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

[Mimi Zohar]

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

-- 
thanks,

Mimi