linux-kernel - fscrypt request

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Message-ID: <3450b67a-6766-d24e-5a84-18fff64d1b73@nod.at>
Date:   Fri, 30 Jun 2017 17:27:34 +0200
From:   Richard Weinberger <richard@....at>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Cc:     linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-fscrypt@...r.kernel.org, David Gstir <david@...ma-star.at>,
        herbert@...dor.apana.org.au
Subject: fscrypt request_module() deadlock

Hi!

David and I faced a deadlock with switch_root when fscrypt was in use.
When /sbin/modprobe is encrypted using fscrypt and no other kernel component
requested an AES cipher before, first access to an encrypted file will trigger the
module_request() function, which will execute usermode helper /sbin/modprobe.
Is /sbin/modprobe also encrypted the kernel will deadlock because executing
it will again enter the module_request() path...

As workaround we currently do something like "ls /new_root > /dev/null" in our
initramfs to make request_module() happen before we change the root directory
to /new_root.

While this workaround is legit we think that this could be handled better.
Is there a way to request these ciphers before first usage? Herbert?
e.g. such that the filesystem can request them upon mount time.

Btw: This happens even when AES modules are builtins.

Thanks,
//richard