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

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

On Thu, 2025-11-13 at 12:06 +0800, Coiby Xu wrote:
> 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

As it turns out, the "collect" rule is unnecessary.  On
READING_MODULE_COMPRESSED, process_measurement() should calculate the compressed
file hash.  Extending the IMA measurement list and verifying the signature can
then be differed to READING_MODULE.

> 
> 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.

Correct, either extending process_measurement() with the read_idmap enum or
defining the fake hook would work.

> 
> 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.

The "re-reading" option fails some of the tests.  As the "collect" rule isn't
needed, let's stick with the first option.

-- 

thanks,

Mimi