Re: [PATCH v9 19/25] integrity: Move integrity_kernel_module_request() to IMA

[Roberto Sassu]

On Mon, 2024-02-12 at 15:28 -0500, Stefan Berger wrote:
> 
> On 2/12/24 12:56, Paul Moore wrote:
> > On Mon, Feb 12, 2024 at 12:48 PM Stefan Berger <stefanb@xxxxxxxxxxxxx> wrote:
> > > On 1/15/24 13:18, Roberto Sassu wrote:
> > 
> > ...
> > 
> > > > +/**
> > > > + * ima_kernel_module_request - Prevent crypto-pkcs1pad(rsa,*) requests
> > > > + * @kmod_name: kernel module name
> > > > + *
> > > > + * We have situation, when public_key_verify_signature() in case of RSA > + * algorithm use alg_name to store internal information in order to
> > > > + * construct an algorithm on the fly, but crypto_larval_lookup() will try
> > > > + * to use alg_name in order to load kernel module with same name.
> > > > + * Since we don't have any real "crypto-pkcs1pad(rsa,*)" kernel modules,
> > > > + * we are safe to fail such module request from crypto_larval_lookup().
> > > > + *
> > > > + * In this way we prevent modprobe execution during digsig verification
> > > > + * and avoid possible deadlock if modprobe and/or it's dependencies
> > > > + * also signed with digsig.
> > > 
> > > This text needs to some reformulation at some point..
> > 
> > There is no time like the present.  If you have a suggestion I would
> > love to hear it and I'm sure Roberto would too.
> > 
> 
> My interpretation of the issue after possibly lossy decoding of the 
> above sentences:
> 
> Avoid a deadlock by rejecting a virtual kernel module with the name 
> "crypto-pkcs1pad(rsa,*)". This module may be requested by 
> crypto_larval_lookup() while trying to verify an RSA signature in 
> public_key_verify_signature(). Since the loading of the RSA module may 
> itself cause the request for an RSA signature verification it will 
> otherwise lead to a deadlock.

I can be even more precise I guess (I actually reproduced it).

Avoid a verification loop where verifying the signature of the modprobe
binary requires executing modprobe itself. Since the modprobe iint-
>mutex is already held when the signature verification is performed, a
deadlock occurs as soon as modprobe is executed within the critical
region, since the same lock cannot be taken again.

This happens when public_key_verify_signature(), in case of RSA
algorithm, use alg_name to store internal information in order to
construct an algorithm on the fly, but crypto_larval_lookup() will try
to use alg_name in order to load a kernel module with same name.

Since we don't have any real "crypto-pkcs1pad(rsa,*)" kernel modules,
we are safe to fail such module request from crypto_larval_lookup(),
and avoid the verification loop.

Roberto