From: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
To: Waiman Long <longman@redhat.com>
Cc: David Howells <dhowells@redhat.com>,
James Morris <jmorris@namei.org>,
"Serge E. Hallyn" <serge@hallyn.com>,
Mimi Zohar <zohar@linux.ibm.com>,
"David S. Miller" <davem@davemloft.net>,
Jakub Kicinski <kuba@kernel.org>,
keyrings@vger.kernel.org, linux-kernel@vger.kernel.org,
linux-security-module@vger.kernel.org,
linux-integrity@vger.kernel.org, netdev@vger.kernel.org,
linux-afs@lists.infradead.org, Sumit Garg <sumit.garg@linaro.org>,
Jerry Snitselaar <jsnitsel@redhat.com>,
Roberto Sassu <roberto.sassu@huawei.com>,
Eric Biggers <ebiggers@google.com>,
Chris von Recklinghausen <crecklin@redhat.com>
Subject: Re: [PATCH v5 2/2] KEYS: Avoid false positive ENOMEM error on key read
Date: Fri, 20 Mar 2020 14:35:47 +0000 [thread overview]
Message-ID: <20200320143547.GB3629@linux.intel.com> (raw)
In-Reply-To: <7dbc524f-6c16-026a-a372-2e80b40eab30@redhat.com>
On Fri, Mar 20, 2020 at 09:27:03AM -0400, Waiman Long wrote:
> 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.
OK, right because it is possible in-between tmpbuflen could be
larger. Got it.
I think that longish key_data and key_data_len would be better
names than tmpbuf and tpmbuflen.
Also the comments are somewat overkill IMHO.
I'd replace them along the lines of
/* Cap the user supplied buffer length to PAGE_SIZE. */
/* Key data can change as we don not hold key->sem. */
/Jarkko
WARNING: multiple messages have this Message-ID (diff)
From: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
To: Waiman Long <longman@redhat.com>
Cc: David Howells <dhowells@redhat.com>,
James Morris <jmorris@namei.org>,
"Serge E. Hallyn" <serge@hallyn.com>,
Mimi Zohar <zohar@linux.ibm.com>,
"David S. Miller" <davem@davemloft.net>,
Jakub Kicinski <kuba@kernel.org>,
keyrings@vger.kernel.org, linux-kernel@vger.kernel.org,
linux-security-module@vger.kernel.org,
linux-integrity@vger.kernel.org, netdev@vger.kernel.org,
linux-afs@lists.infradead.org, Sumit Garg <sumit.garg@linaro.org>,
Jerry Snitselaar <jsnitsel@redhat.com>,
Roberto Sassu <roberto.sassu@huawei.com>,
Eric Biggers <ebiggers@google.com>,
Chris von Recklinghausen <crecklin@redhat.com>
Subject: Re: [PATCH v5 2/2] KEYS: Avoid false positive ENOMEM error on key read
Date: Fri, 20 Mar 2020 16:35:47 +0200 [thread overview]
Message-ID: <20200320143547.GB3629@linux.intel.com> (raw)
In-Reply-To: <7dbc524f-6c16-026a-a372-2e80b40eab30@redhat.com>
On Fri, Mar 20, 2020 at 09:27:03AM -0400, Waiman Long wrote:
> 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.
OK, right because it is possible in-between tmpbuflen could be
larger. Got it.
I think that longish key_data and key_data_len would be better
names than tmpbuf and tpmbuflen.
Also the comments are somewat overkill IMHO.
I'd replace them along the lines of
/* Cap the user supplied buffer length to PAGE_SIZE. */
/* Key data can change as we don not hold key->sem. */
/Jarkko
next prev parent reply other threads:[~2020-03-20 14:35 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-18 22:14 [PATCH v5 0/2] KEYS: Read keys to internal buffer & then copy to userspace Waiman Long
2020-03-18 22:14 ` Waiman Long
2020-03-18 22:14 ` [PATCH v5 1/2] KEYS: Don't write out to userspace while holding key semaphore Waiman Long
2020-03-18 22:14 ` Waiman Long
2020-03-18 22:14 ` [PATCH v5 2/2] KEYS: Avoid false positive ENOMEM error on key read Waiman Long
2020-03-18 22:14 ` Waiman Long
2020-03-19 19:46 ` Jarkko Sakkinen
2020-03-19 19:46 ` Jarkko Sakkinen
2020-03-20 0:07 ` Waiman Long
2020-03-20 0:07 ` Waiman Long
2020-03-20 2:07 ` Jarkko Sakkinen
2020-03-20 2:07 ` Jarkko Sakkinen
2020-03-20 13:27 ` Waiman Long
2020-03-20 13:27 ` Waiman Long
2020-03-20 14:35 ` Jarkko Sakkinen [this message]
2020-03-20 14:35 ` Jarkko Sakkinen
2020-03-20 15:09 ` Waiman Long
2020-03-20 15:09 ` Waiman Long
2020-03-20 23:55 ` David Howells
2020-03-21 0:58 ` Jarkko Sakkinen
2020-03-21 0:58 ` Jarkko Sakkinen
2020-03-20 8:20 ` [PATCH v5 1/2] KEYS: Don't write out to userspace while holding key semaphore David Howells
2020-03-20 13:56 ` Waiman Long
2020-03-20 13:56 ` Waiman Long
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