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

[Roberto Sassu]

On Tue, 2024-02-13 at 11:31 -0500, Stefan Berger wrote:
> 
> On 2/13/24 03:57, Roberto Sassu wrote:
> > 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.
> 
> When ecdsa is used for signing files it could get stuck as well and 
> would need this patch:
> 
> diff --git a/security/integrity/ima/ima_main.c 
> b/security/integrity/ima/ima_main.c
> index 45f1a102c599..2e71dc977d43 100644
> --- a/security/integrity/ima/ima_main.c
> +++ b/security/integrity/ima/ima_main.c
> @@ -1110,7 +1110,9 @@ EXPORT_SYMBOL_GPL(ima_measure_critical_data);
>    */
>   static int ima_kernel_module_request(char *kmod_name)
>   {
> -       if (strncmp(kmod_name, "crypto-pkcs1pad(rsa,", 20) == 0)
> +       if (strncmp(kmod_name, "crypto-pkcs1pad(rsa,", 20) == 0 ||
> +           strncmp(kmod_name, "crypto-ecdsa-nist-p", 19) == 0 ||
> +           strcmp(kmod_name, "cryptomgr") == 0)
>                  return -EINVAL;
> 
>          return 0;
> 
> Rejecting cryptomgr seems necessary in the ecdsa case though I am not 
> sure what the side effects of rejecting it all the time could be.

Thanks. Ok, let's find a proper way once IMA/EVM are moved to the LSM
infrastructure.

Roberto

>     Stefan
> 
> > 
> > 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
> > 
> >