Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Reply-To: crecklin@redhat.com
Subject: Re: [PATCH] security/keyring: avoid pagefaults in
 keyring_read_iterator
From:   Chris von Recklinghausen <crecklin@redhat.com>
To:     David Howells <dhowells@redhat.com>
Cc:     Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
        James Morris <jmorris@namei.org>,
        "Serge E . Hallyn" <serge@hallyn.com>, keyrings@vger.kernel.org,
        linux-security-module@vger.kernel.org,
        linux-kernel@vger.kernel.org, Waiman Long <longman@redhat.com>
References: <20191018184030.8407-1-crecklin@redhat.com>
 <30309.1571667719@warthog.procyon.org.uk>
 <b8aa0f7c-0a90-efae-9fb7-aa85b19a0d9a@redhat.com>
 <3c87bfba-9dc9-665f-17e8-0656e87c658b@redhat.com>
Organization: Red Hat
Message-ID: <3390b0d2-0f8e-3240-2ebb-94400456fdf0@redhat.com>
Date:   Wed, 6 Nov 2019 10:25:55 -0500
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101
 Thunderbird/52.9.1
MIME-Version: 1.0
In-Reply-To: <3c87bfba-9dc9-665f-17e8-0656e87c658b@redhat.com>
Content-Language: en-US
Content-Type: text/plain; charset=WINDOWS-1252
Content-Transfer-Encoding: quoted-printable
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 10/25/2019 07:10 AM, Chris von Recklinghausen wrote:
> On 10/21/2019 11:46 AM, Chris von Recklinghausen wrote:
>> On 10/21/2019 10:21 AM, David Howells wrote:
>>> Chris von Recklinghausen <crecklin@redhat.com> wrote:
>>>
>>>> The put_user call from keyring_read_iterator caused a page fault which
>>>> attempts to lock mm->mmap_sem and type->lock_class (key->sem) in the r=
everse
>>>> order that keyring_read_iterator did, thus causing the circular lockin=
g
>>>> dependency.
>>>>
>>>> Remedy this by using access_ok and __put_user instead of put_user so w=
e'll
>>>> return an error instead of faulting in the page.
>>> I wonder if it's better to create a kernel buffer outside of the lock i=
n
>>> keyctl_read_key().  Hmmm...  The reason I didn't want to do that is tha=
t
>>> keyrings have don't have limits on the size.  Maybe that's not actually=
 a
>>> problem, since 1MiB would be able to hold a list of a quarter of a mill=
ion
>>> keys.
>>>
>>> David
>>>
>> Hi David,
>>
>> Thanks for the feedback.
>>
>> I can try to prototype that, but regardless of where the kernel buffer
>> is allocated, the important part is causing the initial pagefault in the
>> read path outside the lock so __put_user won't fail due to a valid user
>> address but page backing the user address isn't in-core.
>>
>> I'll start work on v2.
> Actually I'm going to back off on a v2 effort at this point and request
> that folks comment on the code as-is. Changing keyctl_read_key to use
> its own kernel buffer might be a worthwhile effort, but it doesn't
> appear to me to have any effects on preventing pagefaults on user pages
> at inopportune points of the code.

Does anyone have any more feedback on v1 of this patch?

Thanks,

Chris

>
> Thanks,
>
> Chris
>
>> Thanks,
>>
>> Chris
>>