Re: [PATCH] ima: Fall back to default kernel module signature verification

[Mimi Zohar <zohar@linux.ibm.com>]

On Mon, 2025-10-20 at 08:21 -0400, Mimi Zohar wrote:
> On Sat, 2025-10-18 at 07:19 +0800, Coiby Xu wrote:
> > > > > 2. Instead of defining an additional process_measurement() argument to identify
> > > > > compressed kernel modules, to simplify the code it might be possible to define a
> > > > > new "func" named COMPRESSED_MODULE_CHECK.
> > > > > 
> > > > > +       [READING_COMPRESSED_MODULE] = MODULE_CHECK,  -> COMPRESSED_MODULE_CHECK
> > > > 
> > > > I also thought about this approach. But IMA rule maps kernel module
> > > > loading to MODULE_CHECK. If we define a new rule and ask users to use
> > > > this new rule, ima_policy=secure_boot still won't work.
> > > 
> > > I don't have a problem with extending the "secure-boot" policy to support
> > > uncompressed kernel modules appended signatures, based on whether
> > > CONFIG_MODULE_SIG is enabled.  The new rule would be in addition to the existing
> > > MODULE_CHECK rule.
> > 
> > I assume once the new rule get added, we can't remove it for userspace
> > backward compatibility, right? And with CPIO xattr supported, it seems
> > there is no need to keep this rule. So if this concern is valid, do you
> > think we shall switch to another approach i.e. to make IMA support
> > verifying decompressed module and then make "secure-boot" to allow
> > appended module signature?
> 
> Yes, once the rule is added, it wouldn't be removed.  As for "to make IMA
> support verifying decompressed module", yes that might be a better solution,
> than relying on "sig_enforce" being enabled. IMA already supports verifying the
> appended signatures.  A new IMA specific or LSM hook would need to be defined
> after module_decompress().

Looking at the code further, decompressing the kernel module in IMA is
redundant.  Instead I think the best approach would be to:
- define DECOMPRESSED_MODULE, in addition to COMPRESSED_MODULE.

id(COMPRESSED_MODULE, compressed-kernel-module) \
id(DECOMPRESSED_MODULE, decompressed-kernel-module)    \

- instead of passing a boolean indicating whether the module is compressed, pass
the kernel_read_file_id enumeration to differentiate between the compressed and
decompressed module.

- define a new IMA hook, probably LSM hook as well, named
ima_decompressed_module().

- call the new ima_decompressed_module() from init_module_from_file()
immediately after decompressing the kernel module.  Something along the lines
of:

err = ima_decompressed_module(f, (char *)info.hdr, info.len,
                              READING_DECOMPRESSED_MODULE);

For testing purposes to see the decompressed appended signature in the
measurement list, modify the MODULE_CHECK measure rule to include "template=ima-
modsig" in ima_efi.c.

-- 
Mimi