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

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Message-ID: <b8aa0f7c-0a90-efae-9fb7-aa85b19a0d9a@redhat.com>
Date:   Mon, 21 Oct 2019 11:46:15 -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 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.

Thanks,

Chris