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

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Message-ID: <30309.1571667719@warthog.procyon.org.uk>
Date:   Mon, 21 Oct 2019 15:21:59 +0100
From:   David Howells <dhowells@...hat.com>
To:     Chris von Recklinghausen <crecklin@...hat.com>
Cc:     dhowells@...hat.com,
        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

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