will
just return the expected length without storing anything into the user
buffer. For the case that buflen >= ret > tmpbuflen, the revised check
above will incorrectly skip the storing step causing the caller to
incorrectly think the key is there in the buffer.

Maybe I should clarify that a bit more in the comment.

Cheers,
Longman