linux-kernel - Re: [PATCH v2 2/2] KEYS: Avoid false positive ENOMEM error on key read

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Date:   Tue, 10 Mar 2020 17:12:23 +0000
From:   David Howells <dhowells@...hat.com>
To:     Waiman Long <longman@...hat.com>
Cc:     dhowells@...hat.com,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Mimi Zohar <zohar@...ux.ibm.com>, keyrings@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-integrity@...r.kernel.org,
        Sumit Garg <sumit.garg@...aro.org>,
        Jerry Snitselaar <jsnitsel@...hat.com>,
        Roberto Sassu <roberto.sassu@...wei.com>,
        Eric Biggers <ebiggers@...gle.com>,
        Chris von Recklinghausen <crecklin@...hat.com>
Subject: Re: [PATCH v2 2/2] KEYS: Avoid false positive ENOMEM error on key read

Waiman Long <longman@...hat.com> wrote:

> That is not as simple as I thought. First of that, there is not an
> equivalent kzvfree() helper to clear the buffer first before clearing.
> Of course, I can do that manually.

Yeah, the actual substance of vfree() may get deferred.  It may be worth
adding a kvzfree() that switches between kzfree() and memset(),vfree().

> With patch 2, the allocated buffer length will be max(1024, keylen). The
> security code uses kmalloc() for allocation. If we use kvalloc() here,
> perhaps we should also use that for allocation that can be potentially
> large like that in big_key. What do you think?

Not for big_key: if it's larger than BIG_KEY_FILE_THRESHOLD (~1KiB) it gets
written encrypted into shmem so that it can be swapped out to disk when not in
use.

However, other cases, sure - just be aware that on a 32-bit system,
vmalloc/vmap space is a strictly limited resource.

David