linux-kernel - Re: [PATCH] security/keyring: avoid pagefaults in keyring_read

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Date:   Wed, 6 Nov 2019 10:25:55 -0500
From:   Chris von Recklinghausen <crecklin@...hat.com>
To:     David Howells <dhowells@...hat.com>
Cc:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E . Hallyn" <serge@...lyn.com>, keyrings@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, Waiman Long <longman@...hat.com>
Subject: Re: [PATCH] security/keyring: avoid pagefaults in
 keyring_read_iterator

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@...hat.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 reverse
>>>> order that keyring_read_iterator did, thus causing the circular locking
>>>> dependency.
>>>>
>>>> Remedy this by using access_ok and __put_user instead of put_user so we'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 in
>>> keyctl_read_key().  Hmmm...  The reason I didn't want to do that is that
>>> 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 million
>>> 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
>>