linux-kernel - Re: [PATCH v2] lsm,ima: new LSM hook security_kernel_module_read_file to access decompressed kernel module

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Message-ID: <42qcfcxxlmwphctzvji76hy5tycfabiiv5u6zw6lgg2p3e2jwv@fp4g2y7ecf2y>
Date: Thu, 13 Nov 2025 12:06:02 +0800
From: Coiby Xu <coxu@...hat.com>
To: Mimi Zohar <zohar@...ux.ibm.com>
Cc: Paul Moore <paul@...l-moore.com>, linux-integrity@...r.kernel.org, 
	linux-security-module@...r.kernel.org, Karel Srot <ksrot@...hat.com>, James Morris <jmorris@...ei.org>, 
	"Serge E. Hallyn" <serge@...lyn.com>, Luis Chamberlain <mcgrof@...nel.org>, 
	Petr Pavlu <petr.pavlu@...e.com>, Daniel Gomez <da.gomez@...nel.org>, 
	Sami Tolvanen <samitolvanen@...gle.com>, Roberto Sassu <roberto.sassu@...wei.com>, 
	Dmitry Kasatkin <dmitry.kasatkin@...il.com>, Eric Snowberg <eric.snowberg@...cle.com>, 
	open list <linux-kernel@...r.kernel.org>, "open list:MODULE SUPPORT" <linux-modules@...r.kernel.org>
Subject: Re: [PATCH v2] lsm,ima: new LSM hook
 security_kernel_module_read_file to access decompressed kernel module

On Fri, Nov 07, 2025 at 02:28:13PM -0500, Mimi Zohar wrote:
>On Thu, 2025-11-06 at 17:15 -0500, Mimi Zohar wrote:
>> On Thu, 2025-11-06 at 21:29 +0800, Coiby Xu wrote:
>> > On Wed, Nov 05, 2025 at 03:47:25PM -0500, Mimi Zohar wrote:
>> > > On Wed, 2025-11-05 at 08:18 +0800, Coiby Xu wrote:
>> > [...]
>> > >
>> > > Hi Coiby,
>> > >
>> > > Based on the conversation with Paul, there is no reason to remove the existing
>> > > security_kernel_post_read_file() call.
>> > >
>> > > The changes are similar to the 2nd link, but a bit different.
>> > > - Define a single enumeration named READING_MODULE_COMPRESSED.
>> > >
>> > > - In module/main.c add a new security_kernel_post_read_file() call immediately
>> > > after decompressing the kernel module.  Like a previous version of this patch,
>> > > call kernel_read_file() with either READING_MODULE or READING_MODULE_COMPRESSED
>> > > based on MODULE_INIT_COMPRESSED_FILE.
>> > >
>> > > - In ima_post_read_file() defer verifying the signature when the enumeration is
>> > > READING_MODULE_COMPRESSED.  (No need for a new function ima_read_kernel_module.)
>> >
>> > Hi Mimi,
>> >
>> > Thanks for summarizing your conversation with Paul! I can confirm Paul's
>> > approach works
>> > https://github.com/coiby/linux/tree/in_kernel_decompression_ima_no_lsm_hook_paul
>> >
>> > While testing the patch today, I realized there is another
>> > issue/challenge introduced by in-kernel module decompression. IMA
>> > appraisal is to verify the digest of compressed kernel module but
>> > currently the passed buffer is uncompressed module. When IMA uses
>> > uncompressed module data to calculate the digest, xattr signature
>> > verification will fail. If we always make IMA read the original kernel
>> > module data again to calculate the digest, does it look like a
>> > quick-and-dirty fix? If we can assume people won't load kernel module so
>> > often, the performance impact is negligible. Otherwise we may have to
>> > introduce a new LSM hook so IMA can access uncompressed and original
>> > module data one time.
>>
>> ima_collect_measurement() stores the file hash info in the iint and uses that
>> information to verify the signature as stored in the security xattr.
>> Decompressing the kernel module shouldn't affect the xattr signature
>> verification.
>
>In the case when the compressed kernel module hasn't previously been measured or
>appraised before loading the kernel module, we need to "collect" the file data
>hash on READING_MODULE_COMPRESSED, but defer appraising/measuring it.
>
>An alternative to your suggestion of re-reading the original kernel module data
>to calculate the digest or defining a new hook, would be to define "collect" as
>a new "action" and pass the kernel_read_file_id enumeration to
>process_measurement().  IMA_COLLECTED already exists.  Only IMA_COLLECT would
>need to be defined.  The new collect "action" should be limited to
>func=MODULE_CHECK.
>
>The downside of this alternative is that it requires a new collect rule:
>collect func=MODULE_CHECK mask=MAY_READ uid=0
>appraise func=MODULE_CHECK appraise_type=imasig|modsig

Thank for suggesting an alternative! I've implemented the idea in
https://github.com/coiby/linux/tree/in_kernel_decompression_ima_collect

Note besides a new collect rule, another change is needed. Currently,
process_measurement only accepts enum ima_hooks thus it can't tell if
it's READING_MODULE_COMPRESSED so to only do collect action. So I
create a fake MODULE_COMPRESSED_CHECK func.

And for the idea of re-reading the original kernel module data, it has
been implemented in 
https://github.com/coiby/linux/tree/in_kernel_decompression_ima_no_lsm_hook_paul

Both branches have applied your requested three changes including
respecting the 80 char line limit. Additionally, I made a change to the
IPE LSM because of the new READING_MODULE_COMPRESSED kernel_read_file_id
enumerate.

After comparing the two implementations, personally I prefer re-reading
the original kernel module data because the change is smaller and it's
more user-friendly. But if there are other reasons I don't know, I'll
post the patches of the new collect action approach to the mailing list.


-- 
Best regards,
Coiby