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fscrypt request_module() deadlock

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  fscrypt request_module() deadlock Richard Weinberger <richard@nod.at> - 2017-06-30 17:30 +0200

#1678905 — 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

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