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

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Date:   Fri, 25 Oct 2019 07:10:29 -0400
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 10/21/2019 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.

Thanks,

Chris

>
> Thanks,
>
> Chris
>